A protein similar to alpha-actinin has been isolated from unfertilized sea urchin eggs. This protein co-precipitated with actin from an egg extract as actin bundles. Its apparent molecular weight was estimated to be approximately 95,000 on an SDS gel: it co-migrated with skeletal-muscle alpha-actinin. This protein also co-eluted with skeletal muscle alpha-actinin from a gel filtration column giving a Stokes radius of 7.7 nm, and its amino acid composition was very similar to that of alpha-actinins. It reacted weakly but significantly with antibodies against chicken skeletal muscle alpha-actinin. We designated this protein as sea urchin egg alpha-actinin. The appearance of sea urchin egg alpha-actinin as revealed by electron microscopy using the low-angle rotary shadowing technique was also similar to that of skeletal muscle alpha-actinin. This protein was able to cross-link actin filaments side by side to form large bundles. The action of sea urchin egg alpha-actinin on the actin filaments was studied by viscometry at a low-shear rate. It gelled the F-actin solution at a molar ratio to actin of more than 1:20, at pH 6-7.5, and at Ca ion concentration less than 1 microM. The effect was abolished by the presence of tropomyosin. Distribution of this protein in the egg during fertilization and cleavage was investigated by means of microinjection of the rhodamine-labeled protein in the living eggs. This protein showed a uniform distribution in the cytoplasm in the unfertilized eggs. Upon fertilization, however, it was concentrated in the cell cortex, including the fertilization cone. At cleavage, it seemed to be concentrated in the cleavage furrow region.

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