Calcium-binding lens membrane proteins.

Calcium-binding membrane-bound proteins are present in the vertebrate eye lens. Among these proteins are a distinct group of immunologically related extrinsic EDTA-extractable proteins (EEP) and calmodulin. The EEP proteins contain calcium-binding sites with a total capacity of 25 mol Ca2+ per mol protein. This high calcium-binding capacity of EEP points to a function of these proteins as intracellular calcium store in the lens. However, EEP undergoes a conformational change upon calcium binding, indicating that these proteins may be involved in the regulation of calcium-dependent cellular processes in the lens. One of these processes is the action of communicating lens fiber junctions, which contain EEP as a main protein component. In addition to EEP, another calcium-binding protein in lens, calmodulin, probably functions as mediator of calcium in the regulation of the structure and function of lens junctions. Like other vertebrate calmodulins, lens calmodulin shows a calcium-dependent mobility shift on SDS-polyacrylamide gels and forms immune complexes with antiserum raised against vertebrate calmodulin. Lens calmodulin binds to the junction proteins MIP (main intrinsic protein, MW 26 Kdalton) and a 17.5 Kdalton polypeptide of lens fiber cells in a calcium-independent manner. Via calmodulin the junctions become calcium-sensitive.