This work was supported by the Burroughs Wellcome Career Award at the Scientific Interface (A.R.D.), the National Institutes of Health through the NIH Director's New Innovator Award Program 1-DP2-OD007078 (A.R.D.), the William Bowes Jr. Stanford Graduate Fellowship (A.S.A.), and the Stanford Cardiovascular Institute Younger Predoctoral Fellowship (J.C.). The authors thank James Spudich for loaning microscopy equipment.

1. Flow Cell Preparation
<ol>
	<li>Coverslips (#1.5, 22x22 mm and 22x40 mm, VWR) are cleaned using sonication.
	<ol>
		<li>Add the coverslips to a small glass container capable of holding coverslips and fitting in the sonicator (see step 2).</li>
		<li>Fill the container with isopropanol and sonicate in a bath sonicator for 20 minutes.</li>
		<li>Discard the isopropanol and rinse the coverslips with copious quantities of deionized water produced by a Barnsted MilliQ apparatus or similar device. Fill the container with ...

<ol>
	<li>Ingber, D. E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Can+cancer+be+reversed+by+engineering+the+tumor+microenvironment.">Can cancer be reversed by engineering the tumor microenvironment.</a> Semin. Cancer Biol. 18, 356-364 (2008).</li><li>Hahn, C., Schwartz, M. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Mechanotransduction+in+vascular+physiology+and+atherogenesis.">Mechanotransduction in vascular physiology and atherogenesis.</a> Nat. Rev. Mol. Cell Biol. 10, 53-62 (2009).</li><li>Laurens, N., Koolwijk, P., de Maat, M. P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Fibrin+structure+and+wound+healing.">Fibrin structure and wound healing.</a> J. Thromb. Haemost. 4, 932-939 (2006).</li><li>Adhikari, A. S., Chai, J., Dunn, A. R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Mechanical+Load+Induces+a+100-Fold+Increase+in+the+Rate+of+Collagen+Proteolysis+by+MMP-1.">Mechanical Load Induces a 100-Fold Increase in the Rate of Collagen Proteolysis by MMP-1.</a> J. Am. Chem. Soc. 133, (2011).</li><li>Zareian, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Probing+collagen/enzyme+mechanochemistry+in+native+tissue+with+dynamic,+enzyme-induced+creep.">Probing collagen/enzyme mechanochemistry in native tissue with dynamic, enzyme-induced creep.</a> Langmuir. 26, (2010).</li><li>Ellsmere, J. C., Khanna, R. A., Lee, J. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Mechanical+loading+of+bovine+pericardium+accelerates+enzymatic+degradation.">Mechanical loading of bovine pericardium accelerates enzymatic degradation.</a> Biomaterials. 20, 1143-1150 (1999).</li><li>Jesudason, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Mechanical+forces+regulate+elastase+activity+and+binding+site+availability+in+lung+elastin.">Mechanical forces regulate elastase activity and binding site availability in lung elastin.</a> Biophys. J. 99, 3076-3083 (2010).</li><li>Neuman, K. C., Block, S. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Optical+trapping.">Optical trapping.</a> Rev. Sci. Instrum. 75, 2787-2809 (2004).</li><li>Lee, C. K., Wang, Y. M., Huang, L. S., Lin, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Atomic+force+microscopy:+determination+of+unbinding+force,+off+rate+and+energy+barrier+for+protein-ligand+interaction.">Atomic force microscopy: determination of unbinding force, off rate and energy barrier for protein-ligand interaction.</a> Micron. 38, 446-461 (2007).</li><li>Smith, S. B., Finzi, L., Bustamante, C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Direct+mechanical+measurements+of+the+elasticity+of+single+DNA+molecules+by+using+magnetic+beads.">Direct mechanical measurements of the elasticity of single DNA molecules by using magnetic beads.</a> Science. 258, 1122-1126 (1992).</li><li>Tanase, M., Biais, N., Sheetz, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Magnetic+tweezers+in+cell+biology.">Magnetic tweezers in cell biology.</a> Methods Cell Biol. 83, 473-493 (2007).</li><li>Neuman, K. C., Nagy, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Single-molecule+force+spectroscopy:+optical+tweezers,+magnetic+tweezers+and+atomic+force+microscopy.">Single-molecule force spectroscopy: optical tweezers, magnetic tweezers and atomic force microscopy.</a> Nat. Methods. 5, 491-505 (2008).</li><li>Frank, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Stabilization+of+short+collagen-like+triple+helices+by+protein+engineering.">Stabilization of short collagen-like triple helices by protein engineering.</a> J. Mol. Biol. 308, 1081-1089 (2001).</li><li>Stetefeld, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Collagen+stabilization+at+atomic+level:+crystal+structure+of+designed+(GlyProPro)10foldon.">Collagen stabilization at atomic level: crystal structure of designed (GlyProPro)10foldon.</a> Structure. 11, 339-346 (2003).</li><li>Chung, L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Collagenase+unwinds+triple-helical+collagen+prior+to+peptide+bond+hydrolysis.">Collagenase unwinds triple-helical collagen prior to peptide bond hydrolysis.</a> EMBO J. 23, 3020-3030 (2004).</li><li>Bell, G. I. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Models+for+the+specific+adhesion+of+cells+to+cells.">Models for the specific adhesion of cells to cells.</a> Science. 200, 618-627 (1978).</li><li>Selvin, P. R., Ha, T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Single-molecule techniques: a laboratory manual. , (2008).</li><li>Graneli, A., Yeykal, C. C., Prasad, T. K., Greene, E. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Organized+arrays+of+individual+DNA+molecules+tethered+to+supported+lipid+bilayers.">Organized arrays of individual DNA molecules tethered to supported lipid bilayers.</a> Langmuir. 22, 292-299 (2006).</li><li>Danilowicz, C., Greenfield, D., Prentiss, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Dissociation+of+ligand-receptor+complexes+using+magnetic+tweezers.">Dissociation of ligand-receptor complexes using magnetic tweezers.</a> Anal. Chem. 77, 3023-3028 (2005).</li><li>Zhang, X., Halvorsen, K., Zhang, C. Z., Wong, W. P., Springer, T. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Mechanoenzymatic+cleavage+of+the+ultralarge+vascular+protein+von+Willebrand+factor.">Mechanoenzymatic cleavage of the ultralarge vascular protein von Willebrand factor.</a> Science. 324, 1330-1334 (2009).</li><li>Woodside, M. T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Nanomechanical+measurements+of+the+sequence-dependent+folding+landscapes+of+single+nucleic+acid+hairpins.">Nanomechanical measurements of the sequence-dependent folding landscapes of single nucleic acid hairpins.</a> Proc. Natl. Acad. Sci. U.S.A. 103, 6190-6195 (2006).</li><li>del Rio, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Stretching+single+talin+rod+molecules+activates+vinculin+binding.">Stretching single talin rod molecules activates vinculin binding.</a> Science. 323, 638-641 (2009).</li><li>Gosse, C., Croquette, V. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Magnetic+tweezers:+micromanipulation+and+force+measurement+at+the+molecular+level.">Magnetic tweezers: micromanipulation and force measurement at the molecular level.</a> Biophys. J. 82, 3314-3329 (2002).</li></ol>

