This paper reviews a recent progress of molecular level studies on the rigidity of surface immobilized as well as membrane bound proteins embedded in the lipid bilayer. Some details and emphasis are given to the work done in our laboratory in the last few years. Immobilization of protein molecules on a solid surface through covalent crosslinkers on one side and to the probe of the atomic force microscope on the other enabled us to pull or push a single protein molecule to specified directions. On pulling, the internal structure of the protein molecule is mechanically opened up and, on pushing, it is compressively deformed until it is flattened out. Such experiments reveal the mechanical rigidity of the folded structure of a protein molecule in two different ways. In the field of ligand-protein interaction, some merits and problems of newly introduced compression free method are discussed. When a protein molecule is embedded in a lipid membrane, information on its anchoring force to the membrane can be obtained by pulling it out from the membrane. Experiments have been done either on a lipid bilayer formed on a solid surface, or on the surface of live cells. The extraction process of membrane proteins is often accompanied by extrusion of a thin lipid tether trailing behind the target protein of the tensile force. A short review of the tether forming process from the red cell membrane surface will be given.
Keywords: Protein rigidity, Protein stretching, Protein pushing, Compression, Atomic force microscopy (AFM), Young's modulus of proteins, Tatara model
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