Calmodulin-binding proteins in the cytosol extract of sea urchin eggs

Calmodulin and calmodulin-binding proteins in the cytosol extract of eggs from the sea urchins Hemicentrotus pulcherrimus and Strongylocentrotus intermedius were studied in an attempt to elucidate the physiological role(s) of calmodulin in eggs. Calmodulin in the cytosol extract was found both in a free form and in complexes with other proteins, either Ca2+-dependently or Ca2+-independently. The extracts contained at least three calcium-dependent calmodulin-binding proteins. One was an NAD kinase of unknown molecular composition. The apparent molecular weights of the other two calmodulin-binding proteins were 50K and 55K + 17K daltons, as estimated by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate. These three proteins formed complexes with calmodulin only in the presence of calcium as demonstrated by gel filtration. The 17K-dalton protein was found to be calmodulin itself; it did not dissociate from the 55K-dalton protein regardless of the presence or absence of calcium. The native molecular weights of these protein-calmodulin complexes obtained by gel filtration through a Sephacryl S-300 column were 190K for the NAD kinase, 130K for the 50K-dalton protein and 100K daltons for the 55K + 17K-dalton protein.