Secretion of foreign proteins from Saccharomyces cerevisiae directed by alpha-factor gene fusions

Fusions between the cloned yeast α -factor structural gene and chemically synthesized DNA segments encoding human protein analogs have been constructed. The gene fusions encode hybrid proteins that include the first 89 amino acids of the native α -factor precursor fused to either a small (β -endorphin, 31 amino acids) or large (α -interferon, 166 amino acids) foreign protein. Proteolytic cleavage sites involved in α -factor maturation from the native precursor immediately precede the foreign peptide in the hybrid protein. The α -factor promoter was utilized to express the gene fusions in Saccharomyces cerevisiae and resulted in the efficient secretion of the foreign proteins into the culture medium. The processing of the hybrid proteins has been characterized by amino acid sequence analysis of the secreted proteins. The proteolytic cleavages involved in the maturation of α -factor peptides from the native precursor also occur accurately in the hybrid protein. In addition, cleavages occurred on the carboxyl side of two lysines within the β -endorphin peptide. Internal cleavages in the interferon protein were also detected. However, in this case, the cleavages occurred at a very low frequency such that >95% of the secreted interferon remained intact.