Properties of alpha-crystallin bound to lens membrane: probing organization at the membrane surface.

Alpha crystallin, one of the three major soluble proteins of the eye lens, appears to be a natural extrinsic protein of lens plasma membrane. Membrane-immobilized alpha-crystallin could provide a template for the increased association of protein with lens membrane seen in aging and cataracts. Alpha-crystallin binds to lens membrane through both a high-affinity saturable and low-affinity nonsaturable process. The organization of alpha-crystallin at the membrane surface was proved by the examination of various chemical reactivities and a functional property of the membrane bound protein. The carboxyl-terminal domain of membrane bound alpha-crystallin appeared to be as readily cleaved by mild trypsinolysis as that of the soluble protein and the cleaved protein remained bound to the membrane. The immobilized protein was more extensively crosslinked by a bifunctional primary amine-reactive agent than the soluble protein. No evidence for crosslinking to membrane intrinsic protein was obtained. Like soluble alpha-crystallin, the membrane bound protein displayed chaperone-like activity, a property dependent upon quaternary structure. These findings were interpreted to indicate that alpha-crystallin binds to lens membrane as an aggregate with only a fraction of each aggregate in direct contact with the membrane's hydrophobic surface. It is suggested that the nonsaturable binding reflects low affinity association of soluble alpha-crystallin with a layer of membrane-immobilised protein.