CA-2+-DEPENDENT ACTIN-BINDING PHOSPHOPROTEIN IN PHYSARUM-POLYCEPHALUM .1. CA-2+/ACTIN-DEPENDENT INHIBITION OF ITS PHOSPHORYLATION

  • 1 January 1983
    • journal article
    • research article
    • Vol. 258  (16) , 144-150
Abstract
When crude extracts of the slime mold P. polycephalum were incubated with ATP and Mg2+ at 35.degree. C, a peptide of .apprx. 42,000 Da [dalton] 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 2 distinct 42,000-Da peptides a and b, was purified to near the 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.