Crystallins are stress-related proteins or enzymes that often maintain dual roles without gene duplication. We have shown that the putative lens promoter of NADPH:quinone oxidoreductase/zelmu-crystallin is highly lens specific and by itself is responsible for the recruitment of this gene as a crystallin. An important functional element (ZPE) that binds different complexes in lens and nonlens-cell extracts has been identified. The full sequence of eta-crystallin, another mammalian enzyme crystallin, has been obtained and shows identify with the enzyme ALDH1, another example of gene recruitment. mu-Crystallin, originally discovered in marsupial lenses, is a novel enzyme with NADPH-binding activity. Immunohistochemistry suggests it is preferentially expressed in vertebrate photoreceptors. The gene for mu has been cloned. Not all important lens proteins are crystallins. We have shown that MIF, a lymphokine, is expressed in differentiating lens cells. The gene for human MIF has been cloned and expression studies are under way. Lens P2 protein, another potential marker for the differentiation process and a possible intermediary messenger has been cloned and shown to belong to a lipid/retinoid-binding family. Members of the C/EBP family, transcription factors with important roles in differentiation and candidates for promoter binding in some crystallin genes, have been detected in the lens, where their pattern of expression is developmentally regulated.
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