Abstract:Portunus trituberculatus, aka the swimming crab, is an important fishery species with high commercial value in China. They mainly inhabit the sandy and muddy bottom underneath the seawater in the coast of countries such as China, Japan and Korea. The growth and development of P. trituberculatus was characterized by periodic molting activities in which the new ectoskeleton grows while the old one molting. Previous studies reported that chitinase was an essential enzyme involved in the molting of crustaceans, and that it was required in chitin digestion and the immunity to the pathogen infection. However, little has been known about the effect of chitinase on molting activities, and the expression of the enzyme during rapid salinity stress is also obscure. To understand the biological function of PtCht3 gene in P. trituberculatus, we cloned and analyzed the full-length cDNA sequence of PtCht3 gene using SMARTTM RACE amplification kit. The cDNA sequence had 1409 bp encoding 394 amino acid residues. The isoelectric point (pI) of the putative peptide was 4.80, and the predicted molecular mass was 43.67 kDa. PtCht3 could be a stable protein with the hydrophilic coefficient total average of –0.097. The homology and systematic evolution analysis revealed that PtCht3 of P. trituberculatus was highly homologous with the protein in other species; for example, it shared 54% similarity with Pandalopsis japonica and 53% similarity with Eriocheir sinensis. The phylogenetic analysis showed that P. trituberculatus PtCht3 was in the same class as other arthropods¢ PtCht3. The expression of PtCht3 in different tissues was analyzed with quantitative real-time PCR. We found that the highest expression level appeared in hepatopancreas, and that the expression was significantly up-regulated in the prophase of the molting cycle. In response to the low salinity stress the expression level fluctuated in gills and hepatopancreas, showing a general up-and-down pattern. Our results suggested that PtCht3 might play an important role in digestion and molting activities, and it could be involved in the regulation of osmotic pressure in P. trituberculatus.
