Ω-Crystallin of the Scallop Lens

Abstract

While many of the diverse crystallins of the transparent lens of vertebrates are related or identical to metabolic enzymes, much less is known about the lens crystallins of invertebrates. Here we investigate the complex eye of scallops. Electron microscopic inspection revealed that the anterior, single layered corneal epithelium overlying the cellular lens contains a regular array of microvilli that we propose might contribute to its optical properties. The sole crystallin of the scallop eye lens was found to be homologous to ⍀-crystallin, a minor crystallin in cephalopods related to aldehyde dehydrogenase (ALDH) class 1/2. Scallop ⍀-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% identical to retinaldehyde dehydrogenase/-crystallin of elephant shrews. Like other enzyme-crystallins, scallop ⍀-crystallin appears to be present in low amounts in nonocular tissues. Within the scallop eye, immunofluorescence tests indicated that ⍀-crystallin expression is confined to the lens and cornea. Although it has conserved the critical residues required for activity in other ALDHs and appears by homology modeling to have a structure very similar to human ALDH2, scallop ⍀-crystallin was enzymatically inactive with diverse substrates and did not bind NAD or NADP. In contrast to mammalian ALDH1 and -2 and other cephalopod ⍀-crystallins, which are tetrameric proteins, scallop ⍀-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 as well as elephant shrews.

The structure of human ADP-ribosylhydrolase 3 (ARH3) provides insights into the reversibility of protein ADP-ribosylation

“…The hits include the JI jellyfish lens crystallins (29) and the zebrafish SelJ selenoprotein (30), indicating that the ARH fold may have been adopted to also serve structural functions. It is interesting to note, however, that neither PSI-BLAST nor secondary-structure prediction analyses revealed any sequence or structural resemblance of PARG to the ARH3͞DRAG protein family (28).…”

“…The ARH fold is completely different from that of arginase (PDB ID code 1RLA) (32), which contains a binuclear Mn center and which has been used in a previous study as a model for DRAG (18). PSI-BLAST analyses indicate that the new ARH fold is widely spread in nature and may serve various enzymatic (i.e., hydrolysis of protein-ADP-ribosyl and polyADP-ribosyl bonds) as well as structural functions (i.e., lens crystallins in jellyfish and zebrafish) (29,30). Importantly, the results of PSI-BLAST, structure prediction, and threading analyses do not show any significant similarities of the ARH fold to the catalytic domain of PARG, indicating that the ARH fold is not a useful model for PARG.…”

Identification of novel components of NAD-utilizing metabolic pathways and prediction of their biochemical functions

“…44 The protein ADP ribosylation systems, in addition to the ARTs, also contain ADP ribose glycohydrolases (ARGs) and polyADP ribose glycohydrolases (PARGs; the above-mentioned Macro domain enzymes) that remove the ADP-ribose moieties from targets by hydrolyzing the glycosidic bonds. 25,45,46 In recent years there has been tremendous progress in terms of structural and biochemical understanding of ARTs, PARTs, sirtuins, MACROs and several NAD biosynthesis enzymes. There have also been several efforts in terms of sequence analysis leading to the discovery of novel ART superfamily enzymes 19,32,47 and tremendous interest in the connections between NAD metabolism and the dynamics of heterochromatin formation, especially in the context of organismal aging.…”

Diversity and functional plasticity of eukaryotic selenoproteins: Identification and characterization of the SelJ family

“…8, which is published as supporting information on the PNAS web site; see also Fig. 7) and are encoded by three intronless genes (26). Crystallins maintain the transparency and proper light diffraction in the eye lens across the animal taxa, but unrelated crystallins are found in vertebrates, cephalopods, and jellyfish.…”

“…As reported in ref. 26, the J1A-, J1B-, and J1C-crystallins have completely different promoter regions. Furthermore, the fish promoters do not show sequence similarity to the jellyfish promoter regions.…”