CRYSTALLINS OF CEPHALOPODS AND VERTEBRATE S ARE EVOLUTIONARY RELATED TO THE HEAT-SHOCK AND STRESS-INDUCED PROTEINS

The lenses of cephalopods and vertebrates are a classical example of c onvergent evolution. In the course of evolution vertebrates and cephal opods used the same strategy of recruitment of ancestral proteins (pri marily enzymes, whose activity is related with stress and detoxificati on) for structural functions in the lens. Crystallins are defined as w ater soluble proteins, that form the lenses of cephalopods and vertebr ates. In vertebrates. only the beta/gamma-crystallin genes are lens-sp ecific. The other crystallin classes (alpha-, beta-, etc) are overexpr essed in the lens, having the various noticeable expression levels in the other tissues as well. Almost all the crystallins, forming squid l ens are referred to a single class, namely S-crystallins. The latters are encoded by a family of genes, consisting of at least 10 members S- crystallins of the squid and the octopus are related, but not identica l to glutathione S-transferases. S-crystallins show no enzymatic activ ity, though they have 42-44% homology with a squid glutathione-S-trans ferase (GST), which we have structurally characterized. Genes of GST a nd S-crystallins have similar ''on-intron structures (5-6 exons), wher eas their 5' nanking regions are different. Promoters of both of S-cry stallin genes and of some vertebrate crystallin genes bear the similar regulatory element (AP1-site), that must take part in regulation of t heir lens-specific expression. Besides S-crystallins, octopus lens con tains a major polypeptide, omega-crystallin, that was characterized in detail. Though omega-crystallin is related to aldehyde dehydrogenase (ALDG), it has no enzymatic activity. Genes coding for omega- and S-cr ystallins were shown to be expressed only in the lens cells of squid a nd octopus. In some mammals ALDG is known to serve as a crystallin (et a-crystallin of elephant shrew). Thus, octopus omega-crystallin and et a-crystallin of elephant shrew - are the first example of similarity b etween vertebrate and cephalopod crystallins.