Homology modeling of cephalopod lens S-crystallin: A natural mutant of sigma-class glutathione transferase with diminished endogenous activity

The soluble S-crystallin constitutes the major lens protein in cephalopods. The primary amino acid sequence of S-crystallin shows an overall 41% ident ity with the digestive gland sigma-class glutathione transferase (GST) of c ephalopod. However, the lens S-crystallin fails to bind to the S-hexylgluta thione affinity column and shows Very little GST activity in the nucleophil ic aromatic substitution reaction between GSH and 1-chloro-2,4-dinitrobenze ne. When compared with other classes of GST, the S-crystallin has an 11-ami no acid residues insertion between the conserved alpha 4 and alpha 5 helice s. Based on the crystal structure of squid sigma-class GST, a tertiary stru cture model for the octopus lens S-crystallin is constructed. The modeled S -crystallin structure has an overall topology similar to the squid sigma-cl ass GST, albeit with longer alpha 4 and alpha 5 helical chains, correspondi ng to the long insertion. This insertion, however, makes the active center region of S-crystallin to be in a more closed conformation than the sigma-c lass GST, The active center region of S-crystallin is even more shielded an d buried after dimerization, which may explain for the failure of S-crystal lin to bind to the immobilized-glutathione in affinity chromatography. In t he active site region, the electrostatic potential surface calculated from the modeled structure is quite different from that of squid GST. The positi vely charged environment, which contributes to stabilize the negatively cha rged Meisenheimer complex, is altered in S-crystallin probably because of m utation of Asn99 in GST to Asp101 in S-crystallin. Furthermore, the importa nt Phe106 in authentic GST is changed to His108 in S-crystallin, Combining the topological differences as revealed by computer graphics and sequence v ariation at these structurally relevant residues provide strong structural evidences to account for the much decreased GST activity of S-crystallin as compared with the authentic GST of the digestive gland.