Calmodulin-binding protein (55K + 17K) of sea urchin eggs has a Ca2+- and calmodulin-dependent phosphoprotein phosphatase activity.

A calmodulin-binding protein from sea urchin eggs consisting of two subunits (55 and 17K-daltons) was identified as a Ca2+-dependent phosphoprotein phosphatase similar to calcineurin in mammalian brain and to phosphatase 2B in skeletal muscle. Peptide mappings showed that the 55K subunit was different from 61K subunit of calcineurin, whereas the 17K subunit was similar to 19K subunit of calcineurin but different from calmodulin. The 55K + 17K protein of sea urchin eggs dephosphorylated 32P-inhibitor-1 in a Ca2+- and calmodulin-dependent manner. Vmax and Km for inhibitor-1 in the presence of Ca2+ and calmodulin were 2,100 pmol Pi/min/mg and 2.7 microM. Ca2+-dependent phosphatase activity for inhibitor-1 was detected in homogenates of both unfertilized and fertilized eggs, but was not detected in isolated cortices and mitotic apparatus.