Abstract
Crustin antibacterial homologues, containing a whey acidic protein (WAP) domain, have been identified from the haemocyte library of the black tiger shrimp, Penaeus monodon. Sequence analysis of these cDNAs indicates the presence of several isoforms of crustin in P. monodon. Crustin Pm1, the most abundant isoform, contains an open reading frame of 435 bp encoding a precursor of 145 amino acids that comprises 17 amino acid signal peptides and 128 amino acid mature peptides. The peptides contain a Gly–Pro rich region at the amino-terminus and a single whey acidic protein (WAP) domain at the carboxyl-terminus. In order to characterize the properties and biological activities of this peptide, crustin Pm1 was overexpressed in Escherichia coli. The recombinant crustin Pm1 has a molecular mass of 14.7 kDa with a predicted p I of 8.3. Antimicrobial assays demonstrated that recombinant crustin Pm1 exhibited antimicrobial activity against only Gram-positive bacteria with strong inhibition against Staphylococcus aureus and Streptococcus iniae. In addition, the study of inhibition mechanism revealed that the antimicrobial activity of crustin Pm1 was a result of bactericidal effect. In situ hybridization with crustin Pm1 antisense probes showed strong hybridization signals in a certain haemocyte population of unchallenged shrimp, indicating that crustin Pm1 transcript is differentially expressed in different subsets of haemocyte cells.
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