Ca2+-dependent actin-binding phosphoprotein in Physarum polycephalum. I. Ca2+/actin-dependent inhibition of its phosphorylation.

When crude extracts of the slime mold Physarum polycephalum were incubated with ATP and Mg2+ at 35 degrees C, a peptide of approximately 42,000 Da was predominantly phosphorylated. The kinase, separated from the phosphorylatable peptide, phosphorylated neither actin nor fragmin, both proteins of 42,000 Da, the latter known to cap and shorten actin filaments in a Ca2+-dependent manner. The phosphorylatable peptide was phosphorylated only at threonine residue(s), and its phosphorylation was almost completely inhibited by micromolar concentrations of Ca2+ in the extracts. The Ca2+-dependent inhibition of the phosphorylation was reversed by the subsequent addition of ethylene glycol bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid but not by trifluoperazine. The Ca2+-dependent inhibition of the phosphorylation required either actin or another, so far unidentified, protein(s) which is distinct from calmodulin. Fragmin reversed the Ca2+/actin-dependent inhibition of the phosphorylation. The Ca2+-dependent actin-binding phosphorylatable protein named Cap 42 (a + b), consisting of two distinct 42,000-Da peptides a and b, was purified to near homogeneity. Peptide b was identified as the phosphorylatable subunit. Substoichiometric amounts of Cap 42 (a + b) reduced the low shear viscosity of F-actin solutions.