This protocol describes a new use for a classical single molecule technique. Magnetic tweezers allow medium to high-throughput single molecule assays in a cost-efficient manner. However, like all experimental techniques there are challenges and potential pitfalls.
Limitations of magnetic tweezers
Compared to an optical trap the spatial and temporal resolution of a MT apparatus is low. Moreover, the forces generated by the simple MT described here are 30 pN or less, sig...

<table border="1">
 <tbody>
 <tr>
 <td>Name of Reagent</td>
 <td>Company</td>
 <td>Catalogue Number</td>
 </tr>
 <tr>
 <td>Micro Cover Glass #1.5 (22x22)</td>
 <td>VWR</td>
 <td>48366-067</td>
 </tr>
 <tr>
 <td>Micro Cover Glass #1.5 (22x40)</td>
 <td>VWR</td>
 <td>48393-048</td>
 </tr>
 <tr>
 <td>Lambda DNA</td>
 <td>Invitrogen</td>
 <td>25250-010</td>
 </tr>
 <tr>
 <td>T4 DNA Ligase</td>
 <td>Invitrogen</td>
 <td>15224-041</td>
 </tr>
 <tr>
 <td>Microcon Ultracel YM-100</td>
 <td>Millipore</td>
 <td>42413</td>
 </tr>
 <tr>
 <td>Anti-Digoxigenin</td>
 <td>Roche Diagnostics</td>
 <td>11-333-089-001</td>
 </tr>
 <tr>
 <td>Tween 20</td>
 <td>Sigma</td>
 <td>P9416-100ML</td>
 </tr>
 <tr>
 <td>Anti-myc Antibody</td>
 <td>Invitrogen</td>
 <td>46-0603</td>
 </tr>
 <tr>
 <td>Bovine Serum Albumin</td>
 <td>Sigma</td>
 <td>B4287-5G</td>
 </tr>
 <tr>
 <td>Dynabeads M-280 Streptavidin</td>
 <td>Invitrogen</td>
 <td>658.01D</td>
 </tr>
 <tr>
 <td>Dynabeads MyOne T1 Streptavidin</td>
 <td>Invitrogen</td>
 <td>658.01D</td>
 </tr>
 <tr>
 <td>p-Aminophenylmercuric Acetate</td>
 <td>Calbiochem</td>
 <td>164610</td>
 </tr>
 <tr>
 <td>Biotin-Maleimide</td>
 <td>Sigma Aldrich</td>
 <td>B1267</td>
 </tr>
 <tr>
 <td>Biotin labeled oligo</td>
 <td>IDT DNA</td>
 <td>Custom synthesis</td>
 </tr>
 <tr>
 <td>Digoxigenin labeled oligo</td>
 <td>IDT DNA</td>
 <td>Custom synthesis</td>
 </tr>
 <tr>
 <td>Collagen peptide gene</td>
 <td>DNA 2.0</td>
 <td>Custom synthesis</td>
 </tr>
 <tr>
 <td>MMP-1 cDNA</td>
 <td>Harvard Plasmid Database</td>
 <td>&nbsp;</td>
 </tr>
 <tr>
 <td>z-translator</td>
 <td>Thorlabs</td>
 <td>MTS50</td>
 </tr>
 <tr>
 <td>Servo controller for translator</td>
 <td>Thorlabs</td>
 <td>TDC001</td>
 </tr>
 </tbody>
 </table>

