Bovine lens calmodulin. Isolation, partial characterization and calcium-independent binding to lens membrane proteins

Abstract

Calmodulin has been isolated from calf lens fiber cells. 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. Via the gel overlay technique radioiodinated calmodulin from lens or bovine brain was found to bind to the main intrinsic protein (MIP) and the 17.5 kDa protein of lens fiber membranes in a calcium-independent manner. After proteolytic digestion of lens fiber membranes with trypsin or Staphylococcus aureus V8 protease the calmodulin-binding activity of MIP is retained. This result indicates that the small polypeptide fragment of MIP, which is accessible to proteolytic attack, apparently is not the attachment point for calmodulin. Two additional calmodulin-binding proteins (MW 14 kDa and 16.5 kDa) are observed in junction-enriched fiber membrane fractions. These junction-specific proteins are bound to the membrane via calcium. In addition to MIP and the 17.5 kDa protein they are possibly involved in the calcium-dependent regulation of lens fiber junctions. The 14 and 16.5 kDa proteins are also present in epithelial membranes, prepared from freshly obtained calf lens epithelia. Whereas in the latter membranes the two proteins form part of the four 14-17 kDa major protein components, these proteins are absent in membranes from cultured lens epithelial cells. The epithelial 14 kDa and 16.5 kDa proteins thus appear to be junction-specific. The capacity of the latter proteins to bind calmodulin in the presence and absence of calcium indicates that these junction-specific proteins are very similar, if not identical, to the corresponding fiber junctional proteins.(ABSTRACT TRUNCATED AT 250 WORDS)