single-molecule imaging of halo-tagged proteins to study protein-condensate interactions

Live Cell Single-Molecule Imaging-Based Quantification of Protein Interactions

A growing body of work suggests that biomolecular condensates play an important role in cellular organization and numerous cellular functions, e.g., transcriptional regulation1,2,3,4,5, DNA damage repair6,7,8, chromatin organization9,10,11,12, X-chromosome inactivation13,14,15, and intracellular signaling16,17,18. In addition, the dysregulation of biomolecular condensates is implicated in many diseases, including cancers19,20,21 and neurodegenerative disorders22,23,24,25,26. Condensate formation is often driven by transient, selective, and multivalent protein-protein, protein-nucleic acid, or nucleic acid-nucleic acid interactions27. Under certain conditions, these interactions can lead to liquid-liquid phase separation (LLPS), a density transition that locally enriches specific biomolecules in membraneless droplets. Such multivalent interactions are often mediated by the intrinsically disordered regions (IDRs) of proteins1,28,29. Biophysical characterization of these interactions at the molecular level is critical to our understanding of numerous healthy and aberrant cellular functions, given the pervasiveness of condensates across them. Although techniques based on confocal fluorescence microscopy, e.g., fluorescence recovery after photobleaching (FRAP)30,31,32, have been widely used to qualitatively show that the molecular exchanges between condensates and the surrounding cellular environment are dynamic, quantifying the interaction dynamics of specific biomolecules within condensates is generally not possible using conventional confocal microscopy or single-molecule microscopy without specialized data analysis methods. The single-particle tracking (SPT) technique described in this protocol is based on live-cell single-molecule microscopy33 and provides a uniquely powerful tool to quantify the interaction dynamics between specific proteins within condensates. The readout of SPT for such measurement is the mean residence time of a protein of interest in the condensates.
The protocol can be broken down into two parts - data acquisition and data analysis. The first step of imaging data acquisition is to express in cells a protein of interest that is fused to a HaloTag34. This enables labeling of the protein of interest with two fluorophores, where a majority of the protein molecules are to be labeled with a non-photoactivatable fluorophore (e.g., JFX549 Halo ligand35) and a small fraction of them are to be labeled with a spectrally distinct, photoactivatable fluorophore (e.g., PA-JF646 Halo ligand36). This allows for the simultaneous acquisition of all condensate locations in the cell and the acquisition of single-molecule movies of the protein of interest binding and unbinding to the condensates. Meanwhile, the same type of cells are modified to stably express Halo-tagged H2B, a histone that is largely immobile on chromatin. The cells are then stained with the PA-JF646 Halo ligand to enable single-molecule imaging of H2B. As will be discussed in detail below, this experiment accounts for the contribution of photobleaching to enable precise quantification of the interaction dynamics of the protein of interest. Cells for imaging experiments must then be cultured on clean coverslips, stained with HaloTag ligand(s), and assembled into a live-cell imaging chamber. From there, the sample is imaged under highly inclined and laminated optical sheet (HILO) illumination on a total internal reflection fluorescence (TIRF) microscope capable of two-channel imaging and single-molecule detection. The emission is then split onto two cameras, one tracking condensate positions and one tracking single molecules. Acquisition is performed with a long integration time (on the order of hundreds of ms) to blur out freely-diffusing proteins and only capture proteins that are less mobile due to binding to stable structures in the cell37.
The first step of data analysis is using an established single-particle tracking (SPT) algorithm38,39 to localize individual protein molecules in each frame of the movie and assemble the localizations into a trajectory for each molecule over its detectable lifetime. The trajectories are then sorted into those representing molecules inside and those representing molecules outside the condensates by comparing the localizations of the molecules throughout their trajectories to the localizations of all the condensates at the corresponding times1.
Next, a survival curve (1 - CDF) is generated using the lengths of all the in-condensate trajectories. The apparent mean residence time of the molecules is then extracted by fitting the survival curve to the following two-component exponential model of protein binding,
<img align="center" alt="Equation 1" src="/files/ftp_upload/66169/66169eq01.jpg" style="margin: auto;" />,
with A as the fraction of molecules non-specifically bound and with kobs,ns and kobs,s as the observed dissociation rates of the non-specifically bound and specifically bound molecules, respectively. Only kobs,s is considered from here onward. The dynamics of both protein dissociation, ktrue,s, and photobleaching of the fluorophore, kpb, contribute to kobs,s as
<img alt="Equation 2" src="/files/ftp_upload/66169/66169eq02.jpg" style="margin: auto;" />;
thus, to isolate the effects of protein dissociation, the specific dissociation rate of H2B-Halo in the cell line mentioned prior is measured.
<img alt="Equation 3" src="/files/ftp_upload/66169/66169eq03.jpg" style="margin: auto;" />
H2B is a protein that is stably integrated into chromatin and that experiences minimal dissociation in the time scale of a single-molecule movie acquisition37. Its specific dissociation rate is then equal to the photobleaching rate of the PA-JF646 Halo ligand, or
<img alt="Equation 4" src="/files/ftp_upload/66169/66169eq04.jpg" style="margin: auto;" />.
The mean in-condensate residence time of the protein of interest, <img alt="Equation 5" src="/files/ftp_upload/66169/66169eq05.jpg" style="margin: auto;" />, is then
<img alt="Equation 6" src="/files/ftp_upload/66169/66169eq06.jpg" style="margin: auto;" />.
Representative results from Irgen-Gioro et al.40 are shown, where this protocol was applied to demonstrate that fusing an oligomerization domain to IDR results in longer residence times of the IDR in its condensates. This result suggests that the added oligomerization domain stabilizes the homotypic interactions of the IDR that drives LLPS. In principle, the same method with slightly modified protocols can be applied to characterize the homotypic or heterotypic interactions of any protein that participates in the formation of any types of condensates.

