Charge and molecular weight heterogeneity of EDTA-extractable proteins from calf lens membranes

Abstract

The EDTA-extractable proteins (EEP) of calf lens fiber cell membranes have been further characterized. Fiber EEP has been purified by gel filtration and resolved into eight bands with molecular weights of 30–38 Kdalton by SDS-polyacrylamide gel electrophoresis. For epithelial EEP the same range has been obtained. In agreement with these findings a value of 33 K has been determined for fiber EEP by Sephadex G200 thin-layer gel filtration, while 34 K dalton was found by high-performance gel permeation chromatography in combination with low-angle laser light scattering (HPGPC-LALLS). The isoelectric focusing patterns of fiber and epithelial EEP show considerable charge heterogeneity. By two-dimensional electrophoresis the relation molecular weight-isoelectric point has been established for most EEP components. Peptide maps of the individual protein bands of fiber EEP differ from each other and from those of the βB p-, βB 1a- and βB 1b-crystallin bands, which have about the same molecular weight. From our results we conclude that EEP is not an oligomeric nor a multisubunit protein, but a collection of different extrinsic membrane proteins, biochemically unrelated to lens crystallins. The fact that removal of the cytoskeleton by urea-treatment of the membranes is a prerequisite for its isolation by EDTA or EGTA suggests that EEP is bound to the inner surface of the plasma membranes, probably via calcium.