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Lipid monolayers have been used as a foundation for forming two-dimensional (2D) protein crystals for structural studies for decades. They are stable at the air-water interface and can serve as a thin supporting material for electron imaging. Here we present the proven steps on preparing lipid monolayers for biological studies.
Electron crystallography is a powerful tool for high-resolution structure determination. Macromolecules such as soluble or membrane proteins can be grown into highly ordered two-dimensional (2D) crystals under favorable conditions. The quality of the grown 2D crystals is crucial to the resolution of the final reconstruction via 2D image processing. Over the years, lipid monolayers have been used as a supporting layer to foster the 2D crystallization of peripheral membrane proteins as well as soluble proteins. This method can also be applied to 2D crystallization of integral membrane proteins but requires more extensive empirical investigation to determine detergent and dialysis conditions to promote partitioning to the monolayer. A lipid monolayer forms at the air-water interface such that the polar lipid head groups remain hydrated in the aqueous phase and the non-polar, acyl chains, tails partition into the air, breaking the surface tension and flattening the water surface. The charged nature or distinctive chemical moieties of the head groups provide affinity for proteins in solution, promoting binding for 2D array formation. A newly formed monolayer with the 2D array can be readily transfer into an electron microscope (EM) on a carbon-coated copper grid used to lift and support the crystalline array. In this work, we describe a lipid monolayer methodology for cryogenic electron microscopic (cryo-EM) imaging.
Electron diffraction through 2D crystals or helical arrays of proteins can achieve sub-nanometer resolutions in favorable cases1,2,3. Of particular interest are reconstituted 2D membrane protein arrays or crystals in their near-native environments1. Because a crystal acts as a signal amplifier enhancing the intensities of the structural factors at specific spatial frequencies, electron crystallography allows probing a target with a smaller size at high resolutions, such as small molecules, than those for single-particle cryo-EM. The electron beam can b....
1. Teflon block preparation
2. Monolayer lipid preparation
NOTE: Estimated operating time: 30- 45 minutes
A lipid monolayer deposited on the EM grid can be visualized under a transmission electron microscope (TEM) without staining. The monolayer presence can be recognized by the contrast difference from the area without any specimen in the beam path. Areas that have lipid monolayer coverage have lower local contrast than the ones with no coverage, since the electron beam through the empty holes has no scattering and shows a brighter illumination (Figure 3).
To screen .......
A lipid monolayer is a powerful tool that facilitates the growth of large 2D crystals for structural studies of biological macromolecules. To successfully prepare an intact lipid monolayer at the air-water interface, it is strongly recommended that the lipids are prepared freshly on the day of the experiment, because oxidization of the lipid acyl chain could lead to packing disruption in the monolayer and adversely affect the resulting crystal formation. Purchased lipids in powder form should be dissolved using a mixture.......
The preparation of this manuscript was partially supported by US Army Research Office (W911NF2010321) and Arizona State University startup funds to P.-L.C.
....Name | Company | Catalog Number | Comments |
14:0 PC (DMPC) | Avanti Lipids | 850345 | 1,2-dimyristoyl-sn-glycero-3-phosphocholine, 1 x 25 mg, 10 mg/mL, 2.5 mL |
Bulb for small pipets | Fisher Scientific | 03-448-21 | |
Chloroform | Sigma-Aldrich | C2432 | |
Desiccator vacuum | Southern Labware | 55207 | |
EM grids | Electron Microscopy Sciences | CF413-50 | CF-1.2/1.3-4C 1.2 µm hole, 1.3 µm space |
Filter paper | GE Healthcare Life Sciences | 1001-090 | Diameter 90 mm |
Glass Pasteur pipets | Fisher Scientific | 13-678-20A | |
Hamilton syringe (25 µL) | Hamilton Company | 80465 | |
Hamilton syringe (250 µL) | Hamilton Company | 81165 | |
Hamilton syringe (5 µL) | Hamilton Company | 87930 | |
Hamilton syringe (500 µL) | Hamilton Company | 203080 | |
Methanol | Sigma-Aldrich | M1775-1GA | |
Petri dish | VWR | 25384-342 | 100 mm × 15 mm |
Teflon block | Grainger | 55UK05 | 60 µL wells with side injection ports, manually made |
Tweezers | Electron Microscopy Sciences | 78325 | Various styles |
Ultra-pure water | |||
Ultrasonic cleaner | VWR | 97043-996 |
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