Biochemical and histochemical characteristics of proteins homologous to calf lens membrane proteins with high calcium-binding capacity.

Proteins with high affinity and capacity for calcium are present in the membranes of calf lens fiber and epithelial cells. They can be extracted from these membranes by means of EDTA or EGTA. The tissue specificity and localization of these 30-38 kD EDTA-extractable proteins (EEP) have been examined. Antibodies raised against calf lens fiber EEP specifically form immune complexes with distinct proteins of 30-38 kD in a great variety of non-lenticular tissues. By indirect immunofluorescence microscopy using anti-EEP antiserum, the EEP-like proteins could be detected in fibroblasts, retinal Müller cells, endothelial cells and some types of epithelial cells. Only covering epithelia (cornea, glomerulus) contained significant amounts of these proteins, irrespective of the shape of the cells. EEP-like proteins were absent in secreting epithelial cells of liver, kidney tubules and pancreas. In addition, they were not detected in muscle, nerve and fat cells, erythrocytes and lymphocytes. The localization and the number of EEP-like proteins varied among different cell types. In fibroblasts, containing only two EEP-like proteins (molecular weight (MW) 33.0 and 31.5 kD in calf tissue), predominantly the nucleus was stained. In vitro studies with permeabilized cultured fibroblasts from several species have shown that the nuclear staining was built up of bright spots around unstained nucleoli. In epithelial and endothelial cells of calf tissue, however, most fluorescent label was found in the plasma membranes. Immunoblotting experiments revealed the presence in these cell types of at least five EEP-like proteins, including a 33.0 and 31.5 kD component. The difference in staining pattern between these cells and fibroblasts might thus indicate that the nature or the localization of some of the EEP-like proteins is cell type-specific. Because of their extractability from various tissue membrane fractions by means of EDTA or EGTA it is suggested that at least part of the EEP-like proteins is bound to membrane structures via calcium. This characteristic feature, together with the MW values and the cross-reactivity with anti-EEP antiserum indicate that these proteins and the lens membrane proteins with high calcium-binding capacity share a very high degree of homology and may even be identical.