THE INTRACELLULAR PRODUCTION AND SECRETION OF HIV-1 ENVELOPE PROTEIN IN THE METHYLOTROPHIC YEAST PICHIA-PASTORIS

The human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein, gp120 (ENV), is required in large quantities for immunological studie s and as a potential vaccine component. We have expressed the DNA enco ding gp120 in a highly efficient expression system based on the methyl otrophic yeast, Pichia pastoris. The native gene was found to contain a sequence which resembled a Saccharomyces cerevisiae polyadenylation consensus and acted as a premature polyadenylation site in P. pastoris , resulting in the production of truncated mRNA. As full-length mRNA w as produced in S. cerevisiae, this indicates differences in mRNA 3'-en d formation between the two yeasts. Inactivation of this site by site- directed mutagenesis revealed several additional fortuitous polyadenyl ation sites within the gene. We have designed and constructed a 69%-sy nthetic gene with increased G+C content which overcomes this transcrip tional problem, giving rise to full-length mRNA. High levels of intrac ellular, insoluble, unglycosylated ENV were produced [1.25 mg/ml in hi gh-density (2 x 10(10) cells per mi) fermentations]. ENV also was secr eted from P. pastoris using the S. cerevisiae a-factor prepro secretio n leader and the S. cerevisiae invertase signal sequence. However, a h igh proportion of the secreted product was found to be hyperglycosylat ed, in contrast to other foreign proteins secreted from P. pastoris. T here also was substantial proteolytic degradation, but this was minimi zed by maintaining a low pH on induction. Insoluble, yeast-derived ENV proteins are being considered as vaccine antigens, and the P. pastori s system offers an efficient method of production.