Soto, N.E.*; Marshall, C.; Cheng, C.-H.C.: The Trypsinogen-like protease ancestor of the Antarctic notothenioid antifreeze glycogen is a novel serine protease

Dominating the Southern Ocean fish fauna is a single suborder of perciform teleost, the notothenioids, which produce antifreeze glycoproteins (AFGPs) to survive the ice-laden freezing Antarctic water. The ancestral gene that gave rise to the AFGP gene was that of a trypsinogen-like protease (TLP), but whether TLP is the trypsinogen that performs digestive function in the notothenioids is not known. We have now cloned and sequenced full length cDNA of both pancreatic TLPs and digestive trypsinogens across notothenioid families, thereby establishing that they are two distinct groups of pancreatic proteases. TLPs are found in all notothenioids (and non-notothenioids) examined, and they share only ~50% amino acid identity with digestive trypsinogens in the respective species. And while TLPs contain the conserved catalytic triad, disulfide bridges, and many of the conserved residues, other amino acid differences as well as structural disparity from digestive trypsinogens based on molecular modeling indicate the likelihood of a non-trypsin specificity. Phylogenetic analyses also reveal separate grouping of digestive trypsinogens, TLPs and other serine proteases. Therefore we believe TLP is a novel member of the serine protease family whose precise function remains to be ascertained.