Recruitment of enzymes and stress proteins as lens crystallins

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The major water-soluble proteins — or crystallins — of the eye lens are either identical to or derived from proteins with non-refractive functions in numerous tissues. In general, the recruitment of crystallins has come from metabolic enzymes (usually with detoxification functions) or stress proteins. Some crystallins have been recruited without duplication of the original gene (i.e., lactate dehydrogenase B and α-enolase), while others have incurred one (i.e., argininosuccinate lyase and a small heat shock protein) or several (i.e., glutathione S-transferase) gene duplications. Enzyme (or stress protein)-crystallins often maintain their non-refractive function in the lens and/or other tissues as well as their refractive role, a process we call gene sharing. α-Crystallin/small heat shock protein/molecular chaperone is of special interest since it is the major crystallin of humans. There are two α-crystallin genes (αA and αB), with αB retaining the full functions of a small heat shock protein. Here we describe recent evidence indicating that αA and αB have kinase activity, which would make them members of the enzyme-crystallins. We also describe various regulatory elements of the mouse α-crystallin genes responsible for their expression in the lens and, for αB, in skeletal muscle. Delineating the control elements for gene expression of these multifunctional protective proteins provides the foundations for their eventual use in gene therapy. Finally, comparison of the mouse and chicken αAcrystallin genes reveals similarities and differences in their functional cis-acting elements, indicative of evolution at the level of gene regulation.