Actin-Modulating Proteins in the Sea Urchin Egg. Ii. Sea Urchin Egg Profilin

A new low molecular weight protein which binds actin and modulates its polymerization was purified from sea urchin eggs. This protein seemed to exist as a form complexed with actin in the crude egg extract. The molar ratio of this protein and actin in the complex was 1 : 1. The molecular weight of this protein was estimated to be about 13,000 by sodium dodecyl sulfate (SDS)-gel electrophoresis. The amino acid composition of this protein was somewhat similar to profilin from mammalian tissues or Acanrhamoeba profilin. This 13 K protein was purified to homogeneity by Sephadex G-150 column chromatography, DEAE-cellulose column chromatography, ammonium sulfate fractionation, hydroxylapatite column chromatography, and Sephadex G-75 column chromatography steps. The actin-13 K protein complex was also obtained from the hydroxylapatite column. From the complex, the 13 K protein was also purified by DEAE-cellulose column chromatography in the presence of 2 M urea. The purified 13 K protein had an ability to bind actin in 0.1 KC1 and was released from acti11 with 0.6 M KC1 or 0.6 M KI. This protein reduced the rate of polymerization of rabbit skeletal muscle actin although the extent of polymerization was not affected. On the other hand, this protein inhibited the extent of polymerization of sea urchin egg actin in a concentration-dependent manner. The 13 K protein also depolymerized sea urchin egg F-actin in a concentrationdependent manner. From the resembrance of its properties to those of profilin from mammalian tissues or Acanrhamoeba profilin, this 13 K protein is regarded as sea urchin egg profilin.