Omega-crystallin of the scallop lens - A dimeric aldehyde dehydrogenase class 1/2 enzyme-crystallin

While many of the diverse crystallins of the transparent lens of vertebrate s are related or identical to metabolic enzymes, much less is known about t he lens crystallins of invertebrates. Here we investigate the complex eye o f scallops. Electron microscopic inspection revealed that the anterior, sin gle layered corneal epithelium overlying the cellular lens contains a regul ar array of microvilli that we propose might contribute to its optical prop erties. The sole crystallin of the scallop eye lens was found to be homolog ous to Omega -crystallin, a minor crystallin in cephalopods related to alde hyde dehydrogenase (ALDH) class 1/2. Scallop Omega -crystallin (officially designated ALDH1A9) is 55-56% identical to its cephalopod homologues, while it is 67 and 64% identical to human ALDH 2 and 1, respectively, and 61% id entical to retinaldehyde dehydrogenase/eta -crystallin of elephant shrews.: like other enzyme-crystallins, scallop Omega -crystallin appears to be pres ent in low amounts in nonocular tissues. Within the scallop eye, immunofluo rescence tests indicated that Omega -crystallin expression is confined to t he lens and cornea, Although it has conserved the critical residues require d for activity in other ALDHs and appears by homology modeling to have a st ructure very similar to human ALDH2, scallop Omega -crystallin was enzymati cally inactive with diverse substrates and did not bind NAD or NADP, In con trast to mammalian ALDH1 and -2 and other cephalopod Omega -crystallins, wh ich are tetrameric proteins, scallop Omega -crystallin is a dimeric protein . Thus, ALDH is the most diverse lens enzyme-crystallin identified so far, having been used as-a lens crystallin in at least two classes of molluscs a s well as elephant shrews.