α-Crystallin Chaperone-like Activity and Membrane Binding in Age-Related Cataracts

Abstract

α-Crystallin, the major protein component of the vertebrate lens, is thought to play a critical role in the maintenance of transparency through its ability to inhibit stress-induced protein aggregation. However, during aging and cataract formation the amount of membrane-bound α-crystallin increases significantly while high molecular weight complexes (HMWCs) comprised of α-crystallin and other lens crystallins accumulate. These and other recent data suggest a possible link between cataract formation, the formation of high molecular weight α-crystallin aggregates, and the progressive increase in membrane association of α-crystallin. To better understand these processes, we characterized the chaperone-like activity (CLA) and subunit exchange of membrane bound αcrystallin. In addition, we measured the membrane binding properties of in vitro constituted HMWCs to understand the mechanism by which increased α-crystallin is bound to the membrane of old and cataractous lens cells in vivo. Membrane-associated α-crystallin complexes have measurably reduced CLA compared to complexes in solution; however, membrane binding does not alter the time required for α-crystallin complexes to reach subunit exchange equilibrium. In addition, HMWCs prepared in vitro have a profoundly increased membrane binding capacity as compared to native α-crystallin. These results are consistent with a model in which increased membrane binding of α-crystallin is an integral step in the pathogenesis of many forms of cataracts.

Lipids and the ocular lens

“…␣ -Crystallin is the only crystallin that binds noncovalently to bovine lens lipid membranes (128)(129)(130)(131)(132)(133)(134)(135)(136)(137)(138)(139)(140) that are devoid of protein and synthetic lipid membranes ( 43,(129)(130)(131). Lens membranes have both a high-affi nity saturable and a low-affi nity nonsaturable ␣ -crystallin binding site (135)(136)(137).…”

“…Lens membranes have both a high-affi nity saturable and a low-affi nity nonsaturable ␣ -crystallin binding site (135)(136)(137). The binding of ␣ -crystallin to lens membranes in vitro may not involve proteins ( 135 ), because ␣ -crystallin binding to native membranes is enhanced when extrinsic proteins are stripped from the membrane surface ( 128,129,138 ), exposing the lipid moiety. These results contradict an earlier study indicating that ␣ -crystallin interacts mostly with MP26 ( 139 ).…”

Role of ATP on the Interaction of α-Crystallin with Its Substrates and Its Implications for the Molecular Chaperone Function

“…␣-Crystallin, like other sHSPs, acts like a molecular chaperone in preventing the aggregation of other proteins (8 -10). An intense research activity on the structure and function of ␣-crystallin has grown since then (11)(12)(13)(14)(15)(16)(17)(18)(19)(20), and the subject matter has been extensively reviewed in recent years (21)(22)(23).…”

Small heat-shock proteins regulate membrane lipid polymorphism

“…␣-Crystallin has been shown to bind to ocular plasma membranes and synthetic phospholipid vesicles (13)(14)(15)(16)(17)(18), but the mechanism of interaction is not yet characterized, and the role these interactions may play in lens biology is not clear. A recent report (19) suggests that increased membrane association of ␣-crystallin is a critical event in the pathogenesis of cataracts.…”