S-crystallin (IPR003083)

Short name: S-crystallin

Overlapping homologous superfamilies

Family relationships


S-crystallins of cephalopod are the major protein constituent of the lens in cephalopods. Their primary protein sequences show a high degree (41%) of identity with the cephalopod digestive gland sigma-class glutathione transferase (GST). In spite of the sequence similarity, lens S-crystallin shows little if any GST activity. S-crystallins fail to bind to an S-hexylglutathione affinity column (marker for glutathione affinity) and have very little GST activity in a typical substitution reaction with glutathione and 1-chloro-2,4-dinitrobenzene [PMID: 9929473]. Nevertheless, pH rate profiles indicate that any reactions that do take place proceed via the same mechanism as GSTs [PMID: 8827456]. Sequence analysis suggests that the S-crystallins arose from gene duplication of a cephalopod sigma-class GST [PMID: 8440736].

The three-dimensional structure of the sigma-class GST from squid has been determined to 2.4A resolution [PMID: 7727393]. The protein is characterised by two domains, one of which has a 3-layer(aba) sandwich architecture, the other being largely helical. The modelled S-crystallin structure has a similar topology to the squid sigma-class GST, with longer helices 4 and 5, corresponding to a long insertion. The insertion causes the active centre to be in a more closed conformation than sigma-class GSTs and may explain the low affinity for glutathione [PMID: 9929473].

The function of glutathione S-transferases (GST) is the conjugation of reduced glutathione to a wide number of exogenous and endogenous hydrophobic electrophiles. The isoenzymes appear to play a central role in the parasite detoxification system. Glutathione S-transferases form homodimers, but in eukaryotes can also form heterodimers of the A1 and A2 or YC1 and YC2 subunits. The GST domain is also found in S-crystallins from squid, and proteins with no known GST activity, such as eukaryotic elongation factors 1-gamma and the HSP26 family of stress-related proteins, which include auxin-regulated proteins in plants and stringent starvation proteins in Escherichia coli.

Contributing signatures

Signatures from InterPro member databases are used to construct an entry.