Antimicrobial peptides are humoral innate immune components of molluscs that provide protection against pathogenic microorganisms. Among these, histone-H2A-derived antimicrobial peptides are known to actively participate in host defense responses of molluscs. Present study deals with identification of putative antimicrobial sequences from the histone-H2A of back-water oyster Crassostrea madrasensis, rock oyster Saccostrea cucullata, grey clam Meretrix casta, fig shell Ficus gracilis, and ribbon bullia Bullia vittata. A 75？bp fragment encoding 25 amino acid residues was amplified from cDNA of these five bivalves and was named “Molluskin.” The 25 amino acid peptide exhibited high similarity to previously reported histone-H2A-derived AMPs from invertebrates indicating the presence of an antimicrobial sequence motif. Physicochemical properties of the peptides are in agreement with the characteristic features of antimicrobial peptides, indicating their potential role in innate immunity of molluscs. 1. Introduction Invertebrates mainly rely on their innate immune defenses to battle a variety of invading microbial pathogens. Among these molecules the antimicrobial peptides (AMPs) play an important role in the humoral innate immune mechanism of invertebrates. AMPs have received increasing attention in recent years as their contribution to host defense mechanisms and their potential as new pharmaceutical substances is becoming increasingly appreciated. This is mainly because of the broad spectrum activity of AMPs and the rapid development of microbial resistance to conventional antibiotics [1]. AMPs derived from histone proteins form an important category of peptide antibiotics [2]. Histone-derived antimicrobial peptides with potent activity have been isolated and reported from various organisms [3–11]. In the case of marine invertebrates histone-H2A-derived AMPs have been reported from Pacific white shrimp Litopenaeus vannamei [8], scallop Chlamys farreri [9], and abalone Haliotis discus discus [11]. From fishes histone-derived antimicrobial peptides have been reported from catfish Parasilurus asotus [4], Atlantic salmon Salmo salar [5], Atlantic halibut Hippoglossus hippoglossus [6], rainbow trout Oncorhynchus mykiss [7], and round whip ray Himantura pastinacoides [12]. Present study was carried out to identify histone-derived antimicrobial peptides from molluscs, namely, back water oyster Crassostrea madrasensis, rock oyster Saccostrea cucullata, grey clam Meretrix casta, fig shell Ficus gracilis, and ribbon bullia Bullia vittata. 2. Materials and Methods
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