HIGH-CAPACITY BINDING OF ALPHA-CRYSTALLINS TO VARIOUS BOVINE LENS MEMBRANE PREPARATIONS

This study examines the high capacity binding of intact and carboxyl-terminal-truncated alpha A(alphaA) crystallin to two types of lens membrane preparations; membrane stripped of extrinsic protein and some lipid by extraction with urea and alkali and unextracted membrane isolated by centrifugation of total water insoluble protein on a sucrose gradient (native membrane). High capacity binding of alphaA crystallin to the urea-treated membrane was seen once the alphaA substrate concentration reached about 1 mg/ml of media. The membrane bound up to one mg of alphaA per mg of intrinsic protein (MP26) at a concentration of 5 mg alphaA/ml media, binding 5 to 10 times greater than that seen by others at saturation of the high affinity but low capacity binding sites. No apparent differences were seen between high capacity binding of carboxyl terminal--truncated alphaA (by trypsin) and intact alphaA, although each crystallin could antagonize binding of the other. However, once membrane bound, neither crystallin appeared to grossly displace the other. Using the carboxyl terminal-truncated alpha crystallin as a model substrate, native membrane was seen to have a higher capacity to bind the truncated alpha crystallin than urea-extracted membrane and binding was better correlated with the preexisting alphaA content of the native membrane than its MP26 content. An artificial native membrane was prepared by prebinding the truncated alphaA to urea-extracted membrane. This preparation bound more intact alphaA than urea-extracted membrane bearing no prebound crystallin. We conclude that lens native membrane possesses a high capacity to bind alpha crystallins and that this binding could be mediated through protein-protein interactions with alpha crystallin bound in situ to the membrane as extrinsic protein.