multiplexed single-molecule force proteolysis measurements using magnetic tweezers

The generation and detection of mechanical forces is a ubiquitous aspect of cell physiology, with direct relevance to cancer metastasis1, atherogenesis2 and wound healing3. In each of these examples, cells both exert force on their surroundings and simultaneously enzymatically remodel the extracellular matrix (ECM). The effect of forces on ECM has thus become an area of considerable interest due to its likely biological and medical importance4-7.
	Single molecule techniques such as optical trapping8, atomic force microscopy9, and magnetic tweezers10,11 allow researchers to probe the function of enzymes at a molecular level by exerting forces on individual proteins. Of these techniques, magnetic tweezers (MT) are notable for their low cost and high throughput. MT exert forces in the range of ~1-100 pN and can provide millisecond temporal resolution, qualities that are well matched to the study of enzyme mechanism at the single-molecule level12. Here we report a highly parallelizable MT assay to study the effect of force on the proteolysis of single protein molecules. We present the specific example of the proteolysis of a trimeric collagen peptide by matrix metalloproteinase 1 (MMP-1); however, this assay can be easily adapted to study other substrates and proteases.

Watch this Scientific Journal Video about Multiplexed Single-molecule Force Proteolysis Measurements Using Magnetic Tweezers at JoVE.com

Multiplexed Single-molecule Force Proteolysis Measurements Using Magnetic Tweezers

In this article we describe the use of magnetic tweezers to study the effect of force on enzymatic proteolysis at the single molecule level in a highly parallelizable manner.

The  generation and detection of mechanical forces is a ubiquitous aspect of cell physiology,  with direct relevance to cancer metastasis1, atherogenesis2 ...

multiplexed-single-molecule-force-proteolysis-measurements-using

Research

JoVE Journal

Bioengineering

11.5K ビュー.  Stanford University. この記事では、高度に並列化可能な方法で単一分子レベルでのタンパク質分解酵素に作用する力の影響を研究するために磁気ピンセットの使用方法について説明します。

ビデオ: Multiplexed Single-molecule Force Proteolysis Measurements Using Magnetic Tweezers

In this article we describe the use of magnetic tweezers to study the effect of force on enzymatic proteolysis at the single molecule level in a highly parallelizable...

analyzing telomeric protein-dna interactions using single-molecule magnetic tweezers

Author Spotlight: Advanced Single-Molecule Techniques for Investigating Telomeric Protein-DNA Interactions

This protocol demonstrates using single-molecule magnetic tweezers to study interactions between telomeric DNA-binding proteins (Telomere Repeat-binding Factor 1 [TRF1] and TRF2) and long telomeres extracted from human cells. It describes the preparatory steps for telomeres and telomeric repeat-binding factors, the execution of single-molecule experiments, and the data collection and analysis...

Analyzing Telomeric Protein-DNA Interactions Using Single-Molecule Magnetic Tweezers

high-speed magnetic tweezers for nanomechanical measurements on force-sensitive elements

High-Speed Magnetic Tweezers for Nanomechanical Measurements on Force-Sensitive Elements

Here, we describe a high-speed magnetic tweezer setup that performs nanomechanical measurements on force-sensitive biomolecules at the maximum rate of 1.2 kHz. We introduce its application to DNA hairpins and SNARE complexes as model systems, but it will be also applicable to other molecules involved in mechanobiological events.

single-molecule manipulation of g-quadruplexes by magnetic tweezers

Single-molecule Manipulation of G-quadruplexes by Magnetic Tweezers

A single-molecule magnetic tweezers platform to manipulate G-quadruplexes is reported, which allows for the study of G4 stability and regulation by various...

