Characterization of actin from fertilized eggs of Ilyanassa obsoleta during polar lobe formation and cytokinesis

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Actin was identified in fertilized eggs of the marine snail, Ilyanassa obsoleta, by heavy meromyosin (HMM) decoration of cortical microfilaments and by tryptic peptide analysis of presumptive egg actin. Eggs beginning to undergo first cleavage after third polar lobe formation were glycerinated, incubated with or without HMM, and examined by electron microscopy. In both sets of eggs, microfilaments were observed in the cleavage furrow and the polar lobe constriction, as well as in the remaining cortical cytoplasm. Microfilaments also were found in the cortex of glycerinated spherical eggs incubated without HMM. Microfilaments in glycerinated eggs treated with HMM had a barbed or fuzzy appearance; however, this type of decoration was not observed in glycerinated eggs incubated without HMM or when ATP or sodium pyrophosphate were present with the HMM. Microfilaments in the cleavage furrow and the third polar lobe constriction of glycerinated Ilyanassa eggs were stabilized by incubation in solutions of skeletal muscle tropomyosin or of 50 m M MgCl 2. Acetone powders prepared from Ilyanassa eggs contained a major protein which comigrated with actin purified from several vertebrate tissues when subjected to gel electrophoresis under denaturing conditions. The presumptive egg actin and purified actin standards were iodinated and compared by two-dimensional tryptic peptide analysis. The resulting peptide patterns from Ilyanassa egg actin were very similar to those of chick brain actin, but were distinctly different from those prepared from mouse skeletal muscle and chick gizzard actins.