[GLOBULAR ACTIN IS THE PARTIALLY INTRINSICALLY DISORDERED PROTEIN WITH QUASI-STATIONARY STRUCTURE]

It is shown that the native globular actin (G-actin) is the thermodynamically unstable (quasi-stationary) form of the protein. This state is stabilized by Mg2+ (in vitro replaced by Ca2+). In vivo this state occurs as a result of complex energy-consuming post-translational folding processes including chaperone Hsp70, prefoldin and CCT complex, providing the formation of the native structure stabilized by Ca2+ and ATP. Structures formed by actin polypeptide chain constantly form complexes with their partners (chaperones Hsp70, prefoldin and chaperonin CCT in folding process, with an Mg ion and ATP in the native state, with numerous actin-binding proteins during the formation and functioning of the cell cytoskeleton, with myosin and other proteins of the muscle contraction in the muscle cells). Actin denaturation is accompanied by self-association of molecules, so the inactivated actin is the thermodynamically stable compact structure consisting of 14-16 protein molecules. Apparently, proteins with quasi-stationary native state are widespread in nature. The emergence of these states is energy-consuming and is conjugated with the inability of the polypeptide chain to form the native compact structure without assistants (complex machinery of protein folding in the cell) and without interaction with their natural partners, in particular with metal ions.