Identification of the EDTA-extractable protein in lens as calpactin I

The EDTA-extractable protein (EEP) is a major extrinsic protein of lens membrane. The 35 kilodalton (kDa) polypeptide of the EEP cross-reacted to antibody prepared against calpactin I, a substrate for the src protein and an inhibitor of phospholipase A2. Calpactin I is also thought to play a structural role in linking cytoskeleton to membrane. The 35 kDa protein in bovine lens contained phosphotyrosine residues that can be detected by affinity purified antibody to this moiety. Although there is some microheterogeneity of EEP using two dimensional gel electrophoresis, at least one of the chick polypeptides, immunoreactive for calpactin I, can be phosphorylated in whole lens culture. These results suggest a regulatory function for the EEP in lens.