Cloning and expression of recombinant shrimp PmRab7 (a virus-binding protein) in Pichia pastoris

White spot syndrome virus (WSSV) is one of the most serious pathogens in shrimp aquaculture. A shrimp WSSV-binding protein called PmRab7 has been isolated and characterized. Since injection of bacterial expressed-rPmRab7 could reduce shrimp mortality caused by WSSV from approximately 95% to 15% mortality, there was potential for its use in protection against WSSV in shrimp aquaculture. To test the feasibility of this, the Pichia pastoris yeast expression system was used for production of rPmRab7 since its expression system has eukaryote post-translational modification capability and since P. pastoris is widely accepted for use in human food or animal feed. Moreover, β-1,3-glucan, a major cell wall component of yeast, has been reported to act as an immunostimulant in shrimp. The recombinant protein was produced intracellularly and the resulting whole yeast cells were lyophilized and stored for supplementation in shrimp feed. The yield of rPmRab7 was 20-30 mg/l of culture medium or 2.67 mg/g yeast dry weight, which was 2-3 times higher than the yield obtained from an Escherichia coli expression system. A two-copy gene expression system was developed to enhance rPmRab7 expression using expression vector pAO815 containing a two-copy PmRab7 expression cassette constructed by site-directed mutagenesis of the PmRab7 gene and two-step overlap, extension PCR. This improved the yield of rPmRab7 2-3 times (40-60 mg/l of culture medium). ELISA was developed to show that the expressed rPmRab7 had WSSV-binding activity. Although some loss of rPmRab7 was found after lyophilization of the yeast cells, projected cost calculations indicated that this production level would make it feasible to use rPmRab7 in shrimp feed for protection against WSSV.