Watching biological nanomotors at work: insights from single-molecule studies
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Part 1: High resolution (≈1 nm lateral resolution) biological AFM imaging has been carried out almost exclusively using freshly cleaved mica as a specimen supporting surface, but mica suffers from a fundamental limitation that has hindered AFM’s broader integration with many modern optical methods. Mica exhibits biaxial birefringence; indeed, this naturally occurring material is used commercially for constructing optical wave plates. In general, propagation through birefringent material alters the polarization state and bifurcates the propagation direction of light in a manner which varies with thickness. This makes it challenging to incorporate freshly cleaved mica substrates with modern optical methods, many of which employ highly focused and polarized laser beams passing through then specimen plane. Using bacteriorhodopsin from Halobacterium salinarum and the Sec-translocon from Escherichia coli, we demonstrate that faithful images of 2D crystalline and non-crystalline membrane proteins in lipid bilayers can be obtained on common microscope cover glass following a straight-forward cleaning procedure. Direct comparison between data obtained on glass and on mica show no significant differences in AFM image fidelity. Repeated association and dissociation of SecA with SecYEG indicated that the proteins remain competent for biological processes on glass substrates for long periods of time. This work opens the door for combining high resolution biological AFM with powerful optical methods that require optically isotropic substrates such as ultra-stable and direct 3D AFM. In turn, this capability should enable long timescale conformational dynamics measurements of membrane proteins in near-native conditions. Part 2: In the second part of this work we studied SecA-ATP hydrolysis and catalase enzyme dynamics. Both of these protein macromolecules were observed to be highly dynamic during catalytic turnover. Single molecule studies of catalase indicated that the enzyme undergoes oligomeric state changes when exposed to H2O2. Conformational dynamics of the SecA-ATPase was visualized at the single molecule level and the protein macromolecule flickers between a compact and expanded state in the presence of ATP, indicating reversible conformational changes. Future studies in the lab will shed more light onto these biological processes.
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