Changes in Phosphorylation of an F-Actin Capping Protein of Physarum polycephalum during the Cell Cycle

Abstract

The Cap 42 (b), a Ca²⁺-dependent F-actin capping phosphoprotein of 42, 000 daltons, was shown to be localized in the cytosol of Physarum polycephalum by measurements of phosphorylatability in the absence of Ca²⁺. The phosphorylation of Cap 42 (b) in the cytosol changed during the cell cycle: it was high in the S and G 2 phase, and low in the M phase and boundary phase between S and G 2 phase. When the isolated Cap 42 (b) was added to M phase cytosol, the phosphorylation of Cap 42 (b) was significantly increased by at least 6-fold. Compared with this result, about 2-fold increase in the phosphorylation of Cap 42 (b) was observed when the Cap 42 (b) kinase was added to M phase cytosol. Therefore, it is likely that the low level of Cap 42 (b) phosphorylation in M phase cytosol is mostly due to the decreased amount of phosphorylatable Cap 42 (b) and to a lesser extent due to a low level of the Cap 42 (b) kinase activity.