direct force measurements of subcellular mechanics in confinement using optical tweezers

Direct Force Measurements of Subcellular Mechanics in Confinement using Optical Tweezers

Here, we present a protocol to investigate the intracellular mechanical properties of isolated embryonic zebrafish cells in three-dimensional confinement with direct force measurement by an optical...

preparation of human telomeric restriction fragments and measurement of telomeric dna using single-molecule magnetic tweezers

Preparation of Human Telomeric Restriction Fragments and Measurement of Telomeric DNA Using Single-Molecule Magnetic Tweezers

magnetic tweezers in a microplate format

Magnetic Tweezers in a Microplate Format

Here, we describe the use of a novel microplate assay to enable mechanical manipulation of biomolecules while performing ensemble biochemical assays. This is achieved using a microplate lid modified with magnets to create multiple static magnetic tweezers across the...

use of dual optical tweezers and microfluidics for single-molecule studies

Use of Dual Optical Tweezers and Microfluidics for Single-Molecule Studies

Visual, single-molecule biochemistry studied through microfluidic chambers is greatly facilitated using glass barrel, gas-tight syringes, stable connections of tubing to flow cells, and elimination of bubbles by placing switching valves between the syringes and tubing. The protocol describes dual optical traps that enable visualization of DNA transactions and intermolecular...

investigating single molecule adhesion by atomic force spectroscopy

Investigating Single Molecule Adhesion by Atomic Force Spectroscopy

A protocol to couple a large variety of single molecules covalently onto an AFM tip is presented. Procedures and examples to determine the adhesion force and free energy of these molecules on solid supports and bio-interfaces are...

Studying TRF1/2 Interactions with Telomeric DNA Using Single-Molecule Assay

https://cloudfront.jove.com/CDNSource/teasers/67251_b.jpg

magnetic tweezers for the measurement of twist and torque

Magnetic Tweezers for the Measurement of Twist and Torque

Magnetic tweezers, a powerful single-molecule manipulation technique, can be adapted for the direct measurements of the twist (using a configuration called freely-orbiting magnetic tweezers) and torque (using a configuration termed magnetic torque tweezers) in biological macromolecules. Guidelines for performing such measurements are given, including applications to the study of DNA and associated nucleo-protein...

making precise and accurate single-molecule fret measurements using the open-source smfbox

Making Precise and Accurate Single-Molecule FRET Measurements using the Open-Source smfBox

This article provides step-by-step instructions for making fully-corrected accurate FRET measurements on individual, freely diffusing biomolecules using the open-source, inexpensive smfBox, from switch on, through alignment and focusing, to data collection and...

covalent immobilization of proteins for the single molecule force spectroscopy

Covalent Immobilization of Proteins for the Single Molecule Force Spectroscopy

This protocol describes the covalent immobilization of proteins with a heterobifunctional silane coupling agent to silicon-oxide surfaces designed for the atomic force microscopy based single molecule force spectroscopy which is exemplified by the interaction of RrgA (pilus-1 tip adhesin of S. pneumoniae) with...

insights into the interactions of amino acids and peptides with inorganic materials using single-molecule force spectroscopy

Insights into the Interactions of Amino Acids and Peptides with Inorganic Materials Using Single-Molecule Force Spectroscopy

Here we present a protocol to measure the force of interactions between a well-defined inorganic surface and either peptides or amino acids by single-molecule force spectroscopy measurements using an atomic force microscope (AFM). The information obtained from the measurement is important to better understand the peptide-inorganic material...

single-molecule imaging of fully reconstituted cmg with correlative dual-beam optical tweezers and confocal microscopy

Single-Molecule Imaging of Fully Reconstituted CMG with Correlative Dual-Beam Optical Tweezers and Confocal Microscopy

oaaep1-mediated enzymatic synthesis and immobilization of polymerized protein for single-molecule force spectroscopy

OaAEP1-Mediated Enzymatic Synthesis and Immobilization of Polymerized Protein for Single-Molecule Force Spectroscopy

Here, we present a protocol to conjugate protein monomer by enzymes forming protein polymer with a controlled sequence and immobilize it on the surface for single-molecule force spectroscopy...

single-molecule surface-enhanced raman scattering measurements enabled by plasmonic dna origami nanoantennas

Author Spotlight: Single-Molecule Surface-Enhanced Raman Scattering Measurements Enabled by Plasmonic DNA Origami Nanoantennas  

This protocol demonstrates single-molecule surface-enhanced Raman scattering (SERS) measurements using a DNA origami nanoantenna (DONA) combined with colocalized atomic force microscopy (AFM) and Raman...

