Abstract: Mytilins are small cationic antibacterial peptides expressed predominantly in the haemocytes of shellfish,including multiple isoforms,and they play an important role in the innate immune system of shellfish. Mytilin-1 is one of the isoforms. The primary structure of Mytilus coruscus mytilin-1 is composed of signal peptide,mature peptide and C-terminal extension peptide. The mytilin-1’s mature peptide consisting of 34 amino acid residues determines its biological activity;8 conserved cysteine residues in the mature peptide may form 4 pairs of disulfide bond to stabilize the structure of mytilin-1 mature peptide. In the present study,having mytilin-1 mature peptide from M. coruscus as the recombinant DNA-expressed protein,a codon-optimized sequence（named as “mMy1”）corresponding to the cDNA encoding mytilin-1 mature peptide was synthesized based on the preferential codon usage of Pichia pastoris,and ligated to the expression vector pPICZαA and electronically transformed into competent P. pastoris X-33. The positive transformants containing multi-copy gene insertions were screened using high-concentration zeocin. The recombinant mMy1 was induced for 96 h with 1.0% methanol at 29℃,250 r/min and pH6.0. Tricine-SDS-PAGE and Western blot analysis demonstrated that the recombinant mMy1 with a molecular mass of 4.8 kD was expressed successfully in P. pastoris X-33. Bacteriostatic test indicated that the culture medium supernatant containing recombinant mMy1 showed antibacterial activity against Gram-positive（i.e.,Staphylococcus aureus and Bacillus subtilis）and Gram-negative（i.e.,Escherchia coli）bacteria.

Key words: Mytilus coruscus, mytilin-1 mature peptide, Pichia pastoris, recombinant expression, antibacterialactivity