Identity of the calmodulin-binding proteins in bovine lens plasma membranes

Two bovine lens plasma membrane proteins have been identified that bind calmodulin in a Ca2(+)-dependent fashion. Electrophoretic analysis of lens membrane proteins photoaffinity-labeled with benzophenone[125I]calmodulin confirmed our previous observation [Louis, Johnson and Turnquist (1985) Eur. J. Biochem. 150, 271-8] that two major products are formed with Mr = 46 kDa and 36 kDa. Limited proteolysis of lens membrane proteins with chymotrypsin resulted in the formation of a 21-kDa-fragment that was derived from the hydrolysis of the major lens membrane protein MP26; this correlated with the loss of the 46-kDa complex, and the formation of a 41-kDa photoaffinity-labeled complex. Use of the [125I]calmodulin gel overlay procedure confirmed that the 46-kDa and 41-kDa photoaffinity-labeled complexes reflect the interaction of calmodulin with both MP26 and its 21-kDa chymotryptic fragment, respectively. Proteolysis of lens membranes with higher concentrations of chymotrypsin resulted in the hydrolysis of the 18-kDa protein which correlated with the generation of a 12-kDa fragment; this paralleled the loss of the 36-kDa photoaffinity labeled complex and the formation of a 28-kDa-complex. The [125I]calmodulin gel overlay procedure demonstrated that the 36-kDa and 28-kDa photoaffinity-labeled complexes reflect the interaction of calmodulin with the 18-kDa lens membrane protein and its likely 12-kDa chymotryptic fragment, respectively. Identification of these calmodulin-binding proteins suggests that MP26 and the 18-kDa membrane protein are likely candidates for the proposed calcium sensitive, calmodulin-dependent gating of lens fiber cell junctions.