Alpha Crystallin Chaperone-like Activity and Membrane Binding

Abstract

Abstract: : Purpose: The abundance of soluble alpha crystallin is known to decline with age and presence of cataract. It is also known that alpha crystallin shifts to a high molecular weight complex (HMWC) and becomes increasingly associated with membrane fractions with age and presence of cataract. The present study was conducted to evaluate whether membrane-associated alpha crystallin was still capable of expressing chaperone-like activity against denaturing proteins. We also studied whether HMWCs such as those formed during chaperone-like function have altered membrane binding properties. Methods: Recombinant human alpha crystallins were used to generate high molecular weight aggregates with DTT-treated insulin or heat-denatured human aldose reductase. Membrane-associated crystallins were formed using either synthetic phospholipid vesicles or plasma membranes extracted from bovine cortical fiber cells. Measurements of subunit exchange experiments were carried out using fluorescence resonance energy transfer. Results: Chaperone-like activity of alphaA and alphaB crystallins was retained in complexes bound to phospholipid vesicles, although at slightly reduced levels when compared with equivalent amounts of crystallins assayed in the absence of vesicles. Membrane-associated alpha crystallin was capable of exchanging subunits with crystallin complexes still in solution. A marked increase in membrane binding capacity was observed with HMWCs formed between alpha crystallin and denatured insulin or aldose reductase compared with native alpha crystallin complexes. Conclusion: Neither loss of chaperone-like activity or loss of subunit exchange capability is evident when alpha crystallin complexes become associated with phospholipid membranes. These results suggest that a redistribution of cytoplasmic alpha crystallin toward the membrane such as seen with aging and presence of cataract does not necessarily result in a concomittant loss of chaperone capability. Increased membrane binding by HMWCs formed from alpha crystallin may be an important factor in mechanisms of cataract.

Keywords: 378 crystallins • 338 cataract • 527 protein structure/function