Calcium Control of Saccharomyces cerevisiae Actin Assembly

Abstract

Low levels of Ca²⁺ dramatically influence the polymerization of Saccharomyces cerevisiae actin in KCl. The apparent critical concentration for polymerization (C_∞) increases eightfold in the presence of 0.1 mM Ca²⁺. This effect is rapidly reversed by the addition of ethylene glycol bis(β-aminoethyl ether)-N,N′-tetraacetic acid or of 0.1 mM Mg²⁺. Furthermore, the addition of Ca²⁺ to polymerized actin causes a reversible increase in the apparent C_∞. In the presence of Ca²⁺, at actin concentrations below the apparent C_∞, particles of 15 to 50 nm in diameter are seen instead of filaments. These particles are separated from soluble actin when Ca²⁺-treated filamentous actin is sedimented at high speed; both the soluble and particulate fractions retain Ca²⁺-sensitive polymerization. The Ca²⁺ effect is S. cerevisiae actin-specific: the C_∞ for rabbit muscle actin is not affected by the presence of Ca²⁺ and S. cerevisiae actin. Ca²⁺ may act directly on S. cerevisiae actin to control the assembly state in vivo.