Carp gamma-crystallins with high methionine content: Cloning and sequencing of the complementary DNA

The nucleotide sequences of γ-crystallin cDNAs cloned from the common carp (Cyprinus carpio) have been determined. The amino-acid sequences derived consist of two polypeptides with 177 and 172 amino-acid residues for γ-m1 and γ-m2, respectively. They exhibit unusually high methionine contents: 12.4% for γ-m1 and 14% for γ-m2. Comparison of both fish γ-crystallins with bovine γ-II crystallin reveals that they are similar in structure. The striking features of both fish γ-crystallins are as follows. (1) Both of them retain the 'conserved' residues, i.e., Tyr-6, Glu-7, Gly-13, Ser-34 and their equivalents in other motifs. (2) they possess the second aromatic residue at position 11. Both of these structural features are considered to be the major factors in stabilizing the folded hairpin structure of the protein. (3) The variable residues in the core region of C-terminal domain are almost all sulfur-containing amino acids, i.e., methionine or cysteine. (4) 30% of the surface hydrophobic groups are composed of methionine. The last two unusual features have been found so far only in these two fish γ-crystallins. The high methionine content may make an important contribution to the protein stability of fish γ-crystallins.