The purification of a 50 kDa protein-actin complex from unfertilized sea-urchin (Strongylocentrotus purpuratus) eggs

The 100,000 g supernatant from the unfertilized egg of the sea urchin Strongylocentrotus purpuratus has been fractionated on DEAE-cellulose and analysed for Ca²⁺-binding activity by the Chelex-100 competitive Ca²⁺-binding activity assay. The major peak of Ca²⁺-binding activity was subjected to further purification and the Ca²⁺-binding protein responsible for this Ca²⁺-binding-activity peak has been isolated and characterized. Non-denaturing polyacrylamide-gel electrophoresis (PAGE) followed by ⁴⁵Ca²⁺ autoradiography suggested a molecular mass of 80 kDa for the Ca²⁺-binding protein. SDS/PAGE revealed that the 80 kDa protein consisted of a 1:1 molar complex of proteins of 50 and 42 kDa. The 42 kDa protein was identified as actin. The complex was not dissociated by extensive dialysis against an EGTA-containing buffer. The EGTA-stable complex was named '50K-A'.