Molecular characterization and phylogenetic analysis of an anti-lipopolysaccharide factor from the crucifix crab, Charybdis feriatus

Abstract

Antimicrobial peptides are small cationic, gene-encoded, amphipathic, host defense peptides with a ubiquitous distribution in all living kingdoms. They are ,10 kDa in size, with 15-100 amino acids having a net positive charge of +2 to +9. Anti-lipopolysaccharide factor (ALF) is a cationic antimicrobial peptide which constitutes one of the key effector molecules in the innate immune system of crustaceans, and is capable of binding and neutralizing lipopolysaccharides. In the present study, an ALF homolog (Charybdis feriatus [Cf]-ALF1)-encoding cDNA sequence from the hemocytes of the crucifix crab, C. feriatus, was cloned, identified, and characterized. The deduced peptide of Cf-ALF1 encoded for a 123 amino acid peptide with a 97 residue mature peptide (11.16 kDa) that had a net charge of +10. Two conserved cysteine residues and a putative lipopolysaccharide binding domain were observed in the Cf-ALF1 mature peptide. BLAST analysis of Cf-ALF1 nucleotides showed a 99% similarity to Scylla serrata. The spatial structure of Cf-ALF1 was composed of three α-helices packed against a four-strand β-sheet. Two of these helices were linked by a disulfide bond to form an amphipathic loop similar to the structure of anti-lipopolysaccharide factor isoform 3 from Penaeus monodon (ALF-Pm3). All these features suggest that Cf-ALF1 could play a significant role in the innate immune defense mechanism of C. feriatus.