PIATIGORSKY, J., J.; National Institues of Health, National Eye Institute: Gene Sharing as an Evolutionary Strategy for Lens Crystallins

Crystallins, the major water-soluble proteins of the lens, are well conserved but unexpectedly diverse and often differ among species. In many cases, crystallins have been recruited for their specialized role in the lens from stress proteins (especially small heat shock proteins) and ubiquitously expressed enzymes in a process we call �gene sharing�. Gene sharing means that a gene encodes a protein that is used for at least two entirely different tasks. Taxon-specific accumulation of ubiquitously expressed proteins suggesting gene sharing also occurs in the cornea. �Borrowing� of diverse proteins to be used as lens crystallins occurs in jellyfish (which have astonishingly complex eyes) and molluscs (cephalopods and scallops) as well as vertebrates. For example, glutathione S-transferase and aldehyde dehydrogenase have been recruited as lens crystallins in mollusks. The cnidarian cubomedusan jellyfish, Tripedalia cystophora, has three lens crystallins. One, J3, is homolgous to vertebrate saposins, which are multifunctional proteins that bridge lysosomal hydrolases to membrane lipids aiding metabolic turnover. The characteristic saposin motif is also found in enzymes and may regulate their activity. J3-crystallin is expressed in the lens, statocyst and tentacle of the jellyfish, suggesting that it is multifunctional. Thus, the jellyfish J3-crystallin reflects both the chaperone and enzyme connections of the vertebrate crystallins. This principle of performing a specialized task (transparency and refraction) with ubiquitous proteins differs from divergent evolution. The similarity in the structure and function of cis-regulatory elements of vertebrate and invertebrate crystallin genes supports the idea that lens-preferred gene expression is a major determinant of crystallin evolution.