Shrimp single WAP domain (SWD)-containing protein exhibits proteinase inhibitory and antimicrobial activities.

Single WAP domain (SWD)-containing proteins are small proteins with a C-terminal region containing a single whey acidic protein (WAP) domain. In the present study, the cDNAs representing three isoforms of SWD proteins (SWDPm1, SWDPm2 and SWDPm3) were identified from hemocytes of the black tiger shrimp, Penaeus monodon. The deduced peptides revealed that they contain a putative signal peptide of 24 amino acids and encode for a mature peptide of 69, 68 and 56 amino acids, respectively, which contain typical characters similar to those of the shrimp SWD proteins (type III crustin) with a Pro-Arg region and a WAP domain towards the C-terminus. Tissue distribution analysis by RT-PCR showed that all three SWDPm transcripts were primarily found in hemocytes. Transcript expression of SWDPm1 was down-regulated upon injection with Staphylococcus aureus whilst there was no change of SWDPm2 and SWDPm3 expression. In contrast, white spot syndrome virus (WSSV) injection resulted in a biphasic response with up-regulation of SWDPm1 and SWDPm2 transcripts at 6h followed by significant down-regulation by 24h after infection. Genomic organization of the SWDPm2 gene revealed the presence of three exons interrupted by two introns. To characterize the biological functions of the SWD protein, the mature SWDPm2 protein encoding cDNA was cloned and expressed in Escherichia coli. Purified recombinant (r)SWDPm2 exhibits antibacterial activity against several Gram-positive, but not Gram-negative, bacteria and is a competitive inhibitor of subtilisin A with an inhibition constant (Ki) of 1.98nM. Thus, rSWDPm2 may contribute to the inhibitory regulation of subtilisin A from bacterial infection and P. monodon SWD protein likely function as immune effectors in defense against invasion of shrimp pathogens.