Two related but distinct forms of the Mr 36,000 tyrosine kinase substrate (calpactin) that interact with phospholipid and actin in a Ca2+-dependent manner.

A method was devised that allows the identification of proteins related to the Mr 36,000 tyrosine kinase substrate calpactin based on their ability to interact with actin and phospholipid in a calcium-dependent manner. Two distinct proteins, detected in human A431 cells and fibroblasts, were resolved by two-dimensional gel electrophoresis. One of these proteins (calpactin I) appears identical to the Mr 34,000-39,000 substrate of the pp60src tyrosine kinase and the second (calpactin II) reacts with antibodies to the Mr 35,000 substrate of the epidermal growth factor receptor. Both proteins interact with phospholipid and actin, are rather basic, and share structural and antigenic determinants. A major difference between the two proteins is noted in their state of association with the Mr 10,000 light chain; i.e., calpactin I is associated with the light chain while calpactin II is not.