[Expression of recombinant human zona pellucida-3 protein (rhZP3) in Pichia pastoris]

Human Zona Pellucida(ZP), which is a complex matrix surrounding oocytes,is comprised of three immunologically distinct glycoproteins(hZP1, hZP2 and hZP3). Because hZP3 possesses the sperm receptor activity and the acrosome-inducing activity, it has long been used as a candidate antigen to develop an immunocontraceptive vaccine. However, a large amount of native hZP3 protein is unavailable. It is an effective way to express hZP3 protein directly in vitro. Nevertheless, it had been reported that the rhZP3 protein produced in Pichia pastoris was not secreted but accumulated in the cells and could only be purified after being solubilized by strong denaturants. More unfortunately, after purification the final product required 6mol/L urea to maintain solubility. An improved project was advanced with the aim to express secreted and soluble rhZP3 protein in yeast. In this study, the fragment of hZP3 cDNA coding for aa 23 - 408, which the N-terminal leader was removed and most of the C-terminal transmembrane-like domain was reserved, was amplified by two PCR primers including EcoR I and Not I sites respectively and a His6 codon cassette was added to 5'-terminal. The hZP3 insert was incorporated into expression vector pPIC9K. The resulting recombinant yeast expression vector was designated pPIC9K-rhZP3. Linearized pPIC9K-rhZP3 was transformed into Pichia pastoris. After G418 selection, the recombinant Pichia pastoris strains were identified by PCR and the rhZP3 was expressed following the manufacturer' s protocol. Following induction with methanol, the rhZP3 protein was secreted and dissolved into the culture supernatant. SDS-PAGE and Western blot analyses showed that the apparent molecular weight of the expressed rhPZ3 proteins in yeast was smaller and a little size heterogeneity than native ones; after purified with Ni-chelating affinity chromatography, the final product's apparent molecular weight was about 32 - 34KD and their yield more than 20mg/L. We supposed that the C-terminal transmembrane-like domain be useful for secretion of rhZP3 into the culture supernatant and the expressed rhZP3 protein be incompletely digested by proteinases of Pichia into shorter fragments which all were glycosylated inhomogeneously. Fortunately, the fragments of rhZP3 protein can be recognized in Western blot by the polyclonal antibodies to porcine ZP3 which has showed a cross-reactivity with human ZP in vitro. It will be expected that the rhZP3 protein expressed in Pichia pastoris not only has immunogencity, say, it can rise antibodies in vivo to prevent spermatozoa-ovum binding, but also does not contain ovarian factors that might be the cause of undesired side effects, e.g. ovaritis and can be used as a safe immunogen in human antifertility vaccine research.