Ca2+-dependent actin-binding phosphoprotein in Physarum polycephalum. II. Ca2+-dependent f-actin-capping activity of subunit a and its regulation by phosphorylation of subunit b.
Cap 42 (a + b), a Ca2+-dependent, actin-binding and phosphorylatable protein consisting of two distinct subunits a and b of 42,000 Da in Physarum polycephalum, has been identified as a new F-actin-capping protein. It capped or bound to the fast growing ends of actin filaments and blocked actin polymerization at this end. The capping activity residing in subunit a and its Ca2+-dependency were regulated by phosphorylation of subunit b; subunit a required Ca2+ for its capping activity when subunit b was phosphorylated, whereas this activity became Ca2+ independent when subunit b was dephosphorylated. Subunit b contained at least two phosphorylatable threonine residues and probably three additional phosphorylation sites. Like cytochalasins and other F-actin-capping proteins, Cap 42 (a + b) was able to induce a rapid depolymerization of actin filaments at the slow growing end, and also to nucleate actin polymerization. However, unlike Physarum fragmin, Cap 42 (a + b) had no severing activity leading to the fragmentation of actin filaments. Our results indicate that Cap 42 (a + b) is the first Ca2+-dependent F-actin-capping phosphoprotein whose phosphorylation regulates its actin-binding and vice versa. A possible mechanism of the capping action of Cap 42 (a + b) in vitro and also its conceivable role in the regulation of the Ca2+/actin-dependent cytoplasmic streaming in plasmodia are discussed.
D000199:Actins / Q000378:metabolism*; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002352:Carrier Proteins / Q000378:metabolism*; D003598:Cytoskeletal Proteins; D046911:Macromolecular Substances; D008854:Microscopy, Electron; D008956:Models, Chemical; D009124:Muscle Proteins / Q000494:pharmacology; D010766:Phosphorylation; D010804:Physarum; D015800:Protozoan Proteins
D000199:Actins; C038781:Cap 42 (a&b) protein, Physarum polycephalum; D002352:Carrier Proteins; D003598:Cytoskeletal Proteins; D046911:Macromolecular Substances; D009124:Muscle Proteins; D015800:Protozoan Proteins; D002118:Calcium