Author Spotlight: Evaluation of Protein-Condensate Dynamics in Live Human Cells

Single-Molecule Measurement of Protein Interaction Dynamics Within Biomolecular Condensates

1 Our lab aims to understand the interaction behaviors<v>2 of intrinsically disordered protein regions 3 and how they play roles in transcriptional regulation 4 in healthy and diseased human cells. 5 In our lab, we develop and use novel 6 single molecule microscopy techniques 7 in combination with molecular biology, 8 biochemical and proteomic approaches 9 to study biomolecular condensates. 10 This protocol enables the quantification of the dynamics 11 by which a specific protein 12 binds to a particular type of condensate 13 in live human cells, 14 and is broadly applicable 15 to measuring the interaction dynamics of any protein 16 that participates in liquid-liquid phase separation.

Single-Molecule Imaging of Halo-Tagged Proteins to Study Protein-Condensate Interactions

1 To begin, obtain ligand stain cells<v>2 expressing halo-tagged protein of interest and halo H2B. 3 After turning on the microscope system, 4 set the live cell incubation parameters 5 and let the system equilibrate. 6 Put a drop of the oil onto 7 a 100x TIRF objective on the microscope 8 and load the cell sample onto the microscope stage.9 To identify a morphologically healthy cell, 10 adjust the Z position 11 and image the cell sample under brightfield illumination. 12 Crop the field of view to capture the target cell nucleus. 13 Using laser illumination, adjust the TIRF angle 14 to achieve hilo illumination with an optimal 15 signal to noise ratio of a single molecule.16 Then, set the exposure time to 500 milliseconds. 17 During the dead time between frames, 18 photo activate the molecules 19 with a 111 microwatt 405 nanometer beam, 20 and during each exposure, excite H2B halo molecules 21 with a 9.1 milliwatt 640 nanometer beam. 22 Initiate image capturing for 2000 frames continuously 23 and gradually increase the power of the 405 nanometer beam 24 upon reduction of photo activated molecules.25 For a halo-tagged protein of interest, 26 split emission wavelengths between two cameras 27 using a long pass dichroic mirror. 28 To the same acquisition parameters demonstrated earlier, 29 add JFX549 channel to acquire one frame every 10 seconds 30 and capture 2000 frames continuously.

single-molecule-imaging-halo-tagged-proteins-to-study-protein

Research

JoVE Journal

Biology

55 ビュー. 1 まず、目的のハロータグ付きタンパク質とハローH2Bを発現するリガンド染色細胞2を取得します。3 顕微鏡システムをオンにした後、4 生細胞のインキュベーションパラメータ5を設定し、システムを平衡化させます。6 顕微鏡8上の100x TIRF対物レンズ7にオイルを一滴垂らし、細胞サンプルを顕微鏡ステージにロードします。9 形態学的に...