Evidence indicating the existence of a novel family of serine protease inhibitors that may be involved in marine invertebrate immunity

• Published in: Fish & shellfish immunology 2009-08, Vol.27 (2), p.250-259
• Main Authors: Xue, Qinggang, Itoh, Naoki, Schey, Kevin L., Cooper, Richard K., La Peyre, Jerome F.
• Format: Article
• Language: English
• Subjects: Amino Acid Sequence; Animals; Base Sequence; Bivalve mollusc; Bivalvia; Crassostrea - genetics; Crassostrea - immunology; Crassostrea virginica; Gene Expression Profiling; Gene Expression Regulation; Invertebrata; Invertebrates - genetics; Invertebrates - immunology; Kinetics; Marine; Marine Biology; Molecular Sequence Data; Oyster; Perkinsus marinus; Protease inhibitor family; Sequence Alignment; Serine protease inhibitor; Serine Proteinase Inhibitors - chemistry; Serine Proteinase Inhibitors - genetics; Serine Proteinase Inhibitors - metabolism; Subtilisins - antagonists & inhibitors; Tunicate
• ISSN: 1050-4648; 1095-9947
• DOI: 10.1016/j.fsi.2009.05.006

A new serine protease inhibitor, designated cvSI-2, was purified and characterized from the plasma of the eastern oyster, Crassostrea virginica. CvSI-2 inhibited the serine protease subtilisin A in a slow-tight binding manner, with an overall dissociation constant Ki* of 0.18 nM. It also inhibited perkinsin, the major extracellular protease of the oyster protozoan parasite Perkinsus marinus. Sequencing of cvSI-2 cloned cDNA revealed an open reading frame of 258 bp encoding a polypeptide of 85 amino acids, with the 18 N-terminal amino acids forming a signal peptide. The mature cvSI-2 molecule predicted consisted of 67 amino acids with 12 cysteine residues and a calculated molecular mass of 7202.96 Da. Overall 91% of the cvSI-2 amino acid sequence predicted from cDNA was confirmed by tandem mass spectrometry sequencing of purified cvSI-2. In addition, serine 43 and a threonine substitution at this position were observed. CvSI-2 amino acid sequence showed a 38% identity and 54% similarity with that of cvSI-1, the first protease inhibitor purified and characterized from a bivalve mollusc. Like cvSI-1, cvSI-2 gene was expressed in the basophil cells of digestive tubules. BLAST search found multiple ESTs from the eastern oyster, Pacific oyster, Mediterranean mussel, and sea vase, a tunicate, which could encode proteins with sequences similar to cvSI-1 and cvSI-2. Our findings indicate that cvSI-1 and cvSI-2 are members of a novel family of serine protease inhibitors in bivalve molluscs and perhaps other marine invertebrates, which share the characteristic cysteine array C-X 4–9-C-X 4–6-C-X 7-C-X 4-C-T-C-X 6–9-C-X 5-C-X 3–7-C-X 6–10-C-X 4-C-X-C.