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Increased expression of recombinant chitosanase by co-expression of Hac1p in the yeast Pichia pastoris.

BACKGROUND
Pichia pastoris is one of the most popular eukaryotic hosts for producing heterologous proteins, while increasing secretion of target proteins is still a top priority for their application in industrial fields. Recently, the research effort to enhance protein production therein has focused on up-regulating the unfolded protein response (UPR).


OBJECTIVE
We evaluated the effects of activated UPR via Hac1p co-expression with the promoter AOX1 (PAOX1) or GAP (PGAP) on expression of recombinant chitosanase (rCBS) in P. pastoris.


METHOD
The DNA sequence encoding the chitosanase was chemically synthesized and cloned into pPICZαA and the resulted pPICZαA/rCBS was transformed into P. pastoris for expressing rCBS. The P. pastoris HAC1i cDNA was chemically synthesized and cloned into pPIC3.5K to give pPIC3.5K/Hac1p. The HAC1i cDNA was cloned into pGAPZB and then inserted with HIS4 gene from pAO815 to construct the vector pGAPZB/Hac1p/HIS4. For co-expression of Hac1p, the two plasmids pPIC3.5K/Hac1p and pGAPZB/Hac1p/HIS4 were transformed into P. pastoris harboring the CBS gene. The rCBS was assessed based on chitosanase activity and analyzed by SDS-PAGE. The enhanced Kar2p was detected with western blotting to evaluate UPR.


RESULTS
Hac1p co-expression with PAOX1 enhanced rCBS secretion by 41% at 28°C. Although the level of UPR resulted from Hac1p co-expression with PAOX1 was equivalent to that with PGAP in terms of the quantity of Kar2p (a hallmark of the UPR), substitution of PGAP for PAOX1 further increased rCBS production by 21%. The methanol-utilizing phenotype of P. pastoris did not affect rCBS secretion with co-expression of Hac1p or not. Finally, Hac1p co-expression with PAOX1 or PGAP promoted rCBS secretion from 22 to 30°C and raised the optimum induction temperature.


CONCLUSION
The study indicated that Hac1p co-expression with PAOX1 or PGAP is an effective strategy to trigger UPR of P. pastoris and a feasible means for improving production of rCBS therein.