Proteins: From chemical properties to cellular function: A practical review of actin dynamics

Proteins are the most abundant polymers in living organisms and major constituents of biological nanocomposites (bio-nanocomposites). Twenty types of various amino acids, differing only by the nature of their sidechains, constitute the basic building blocks of proteins and are linked together by peptide bonds. The specific sequential order of amino acid types encountered within the protein chain is called the primary structure. Folding of secondary structures, such as a-helices or ß-sheets, is ruled by the dihedral angles of the protein backbone structure, which are partially constrained by the specific steric hindrance effects of the various aminoacid sidechains. The 3D folding of these secondary elements to enable formation of the native structure of proteins (i.e., tertiary and quaternary structures) depends on several physicochemical parameters and determines in most cases protein functions. Individual protein interactions depend on non-covalent interactions governed by kinetic and equilibrium parameters. High-throughput mapping of protein-protein interaction networks enables exploring the various cellular functions of the genome-encoded proteins. Among the diverse cellular functions, enzymes have the property to catalyze reactions that would not occur in physiological conditions, by reducing the activation energy required for the chemical reaction to occur through the stabilization of a transition state. Molecular motors constitute a subclass of enzyme that convert the energy provided by the hydrolysis of ATP into a mechanical force that allows movements at the nanoscopic scale. The self-assembly properties of cytoskeletal proteins are crucial to build polymers organizing the cell architecture. Auto-assembly of globular actin monomers into filaments requires the neutralization of repulsive charges at the surface of each subunit by Mg2+, Ca2+, or K+, determined by well-characterized kinetic parameters. A whole set of regulatory proteins modulates filament assembly. This fine-tuning of polymer dynamics plays key functions for cellular processes such as adhesion or cell migration, and reconstituted system using their properties will lead to innovative applications in bionanotechnologies.