Characterization of cDNA clones encoding two distinct cathepsins with restricted expression pattern in a marine pelagic fish.

Cathepsin L (EC 3.4.22.15) from aquatic animals are quite stable and active at neutral or alkaline pH values while their mammalian equivalents work at an acidic environment of the lysosomes. To understand the molecular properties at the gene level we employed a PCR-based strategy using degenerate oligonucleotide primers to isolate cathepsin L-like genes from anchovy Engraulis japonicus. As a result, we obtained two closely related genes encoding cathepsins (aCat1 and aCat2) similar to both cathepsins L and S from other organisms. The predicted precursor protein of 324 amino acid residues for genes differed in six residues and contained conserved residues characteristic of cathepsin L-like cysteine proteases. Phylogenetic analyses failed to produce any precise relationships of aCat1 and aCat2 with other cysteine proteases. However, with a bootstrap value less than 50, these two fish cathepsins formed a separate group to that bearing cathepsins L and S of various organisms. Interestingly, unlike mammalian cathepsin L transcripts of aCat1 and aCat2 were almost exclusively detected in the stomach suggesting that the fish homologues are non-lysosomal secretory enzymes present in the extracellular acidic environment of the stomach and that marine teleosts developed digestive cysteine proteases as a result of evolutionary pressure in response to varying dietary conditions.