Expression of env sequences of the bovine leukemia virus (BLV) in the yeast Saccharomyces cerevisiae

Expression of the avian gag-myc oncogene in Saccharomyces cerevisiae

The gag-myc oncogenic sequence of the avian retrovirus MC29 was first inserted in a multicopy expression vector allowing its expression in Saccharomyces cerevisiae. The oncogene transcripts were detected in yeast by Northern blot hybridization and gag-myc proteins were revealed by immunoprecipitation. On solid medium, the average size of gag-myc transformant colonies was smaller than control. In liquid cultures, the gag-myc strains had a doubling time of 4.7 h compared with 3.1 h in the controls. In one of the recipient strains, and after an initial transient period of 5 days, the gag-myc transformants became physiologically indistinguishable from control. In another recipient strain, the slow-growth phenotype is permanent. Plasmid instability is increased in gag-myc transformants. When a single copy of the gag-myc gene was inserted in a yeast chromosome, no phenotype was observed, showing that slow growth is the consequence of plasmid loss.

Biochemical and immunological characterization of the bovine leukemia virus (BLV) envelope glycoprotein (gp51) produced in Saccharomyces cerevisiae

The nucleotide sequence coding for bovine leukemia virus (BLV) envelope glycoprotein gp51 was inserted into a yeast-Escherichia coli shuttle vector carrying the promoter and secretion signal sequence of PHO5 (the yeast gene coding for repressible acid phosphatase) and the CYC1 transcriptional terminator. Yeast cells transformed by this construction synthesized gp51 after PHO5 induction by inorganic phosphate deprivation. The yeast-expressed gp51 was partially glycosylated into heterodisperse protein molecules ranging from 40 to 48 kDa. No gp51 was excreted in the culture medium. The amount of protein accumulated in yeast cells was estimated to reach 0.06% of soluble proteins. This modest level of expression seemed to be due to the toxicity of gp51 to the yeast cell. The yeast-expressed gp51 products were used in enzyme-linked immunosorbent assays for the detection of antibodies in sera from BLV-infected animals; they were also screened for the presence of well-defined biological epitopes. In both studies poor reactivity was observed. Rabbits immunized with the recombinant gp51 showed high antibody titers to native BLV gp51. However, these antibodies did not neutralize BLV in vitro.

Expression of proteins encoded by foreign genes in Saccharomyces cerevisiae

The yeast, Saccharomyces cerevisiae is currently used for the production of recombinant DNA-generated proteins derived from a variety of eukaryotic organisms. The applications of a yeast-based technology in the production of proteins for pharmaceutical and industrial purposes is discussed including current methods for introducing recombinant genes into yeast and strategies for maximizing their expression.