ALDEHYDE DEHYDROGENASE-DERIVED OMEGA-CRYSTALLINS OF SQUID AND OCTOPUS- SPECIALIZATION FOR LENS EXPRESSION

OMEGA-crystallin of the octopus lens is related to aldehyde dehydrogen ases (ALDH) of vertebrates (Tomarev, S. I., Zinovieva, R. D., and Piat igorsky, J. (1991) J. Biol. Chem. 266, 24226-24231) and ALDH1/eta-crys tallin of elephant shrews (Wistow, G., and Kim, H. (1991) J. Mol. Evol . 32, 262-269). Only very low amounts of OMEGA-crystallin are present in the squid lens. Here, we have cloned OMEGA-crystallin cDNAs of the octopus (Octopus dofleini) and squid (Ommastrephes sloani pacificus) l enses. The deduced amino acid sequences of OMEGA-crystallin from these species are 78% identical to each other, 56-58% identical to cytoplas mic ALDH1 and mitochondrial ALDH2 of vertebrates (which are 66-68% ide ntical to each other), and 40% identical to Escherichia coli and spina ch ALDHs. These data are consistent with the idea that the ALDH1/ALDH2 gene duplication in vertebrates occurred after divergence of cephalop ods from the line giving rise to vertebrates, but before the separatio n of squid and octopus. Southern blot hybridization indicated that OME GA-crystallin is encoded by few genes (possibly just one) in octopus a nd squid. Northern blot hybridization revealed two bands (2.7 and 9.0 kilobases) of OMEGA-crystallin RNA in the octopus lens and one band (4 .2 kilobases) in the squid lens; OMEGA-crystallin RNAs were undetectab le in numerous non-lens tissues of octopus and squid, suggesting lens- specific expression of this gene(s). Finally, extracts of the octopus lens had no detectable ALDH activity using different substrates, consi stent with OMEGA-crystallin having no enzymatic activity. Taken togeth er, our results suggest that OMEGA-crystallin evolved by duplication o f an ancestral gene encoding ALDH and subsequently specialized for ref raction in the transparent lens while losing ALDH activity and express ion in other tissues.