An 100-kDa Ca2+-sensitive actin-fragmenting protein from unfertilized sea urchin egg

An actin-modulating protein was purified from unfertilized eggs of sea urchin, Hemicentrotus pulcherrimus, by means of DNase I affinity and DEAE-cellulose column chromatographies. This protein was a globular protein with a Stokes radius of 41-42 nm and consisted of a single polypeptide chain having an apparent molecular mass of 100 kDa on sodium dodecyl sulfate polyacrylamide gel electrophoresis. Gel filtration chromatography revealed that one 100-kDa protein molecule binds two or three actin monomers in the presence of Ca2+, but such binding was not observed in the absence of Ca2+. The effect of the 100-kDa protein on the polymerization of actin was studied by viscometry, spectrophotometry and electron microscopy. The initial rate of actin polymerization was decreased at a very low molar ratio of 100-kDa protein/actin. Acceleration of the initial rate of polymerization occurred at a relatively high, but still substoichiometric, molar ratio of 100-kDa protein/actin. The 100-kDa protein produced fragmentation of muscle actin filaments at Ca2+ concentrations greater than 0.3 microM as revealed by viscometry and electron microscopy. Evidence was also presented that the 100-kDa protein binds to the barbed end of the actin filament.