Enhancement of protein secretion in Saccharomyces cerevisiae by overproduction of Sso protein, a late-acting component of the secretory machinery

Increased production of secreted proteins in Saccharomyces cerevisiae was achieved by overexpressing the yeast syntaxins, Sso1 or Sso2 protein, the t‐SNAREs functioning at the targeting/fusion of the Golgi‐derived secretory vesicles to the plasma membrane. Up to four‐ or six‐fold yields of a heterologous secreted protein, Bacillus α‐amylase, or an endogenous secreted protein, invertase, were obtained respectively when expressing either one of the SSO genes, SSO1 or SSO2, from the ADH1 promoter on a multicopy plasmid. Direct correlation between the Sso protein level and the amount of secreted α‐amylase was demonstrated by modulating the expression level of the SSO2 gene. Quantitation of the α‐amylase activity in the culture medium, periplasmic space and cytoplasm suggests that secretion into the periplasmic space is the primary stage at which the SSO genes exert the secretion‐enhancing function. Pulse‐chase data also support enhanced secretion efficiency obtained by SSO overexpression. Our data suggest that the Sso proteins may be rate‐limiting components of the protein secretion machinery at the exocytosis step in yeast.