Single-Molecule Surface-Enhanced Raman Scattering Measurements Enabled by Plasmonic DNA Origami Nanoantennas

automated system for single molecule fluorescence measurements of surface-immobilized biomolecules

Automated System for Single Molecule Fluorescence Measurements of Surface-immobilized Biomolecules

In this article we describe how we obtain FRET traces from individual DNA molecules immobilized to a surface using an automated scanning confocal microscope.

in situ nucleosome assembly for single-molecule correlative force and fluorescence microscopy

Author Spotlight: Efficient Nucleosome Reconstitution for Single-Molecule Techniques

This article presents a detailed experimental procedure for reconstituting nucleosome-containing DNA tethers for single-molecule correlative force and fluorescence microscopy. It further describes several downstream experiments that can be conducted to visualize the binding behavior of chromatin-interacting proteins and analyze changes in the physical properties of...

In Situ Nucleosome Assembly for Single-Molecule Correlative Force and Fluorescence Microscopy

multiplexed quantitative bacterial detection using dye-labeled magnetic microspheres

This video demonstrates multiplexed quantitative bacterial detection employing carboxylated magnetic polystyrene microspheres dyed into distinct sets. The procedure encompasses the formation of bead-bacteria complexes, magnetic separation, and flow-based analysis, enabling precise and simultaneous quantification of multiple bacteria in the same reaction...

Multiplexed Quantitative Bacterial Detection using Dye-Labeled Magnetic Microspheres

Stretching Short Sequences of DNA with Constant Force Axial Optical Tweezers

Single-molecule techniques for stretching DNA of contour lengths less than a kilobase are fraught with experimental difficulties. However, many interesting biological events such as histone binding and protein-mediated looping of DNA1,2, occur on this length scale. In recent years, the mechanical properties of DNA have been shown to play a significant role in fundamental cellular processes like the packaging of DNA into compact nucleosomes and chromatin fibers3,4. Clearly, it is then important to understand the mechanical properties of short stretches of DNA. In this paper, we provide a practical guide to a single-molecule optical tweezing technique that we have developed to study the mechanical behavior of DNA with contour lengths as short as a few hundred basepairs.
The major hurdle in stretching short segments of DNA is that conventional optical tweezers are generally designed to apply force in a direction lateral to the stage5,6, (see Fig. 1). In this geometry, the angle between the bead and the coverslip, to which the DNA is tethered, becomes very steep for submicron length DNA. The axial position must now be accounted for, which can be a challenge, and, since the extension drags the microsphere closer to the coverslip, steric effects are enhanced. Furthermore, as a result of the asymmetry of the microspheres, lateral extensions will generate varying levels of torque due to rotation of the microsphere within the optical trap since the direction of the reactive force changes during the extension.
Alternate methods for stretching submicron DNA run up against their own unique hurdles. For instance, a dual-beam optical trap is limited to stretching DNA of around a wavelength, at which point interference effects between the two traps and from light scattering between the microspheres begin to pose a significant problem. Replacing one of the traps with a micropipette would most likely suffer from similar challenges. While one could directly use the axial potential to stretch the DNA, an active feedback scheme would be needed to apply a constant force and the bandwidth of this will be quite limited, especially at low forces.
We circumvent these fundamental problems by directly pulling the DNA away from the coverslip by using a constant force axial optical tweezers7,8. This is achieved by trapping the bead in a linear region of the optical potential, where the optical force is constant-the strength of which can be tuned by adjusting the laser power. Trapping within the linear region also serves as an all optical force-clamp on the DNA that extends for nearly 350 nm in the axial direction. We simultaneously compensate for thermal and mechanical drift by finely adjusting the position of the stage so that a reference microsphere stuck to the coverslip remains at the same position and focus, allowing for a virtually limitless observation period.

We illustrate the use of a constant force axial optical tweezers to explore the mechanical properties of short DNA molecules. By stretching DNA axially, we minimize steric hindrances and artifacts arising in conventional lateral manipulation, allowing us to study DNA molecules as short as ~100 nm.

Single-molecule techniques for stretching DNA of contour lengths  less than a kilobase are fraught with experimental difficulties. However, many ...

