Mechanical properties of elastomeric proteins studied by single molecule force spectroscopy
Authors: Zhou Hao-Tian  , Gao Xiang  , Zheng Peng  , Qin Meng  , Cao Yi  , Wang Wei  ,   School of Physics, Nanjing University, Nanjing 210093, China;,   School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210046, China
Published: December 2020

Elastomeric proteins are a special class of proteins with unique mechanical functions. They bear, transduce mechanical forces inside cell, and serve as biomaterials of high elasticities and strengths outside cell. Depending on their functions, the mechanical properties of elastomeric proteins are very diverse. Some of them are of high mechanical stability and the others are of high extensibility and toughness. Although many elastomeric proteins are engineered for the applications in the fields of biomaterials and nanotechnology, the molecular determinant of the mechanical stability remains elusive. In this review, we summarize recent advances in the field of protein mechanics studied by using single molecule force spectroscopy. Force spectroscopy enables people to probe the unfolding properties of protein domains, thus paving the way for building special proteins with characteristic mechanical functions. To begin with, it is necessary to clarify the factors and their relations with the unfolding force, which is deduced based on Bell's expression. It turns out that the unfolding force is proportional to pulling speed when the speed is relatively small, and has a logarithmic relation in the high-speed approximation. After the external determinant of the force probe is clarified, some intrinsic factors are to be discussed. Hydrogen bound and electrostatic force, rather than covalent bond, contribute to the mechanical performances of proteins. Those interactions rely on the topology structures of protein molecules. By changing the structures of proteins, researchers now manage to change the mechanical characteristics of certain proteins. Since single protein is unable to be detected by traditional optic microscope, three devices used to observe and manipulate single protein are introduced in the present paper. These include atomic force microscopy, magnetic tweezers and optical tweezers. Among them, a more detailed explanation of atomic force microscope (AFM) is provided, which briefly describes the basic mechanism and structure of AFM and possible explanation for the formation of force-extension curves. After that, several recent advances for improving the AFM based single molecule force spectroscopy techniques are highlighted. For example, Tom Perkins group [Sullan R M A, Churnside A B, Nguyen D M, Bull M S, Perkins T T 2013 Methods 60 131] has discovered that the gold-stripped tip gives more accurate and reproducible results than a gold-coated one. Matthias Rief group [Schlierf M, Berkemeier F, Rief M 2007 Biophys. J. 93 3989] has managed to increase the resolution of AFM, pushing it in pair with optical tweezers. Hermann Gaub et al. [Otten M, Ott W, Jobst M A, Milles L F, Verdorfer T, Pippig D A, Nash M A, Gaub H E 2014 Nat. Methods 11 1127] combined the microfluidic chip and DNA expression in vitro to increase the yields of interpretable single-molecule interaction traces. Toshio Ando et al. [Ando T, Uchihashi T, Fukuma T 2008 Prog. Surf. Sci. 83 337] have developed methods to increase the imaging speed of AFM. Finally, the rationally designing the mechanical properties of protein-based materials pioneered by Hongbin Li group is highlighted. They have discovered direct relationship between the mechanical properties of individual proteins and those of the protein materials. To sum up, with AFM, scientists now can explore mechanical properties of a wide range of proteins, which enables them to build biomaterials with exceptional mechanical features.