PROSEGMENT OF YEAST ALPHA-FACTOR DIRECTS A HETEROLOGOUS PROTEIN (HUMAN GROWTH-HORMONE) TO THE CULTURE-MEDIUM OF SACCHAROMYCES-CEREVISIAE

The role of the prosegment of yeast pheromone (or-factor) in the expor t of the heterologous protein human growth hormone (hGH) into the cult ure medium of the yeast Saccharomyces cerevisiae has been studied. Wit h the help of certain genetic engineering constructions it was shown t hat different N-terminal signal peptides (SP) are unable to provide hG H export by themselves. Cells of the transformant that are carrying th e plasmid with a hybrid sequence encoding prosegment-hGH (without SP) accumulate non- glycosylated pro-hGH in the cytosol. Only the combinat ion of SP and prosegment as N-terminal fragment of hGH precursor resul ts in the processing and export of the mature hormone form. The origin (or type) of SP has no significance. Glycosylation inhibitors (2-deox y-D-glucose and tunicamycin) suppress export but do not prevent hGH fr om entering the periplasm; this suggests that intactness of the polyma nnose chains: of the prosegment is of critical importance for the effi cient export of heterologous protein. From these results, we conclude that the roles of the two preprosegment parts are different. The SP co nstituent of prepro-hGH introduces pro-hGH into the general secretory pathway, while the prosegment after SP cleavage directs hGH from the c ell into the culture medium.