Nanomechanics of Protein-Based Biostructures

In this article, we review recent studies on nanomechanics of biostructures performed in the Laboratory of Biodynamics at Tokyo Institute of Technology. We employed the force spectroscopy mode of the atomic force microscope, to determine the hidden mechanical properties of protein-based biostructures that have made life on the earth so successful. We investigated the mechanical heterogeneity of the internal structure of globular proteins and cell membranes. Single molecules of globular proteins were stretched from their two ends after being sandwiched between the probe of the atomic force microscope and the substrate through a covalent crosslinking system. The resulting force-extension curve revealed mechanical heterogeneities in the conformation of globular proteins. The covalent crosslinking system withstood a tensile force of up to 1.8±0.33 nN (loading rate = 11.7 nN/s) while most of the noncovalently folded protein sub-structures were completely stretched out with less force. The result of force spectroscopy supported a long-standing conjecture that an enzyme cannot simply be a soft material because it must catalyze chemical reactions involving the formation and breakdown of mechanically rigid covalent molecules. Next, the AFM force spectroscopy was applied to determine the force needed to disrupt noncovalently assembled biostructures such as composite biomembranes composed of lipids and proteins. We were able to show that intrinsic membrane proteins that are securely anchored to a lipid bilayer could be pulled out of the membrane with a significantly less force than that required for covalent bond breakdown, but with a force in the comparable range required for the disruption of the internal structures of globular proteins. From the available results from our group and other groups, a new concept of force-based biostructure assembly is emerging.