Cathepsin L proteases of the parasitic copepod, Lepeophtheirus salmonis

The salmon louse, Lepeophtheirus salmonis, is a parasitic copepod that feeds on the mucus, skin and blood of salmonids. We describe the identification of two complete L. salmonis cathepsin L-like gene sequences and their molecular characterisation. L. salmonis cathepsin L1 (LsCL1), is 978 base pairs in length, encoding a protein of 325 amino acid residues while L. salmonis cathepsin L2 (LsCL2) is 1149 base pairs in length, encoding a protein of 382 amino acid residues. The predicted molecular weights of LsCL1 and LsCL2 are 35,964. Da and 42,150. Da respectively. The two proteases share only 25% identity in the primary sequences; however, the catalytic triad of cysteine, histidine and asparagine is highly conserved for both. Biochemical analysis of L. salmonis extracts revealed that cathepsin L has an optimum activity at pH 6.5, at 15 °C and remains stable at this temperature. Cathepsin L activity is present in all of the parasite life stages assayed, with the chalimus life stage extract exhibiting the most activity. Cathepsin L activity was also observed in the secretory/excretory products possibly indicating a role for this protease in immunoevasion and establishment of the parasite on the host.