Flexural Rigidity Measurements of Biopolymers Using Gliding Assays

Microtubules are cytoskeletal polymers which play a role in cell division, cell mechanics, and intracellular transport. Each of these functions requires microtubules that are stiff and straight enough to span a significant fraction of the cell diameter. As a result, the microtubule persistence length, a measure of stiffness, has been actively studied for the past two decades1. Nonetheless, open questions remain: short microtubules are 10-50 times less stiff than long microtubules2-4, and even long microtubules have measured persistence lengths which vary by an order of magnitude5-9.
	Here, we present a method to measure microtubule persistence length. The method is based on a kinesin-driven microtubule gliding assay10. By combining sparse fluorescent labeling of individual microtubules with single particle tracking of individual fluorophores attached to the microtubule, the gliding trajectories of single microtubules are tracked with nanometer-level precision. The persistence length of the trajectories is the same as the persistence length of the microtubule under the conditions used11. An automated tracking routine is used to create microtubule trajectories from fluorophores attached to individual microtubules, and the persistence length of this trajectory is calculated using routines written in IDL.
	This technique is rapidly implementable, and capable of measuring the persistence length of 100 microtubules in one day of experimentation. The method can be extended to measure persistence length under a variety of conditions, including persistence length as a function of length along microtubules. Moreover, the analysis routines used can be extended to myosin-based acting gliding assays, to measure the persistence length of actin filaments as well.

A method to measure the persistence length or flexural rigidity of biopolymers is described. The method uses a kinesin-driven microtubule gliding assay to experimentally determine the persistence length of individual microtubules and is adaptable to actin-based gliding assays.

Microtubules are cytoskeletal polymers which  play a role in cell division, cell mechanics, and intracellular transport. Each of these functions requires ...

Investigating Receptor-ligand Systems of the Cellulosome with AFM-based Single-molecule Force Spectroscopy

Cellulosomes are discrete multienzyme complexes used by a subset of anaerobic bacteria and fungi to digest lignocellulosic substrates. Assembly of the enzymes onto the noncatalytic scaffold protein is directed by interactions among a family of related receptor-ligand pairs comprising interacting cohesin and dockerin modules. The extremely strong binding between cohesin and dockerin modules results in dissociation constants in the low picomolar to nanomolar range, which may hamper accurate off-rate measurements with conventional bulk methods. Single-molecule force spectroscopy (SMFS) with the atomic force microscope measures the response of individual biomolecules to force, and in contrast to other single-molecule manipulation methods (i.e.&#160;optical tweezers), is optimal for studying high-affinity receptor-ligand interactions because of its ability to probe the high-force regime (&#62;120 pN). Here we present our complete protocol for studying cellulosomal protein assemblies at the single-molecule level. Using a protein topology derived from the native cellulosome, we worked with enzyme-dockerin and carbohydrate binding module-cohesin (CBM-cohesin) fusion proteins, each with an accessible free thiol group at an engineered cysteine residue. We present our site-specific surface immobilization protocol, along with our measurement and data analysis procedure for obtaining detailed binding parameters for the high-affinity complex. We demonstrate how to quantify single subdomain unfolding forces, complex rupture forces, kinetic off-rates, and potential widths of the binding well. The successful application of these methods in characterizing the cohesin-dockerin interaction responsible for assembly of multidomain cellulolytic complexes is further described.

Cellulosomes are multienzyme complexes designed for digesting cellulose. AFM-based SMFS was used to study the mechanical properties and folding configuration of cellulosome-associated protein assemblies. We present a complete workflow for protein immobilization, data acquisition, and data analysis to study the interactions of individual receptor-ligand complexes involved in cellulosome assembly.

Cellulosomes are discrete multienzyme complexes used by a subset of anaerobic bacteria and fungi to digest lignocellulosic substrates. Assembly of the ...

Single-molecule techniques make it possible to investigate the behavior of individual biological molecules in solution in real time. These techniques include so-called force spectroscopy approaches such as atomic force microscopy, optical tweezers, flow stretching, and magnetic tweezers. Amongst these approaches, magnetic tweezers have distinguished themselves by their ability to apply torque while maintaining a constant stretching force. Here, it is illustrated how such a &#8220;conventional&#8221; magnetic tweezers experimental configuration can, through a straightforward modification of its field configuration to minimize the magnitude of the transverse field, be adapted to measure the degree of twist in a biological molecule. The resulting configuration is termed the freely-orbiting magnetic tweezers. Additionally, it is shown how further modification of the field configuration can yield a transverse field with a magnitude intermediate between that of the &#8220;conventional&#8221; magnetic tweezers and the freely-orbiting magnetic tweezers, which makes it possible to directly measure the torque stored in a biological molecule. This configuration is termed the magnetic torque tweezers. The accompanying video explains in detail how the conversion of conventional magnetic tweezers into freely-orbiting magnetic tweezers and magnetic torque tweezers can be accomplished, and demonstrates the use of these techniques. These adaptations maintain all the strengths of conventional magnetic tweezers while greatly expanding the versatility of this powerful instrument.

Magnetic tweezers, a powerful single-molecule manipulation technique, can be adapted for the direct measurements of the twist (using a configuration called freely-orbiting magnetic tweezers) and torque (using a configuration termed magnetic torque tweezers) in biological macromolecules. Guidelines for performing such measurements are given, including applications to the study of DNA and associated nucleo-protein filaments.

