Mytichitin-A is an antimicrobial peptide isolated from the serum of Mytilus coruscus and is reported to inhibit bacterial growth as tested on several Gram-positive bacteria. To produce large quantity of Mytichitin-A to further investigate its biological activity, nucleotide sequence encoding a recombinant 6 x His-Mytichitin-A (rMytichitin-A) peptide was synthesized and inserted into the inducible yeast expression vector pPICZ alpha A. With the availability of such an expression vector called pPICZ alpha A-MytichitinA, we transformed Pichia pastoris GS115 cells with a SacI-linearized pPICZ alpha A-Mytichitin-A by electroporation. Transgenic strains secreting rMytichitin-A with a molecular weight of approximate 10 KDa as expected were obtained. The optimal culture condition for rMytichitin-A expression was determined to be 1.0% methanol induction, 96 h incubation at 28 degrees C and the amount of rMytichitin-A reached 45.5 mu g/ml. The percentage of rMytichitin-A was estimated to be 73.6% of the total protein. After rMytichitin-A was purified using nickel ions affinity chromatography, approximate 9.1 mg pure rMytichitin-A was obtained from 500 ml of cell culture medium with 97.8% purity. More importantly, both the culture supernatant and purified rMytichitin-A inhibited the growth of Gram-positive bacteria, especially Staphylococcus aureus and Bacillus subtilis with a minimum inhibition concentration of as low as 31 and 48 g/ml, respectively. Differently from the native protein, however, the rMytichitin-A is not active against Gram-negative bacteria. Taken together, this is the first report on the heterologous expression of Mytichitin-A in P. pastoris. Our study showed that P. pastoris is an effective expression system for producing large quantities of biologically active Mytichitin-A for both research and application purposes. (C) 2016 Elsevier Inc. All rights reserved.