Polymerization of plasmodium actin

Abstract

1. (1)|On the addition of monovalent cations, plasmodium G-actin is polymerized to a fibrous polymer, termed plasmodium F-actin. The F-actin has a high viscosity, and shows a positive flow birefringence. During polymerization, 1 mole of inorganic phosphate is liberated from 1 mole of G-actin. Thus: G-actin-ATP ⇄ ATP 0.1 M KCl F-actin-ADP + P i Kinetic and structural analyses shows the F-actin to be a microfilament, 60 Å in diameter, having a two-stranded helical structure with a half pitch of about 290 Å containing about 13 spherical units. 2. (2)|Addition of divalent cations forms another kind of polymer, termed Mg-polymer, which has a relatively low viscosity. During the polymerization, 1 mole of inorganic phosphate is liberated from 1 mole of G-actin, suggesting the reaction: G-actin-ATP ⇄ ATP 2 mM MgCl 2 Mg-polymer-ADP + P i Moreover, this polymer itself seems to have ATP-splitting activity. Mg-polymer appears as a globular aggregate in electron micrographs, with a diameter of about 100–600 Å. 3. (3)|F-Actin and Mg-polymer are depolymerized to G-actin by removing salts from their solutions in the presence of ATP and cysteine, for example, by dialysis. The resultant G-actin may be polymerized again to one of the two polymer forms. Moreover, it is suggested that F-actin and Mg-polymer can transform into each other directly, not passing through the G-actin state. 4. (4)|Plasmodium G-actin can be copolymerized with rabbit striated muscle G-actin on the addition of monovalent cations or divalent cations.