Single-molecule techniques make it possible to investigate the behavior of individual biological molecules in solution in real time. These techniques ...

High-resolution Spatiotemporal Analysis of Receptor Dynamics by Single-molecule Fluorescence Microscopy

Single-molecule microscopy is emerging as a powerful approach to analyze the behavior of signaling molecules, in particular concerning those aspect (e.g., kinetics, coexistence of different states and populations, transient interactions), which are typically hidden in ensemble measurements, such as those obtained with standard biochemical or microscopy methods. Thus, dynamic events, such as receptor-receptor interactions, can be followed in real time in a living cell with high spatiotemporal resolution. This protocol describes a method based on labeling with small and bright organic fluorophores and total internal reflection fluorescence (TIRF) microscopy to directly visualize single receptors on the surface of living cells. This approach allows one to precisely localize receptors, measure the size of receptor complexes, and capture dynamic events such as transient receptor-receptor interactions. The protocol provides a detailed description of how to perform a single-molecule experiment, including sample preparation, image acquisition and image analysis. As an example, the application of this method to analyze two G-protein-coupled receptors, i.e., &#946;2-adrenergic and &#947;-aminobutyric acid type B (GABAB) receptor, is reported. The protocol can be adapted to other membrane proteins and different cell models, transfection methods and labeling strategies.

This protocol describes how to use total internal reflection fluorescence microscopy to visualize and track single receptors on the surface of living cells and thereby analyze receptor lateral mobility, size of receptor complexes as well as to visualize transient receptor-receptor interactions. This protocol can be extended to other membrane proteins.

Single-molecule microscopy is emerging as a powerful approach to analyze the behavior of signaling molecules, in particular concerning those aspect (e.g., ...

Single Molecule Fluorescence Microscopy on Planar Supported Bilayers

In the course of a single decade single molecule microscopy has changed from being a secluded domain shared merely by physicists with a strong background in optics and laser physics to a discipline that is now enjoying vivid attention by life-scientists of all venues 1. This is because single molecule imaging has the unique potential to reveal protein behavior in situ in living cells and uncover cellular organization with unprecedented resolution below the diffraction limit of visible light 2. Glass-supported planar lipid bilayers (SLBs) are a powerful tool to bring cells otherwise growing in suspension in close enough proximity to the glass slide so that they can be readily imaged in noise-reduced Total Internal Reflection illumination mode 3,4. They are very useful to study the protein dynamics in plasma membrane-associated events as diverse as cell-cell contact formation, endocytosis, exocytosis and immune recognition. Simple procedures are presented how to generate highly mobile protein-functionalized SLBs in a reproducible manner, how to determine protein mobility within and how to measure protein densities with the use of single molecule detection. It is shown how to construct a cost-efficient single molecule microscopy system with TIRF illumination capabilities and how to operate it in the experiment.

Preparation of protein-functionalized planar glass-supported lipid bilayers, determination of protein mobility within and measurement of protein densities is shown here. A roadmap to building a noise-reduced Total Internal Reflection microscope is outlined, which allows visualizing single bilayer-resident fluorochromes with high spatiotemporal resolution.

In the course of a single decade single molecule microscopy has changed from being a secluded domain shared merely by physicists with a strong background ...

Label-free Single Molecule Detection Using Microtoroid Optical Resonators

Detecting small concentrations of molecules down to the single molecule limit has impact on areas such as early detection of disease, and fundamental studies on the behavior of molecules. Single molecule detection techniques commonly utilize labels such as fluorescent tags or quantum dots, however, labels are not always available, increase cost and complexity, and can perturb the events being studied. Optical resonators have emerged as a promising means to detect single molecules without the use of labels. Currently the smallest particle detected by a non-plasmonically-enhanced bare optical resonator system in solution is a 25 nm polystyrene sphere1. We have developed a technique known as Frequency Locking Optical Whispering Evanescent Resonator (FLOWER) that can surpass this limit and achieve label-free single molecule detection in aqueous solution2. As signal strength scales with particle volume, our work represents a &#62; 100x improvement in the signal to noise ratio (SNR) over the current state of the art. Here the procedures behind FLOWER are presented in an effort to increase its usage in the field.

We have developed a label-free biosensing system based on optical resonator technology known as Frequency Locking Optical Whispering Evanescent Resonator (FLOWER) that is capable of detecting single molecules in solution. Here the procedures behind this work are described and presented.

Detecting small concentrations of molecules down to the single molecule limit has impact on areas such as early detection of disease, and fundamental ...

