Diphasic transformations of F-Actin. Effects of urea and MgCl 2 on F-actin

Abstract

Asakura, Taniguchi and Oosawa [1] proposed that muscle actin polymer under sonic vibration is in a different state from that of the ordinary double stranded helical structure (F-actin), characterised by partially interrupted structures of F-actin, a state of “f-actin”. In order to confirm different states for actin polymers [1, 2], physicochemical studies were made by measurements of viscosity, flow birefringence, electric birefringence, fluorescence, electron microscopy, quasielastic light scattering and ATP splitting. The following results were obtained. 1. (1) F-actin polymers can undergo two processes of depolymerization upon treatment with urea and various salts as judged by measurements of flow birefringence and viscosity: one is a rapid process in a solution containing K 1 or Ca 2+ and urea; the other is a slow process following a brief rapid one in a solution containing Mg 2+ and urea. 2. (2) In the presence of Mg 2+ and a suitable concentration of urea, F-actin (F mu -actin) appeared to exhibit different properties than ordinary F-actin; it had lower viscosity and lower flow birefringence and it had on the whole a more flexible polymer structure, also judging from experiments of quasielastic light scattering, electric birefringence. The different structure was confirmed directly by electron microscopic observation. The aromatic side chains of F mu -actin were also more mobile than those of F-actin judging from fluorescence measurements. The transformation between F-actin and F mu -actin was reversible. 3. (3) The state of F mu -actin polymers was also characterized by ATP splitting; F mu -actin split about one mole of ATP into ADP and inorganic phosphate per mole of actin monomer at room temperature, where F-actin did not. A molar excess of Mg 2+ with respect to actin monomer is required for ATP splitting. F-actin in solutions containing K + or Ca 2+ and urea did not split ATP. F mu -actin activated on Mg•ATPase of myosin at nearly the same rate as that of F-actin. 4. (4) We have postulated a flexible filament model (f-actin). The relationships between the structure of f-actin and its functional role for force generation during contraction are discussed.