Molecular diversity and genomic organisation of the α, β and γ eye lens crystallins from the Antarctic toothfish Dissostichus mawsoni

The eye lens of the Antarctic toothfish living in the - 2 °C Southern Ocean is cold-stable. To investigate the molecular basis of this cold stability, we isolated, cloned and sequenced 22 full length crystallin cDNAs. We found two α crystallins (αA, αB), six β crystallins (βA1, βA2, βA4, βB1, βB2, βB3) and 14 γ crystallins (γN, γS1, γS2, γM1, γM3, γM4, γM5, γM7, γM8a, γM8b, γM8c, γM8d, γM8e, and γM9). Alignments of α, β and γ with other known crystallin sequences indicate that toothfish α and β crystallins are relatively conserved orthologues of their vertebrate counterparts, but the toothfish and other fish γM crystallins form a distinct group that are not orthologous to mammalian γ crystallins. A preliminary Fingerprinted Contig analysis of clones containing crystallin genes screened from a toothfish BAC library indicated α crystallin genes occurred in a single genomic region of ~ 266 kbp, β crystallin genes in ~ 273 kbp, while the γ crystallin gene family occurred in two separate regions of ~ 180 and ~ 296 kbp. In phylogenetic analysis, the γM isoforms of the ectothermic toothfish displayed a diversity not seen with endothermic mammalian γ crystallins. Similar to other fishes, several toothfish γ crystallins are methionine-rich (γM isoforms) which may have predisposed the toothfish lens to biochemically attenuate γ crystallin hydrophobicity allowing for cold adaptation. In addition to high methionine content, conservation of αβ crystallins both in sequence and abundance suggests greater functional constraints relative to γ crystallins. Conversely, reduced constraints upon γ crystallins could have allowed for greater evolutionary plasticity resulting in increased polydispersity of γ crystallins contributing to the cold-stability of the Antarctic toothfish lens.