Short title: Characterization of glutathione S-transferases from Cyphoma gibbosumFootnote: The nucleotide and translated amino acid sequences for C. gibbosum GSTs have been deposited in GenBank with the following accession nos. EU008563 (CgGSTM1) and EU008562 (CgGSTM2).Abbreviations footnote: CDNB, 1-chloro-2,4-dinitrobenzene; GST, glutathione S-transferase; nrDB, non-redundant database; RACE, rapid amplification of cDNA ends; RT-PCR, reverse transcriptase polymerase chain reaction 2 ABSTRACT Glutathione S-transferases (GST) were characterized from the digestive gland of Cyphoma gibbosum (Mollusca; Gastropoda), to investigate the possible role of these detoxification enzymes in conferring resistance to allelochemicals present in its gorgonian coral diet. We identified the collection of expressed cytosolic Cyphoma GST classes using a proteomic approach involving affinity chromatography, HPLC and nanospray liquid chromatography-tandem mass spectrometry (LC-MS/MS). Two major GST subunits were identified as putative mu-class GSTs; while one minor GST subunit was identified as a putative theta-class GST, apparently the first theta-class GST identified from a mollusc. Two Cyphoma GST cDNAs (CgGSTM1 and CgGSTM2) were isolated by RT-PCR using primers derived from peptide sequences. Phylogenetic analyses established both cDNAs as mu-class GSTs and revealed a mollusc-specific subclass of the GST-mu clade. These results provide new insights into metazoan GST diversity and the biochemical mechanisms used by marine organisms to cope with their chemically defended prey.