4.15: Mechanical Protein Functions
Proteins perform many mechanical functions in a cell. These proteins can be classified into two general categories- proteins that generate mechanical forces and proteins that are subjected to mechanical forces. Proteins providing mechanical support to the structure of the cell, such as keratin, are subjected to mechanical force, whereas proteins involved in cell movement and transport of molecules across cell membranes, such as an ion pump, are examples of generating mechanical force.
Functions such as cell movement and muscle contraction require the conversion of chemical energy to a mechanical, usually through conformational changes. For example, hydrolysis of nucleoside triphosphates, such as ATP and GTP, can result in a small conformational change that gets amplified to major structural changes. For example, EF-Tu is a protein with three distinct domains that transfers a tRNA molecule to the ribosome. One of the domains binds GTP, and the hydrolysis of GTP to GDP results in a conformational change in the nucleotide-binding site due to the released inorganic phosphate. This triggers the movement of an alpha-helix that is located at the interface of the GTP domain and the other two domains changing the relative position of the domains to each other. This allows the protein to release the tRNA that is held at the interface by the three domains, thereby allowing it to move into the ribosome.
Some proteins, such as actin, provide many types of mechanical functions. For example, actin acts as a track for the mechanical protein myosin to walk along. Depending on the type, myosin can perform various functions, such as either pulling on the actin filaments or transporting an attached organelle along the filament. As part of the cytoskeleton, actin filaments act as a mechanical support for the cell structure. During cell movement, these filaments exert pressure on the cell membrane causing the cell to form filopodia and lamellipodia, extensions of the cell membrane that allow the cell to migrate to a new location. Scientists have developed techniques, such as optical tweezers, that can measure the force that actin produces when deforming the membrane.