Ca2+-dependent actin-binding phosphoprotein in Physarum polycephalum. Subunit b is a DNase I-binding and F-actin capping protein.

Physarum contains at least two distinct DNase I-binding proteins, i.e. actin and Cap 42 (a + b). The latter, a tight (1:1) complex of Cap 42 (a) and Cap 42 (b) (Maruta, H., Isenberg. G., Schreckenbach, T., Hallmann, R., Risse, G., Schibayama, T., and Hesse, J. (1983) J. Biol. Chem. 258, 10144-10150), is a Ca2+-dependent F-actin capping protein. DNase I binds to Cap 42 (b) but not to Cap 42 (a). Consequently, DNase I-agarose was used for an affinity-purification of Cap 42 (a + b), after its separation from actin by DEAE-cellulose chromatography. Cap 42 (a + b) was dissociated into its subunits when released from DNase I-agarose by 8.8 M formamide. The two subunits were subsequently separated from each other on hydroxylapatite. Both Cap 42 (a) and Cap 42 (b) were Ca2+-dependent F-actin capping proteins that cap the fast growing end of actin filaments and block actin polymerization at this end. Like Cap 42 (a + b), Cap 42 (b) required Ca2+ for its capping activity only when phosphorylated. The phosphorylation of Cap 42 (b) was completely blocked by DNase I or a tertiary complex of Cap 42 (a), actin, and Ca2+. Cap 42 (b) is not identical with native (= polymerizable) actin because (i) Cap 42 (b) was unable to form filaments, (ii) the Cap 42 (b) kinase did not phosphorylate native actin, and (iii) fragmin formed a tight (1:1) complex with native actin but not with Cap 42 (b). Although it is unlikely that Cap 42 (b) is simply a denatured form of actin that has lost its polymerizability during the preparation, it still remains to be clarified whether Cap 42 (b) is a nonpolmerizable actin variant derived from a distinct actin gene or a post-translationally modified form of polymerizable actin.