Actin modulating proteins in the sea urchin egg. I. Analysis of G-actin-binding proteins by DNase I-affinity chromatography and purification of a 17,000 molecular weight component.

Two groups of protein species which interact with G-actin were detected in unfertilized sea urchin eggs by DNase-I affinity chromatography in the presence of Ca2+. One of the protein groups, which comprised of six major proteins, was eluted by EGTA. One of these proteins was tentatively identified as calmodulin. The other protein group, comprising of four major proteins, could be dissociated from the immobilized DNase I at a higher ionic strength. One of these proteins, showing a molecular weight of 17,000 (17 K protein), was purified to homogeneity. In its action on actin, 17 K protein revealed properties quite similar to those of a protein called depactin isolated from unfertilized starfish oocytes, but different from those of profilins isolated from mammalian tissues or Acanthamoeba. 17 K protein co-migrated with depactin on an SDS-gel. It inhibited actin polymerization and quickly depolymerized F-actin. When added to G-actin before polymerization, 17 K protein suppressed the final extent of actin polymerization. This inhibition was not released by the addition of sonicated F-actin nuclei. When added to F-actin, 17 K protein rapidly reduced the viscosity and increased the G-actin concentration of the actin solution. In both cases, the final extent of actin polymerization strictly depended on the molar ratio of 17 K protein to actin, indicating a stoichiometric association between 17 K protein and actin.