Poly(l‐proline)‐binding proteins from chick embryos are a profilin and a profilactin

Abstract

Two poly(l‐proline)‐binding proteins (PBP‐1 and PBP‐2) were purified from chick embryos by using a poly(l‐proline)‐agarose column. PBP‐1 was composed of two different polypeptides (molecular masses of 42 kDa and 15 kDa). The molar ratio of the two proteins in the complex was 1:1. The other poly(l‐proline)‐binding protein, PBP‐2, was the 15‐kDa protein itself. The 42‐kDa protein was confirmed to be an actin from the amino acid composition, by immunochemical evidence and by its ability to self‐polymerize. In addition, the 42 + 15‐kDa protein complex (PBP‐1) inhibited DNase I, just as a monomeric actin did. The amino acid composition of the 15‐kDa protein was similar to that of mammalian profilin and it inhibited the salt‐induced polymerization of rabbit skeletal muscle actin. Therefore, we conclude that the two poly(l‐proline)‐binding proteins from the chick embryo are a profilactin and a profilin in chick embryo. The ability of profilactin to bind poly(l‐proline) must be due to profilin itself, because the profilin has a greater affinity for poly(l‐proline) than does profilactin. Additionally, both the monomeric and filamentous actin from rabbit skeletal muscle have no affinity for poly(l‐proline).