The C-terminal domains of γS-crystallin pair about a distorted twofold axis

The 2-domain gamma S-crystallin, a highly conserved early evolutionary off-shoot of the gamma-crystallin family, is located in the water-rich region of eye lenses. The expressed C-terminal domain, gamma S-C, has been crystallized and the 2.56 Angstrom X-ray structure determined. There are two domains in the asymmetric unit which pair about a distorted twofold axis. One of the domains has an altered conformation in a highly conserved region of the protein, the tyrosine corner. The distorted gamma S-C dimer of domains is compared with the highly symmetrical, equivalent recombinant dimer of C-terminal domains from gamma B-crystallin. Sequence changes close to the interface, that distinguish gamma S from the other gamma-crystallins, are examined in order to evaluate their role in symmetrical domain pairing.