The 45K molecular weight actin-modulating protein from sea urchin eggs forms a complex with actin in the presence of calcium ions.

The 45K protein from sea urchin eggs forms a complex with actin in the presence of Ca2+. The fraction was not readily dissociated on depletion of Ca2+ by gel filtration, but was dissociated on dialysis against an EGTA-containing solution. The free 45K protein and the 45K protein-actin complex affect F-actin in different manners in the presence of Ca2+. The former severs F-actin in the presence of Ca2+ but not in its absence, while the latter caps the barbed end of F-actin in a Ca2+-independent manner thereby lowering the viscosity of F-actin slowly.