Rapid Scan Electron Paramagnetic Resonance Opens New Avenues for Imaging Physiologically Important Parameters In Vivo

We demonstrate a superior method of 2D spectral-spatial imaging of stable radical reporter molecules at 250 MHz using rapid-scan electron-paramagnetic-resonance (RS-EPR), which can provide quantitative information under in vivo conditions on oxygen concentration, pH, redox status and concentration of signaling molecules (i.e., OH&#8226;, NO&#8226;). The RS-EPR technique has a higher sensitivity, improved spatial resolution (1 mm), and shorter acquisition time in comparison to the standard continuous wave (CW) technique. A variety of phantom configurations have been tested, with spatial resolution varying from 1 to 6 mm, and spectral width of the reporter molecules ranging from 16 &#181;T (160 mG) to 5 mT (50 G). A cross-loop bimodal resonator decouples excitation and detection, reducing the noise, while the rapid scan effect allows more power to be input to the spin system before saturation, increasing the EPR signal. This leads to a substantially higher signal-to-noise ratio than in conventional CW EPR experiments.

Rapid Scan Electron Paramagnetic Resonance Opens New Avenues for Imaging Physiologically Important Parameters In Vivo

A new electron paramagnetic resonance (EPR) method, rapid scan EPR (RS-EPR), is demonstrated for 2D spectral spatial imaging which is superior to the traditional continuous wave (CW) technique and opens new venues for in vivo imaging. Results are demonstrated at 250 MHz, but the technique is applicable at any frequency.

We demonstrate a superior method of 2D spectral-spatial imaging of stable radical reporter molecules at 250 MHz using rapid-scan ...

Characterizing Single-Molecule Conformational Changes Under Shear Flow with Fluorescence Microscopy

Single-molecule behavior under mechanical perturbation has been characterized widely to understand many biological processes. However, methods such as atomic force microscopy have limited temporal resolution, while F&#246;rster resonance energy transfer (FRET) only allow conformations to be inferred. Fluorescence microscopy, on the other hand, allows real-time in situ visualization of single molecules in various flow conditions. Our protocol describes the steps to capture conformational changes of single biomolecules under different shear flow environments using fluorescence microscopy. The shear flow is created inside microfluidic channels and controlled by a syringe pump. As demonstrations of the method, von Willebrand factor (VWF) and lambda DNA are labeled with biotin and fluorophore and then immobilized on the channel surface. Their conformations are continuously monitored under variable shear flow using total internal reflection (TIRF) and confocal fluorescence microscopy. The reversible unraveling dynamics of VWF are useful for understanding how its function is regulated in human blood, while the conformation of lambda DNA offers insights into the biophysics of macromolecules. The protocol can also be widely applied to study the behavior of polymers, especially biopolymers, in varying flow conditions and to investigate the rheology of complex fluids.

We present a protocol for immobilizing single macromolecules in microfluidic devices and quantifying changes in their conformations under shear flow. This protocol is useful for characterizing the biomechanical and functional properties of biomolecules such as proteins and DNA in a flow environment.

Single-molecule behavior under mechanical perturbation has been characterized widely to understand many biological processes. However, methods such as ...

Fabrication of Zero Mode Waveguides for High Concentration Single Molecule Microscopy

In single molecule fluorescence enzymology, background fluorescence from labeled substrates in solution often limits fluorophore concentration to pico- to nanomolar ranges, several orders of magnitude less than many physiological ligand concentrations. Optical nanostructures called zero mode waveguides (ZMWs), which are 100&#8722;200 nm in diameter apertures fabricated in a thin conducting metal such as aluminum or gold, allow imaging of individual molecules at micromolar concentrations of fluorophores by confining visible light excitation to zeptoliter effective volumes. However, the need for expensive and specialized nanofabrication equipment has precluded the widespread use of ZMWs. Typically, nanostructures such as ZMWs are obtained by direct writing using electron beam lithography, which is sequential and slow. Here, colloidal, or nanosphere, lithography is used as an alternative strategy to create nanometer-scale masks for waveguide fabrication. This report describes the approach in detail, with practical considerations for each phase. The method allows thousands of aluminum or gold ZMWs to be made in parallel, with final waveguide diameters and depths of 100&#8722;200 nm. Only common lab equipment and a thermal evaporator for metal deposition are required. By making ZMWs more accessible to the biochemical community, this method can facilitate the study of molecular processes at cellular concentrations and rates.

Described here is a nanosphere lithography method for parallel fabrication of zero mode waveguides, which are arrays of nanoapertures in a metal-clad glass microscopy coverslip for single molecule imaging at nano- to micromolar concentrations of fluorophores. The method takes advantage of colloidal crystal self-assembly to create a waveguide template.

In single molecule fluorescence enzymology, background fluorescence from labeled substrates in solution often limits fluorophore concentration to pico- to ...