The yield of heterologous
proteins is often limited by several bottlenecks in the secretory pathway of yeast Saccharomyces cerevisiae. It was shown earlier that synthesis of measles virus
hemagglutinin (MeH) is inefficient mostly due to a bottleneck in the translocation of
viral protein precursors into the endoplasmic reticulum (ER) of yeast cells. Here we report that heat shock with subsequent induction of MeH expression at 37°C improved translocation of MeH precursors when applied at higher cell densities. The amount of MeH
glycoprotein increased by about 3-fold after heat shock in the late-log phases of both
glucose and
ethanol growth. The same temperature conditions increased both secretion titer and yield of another heterologous
protein human
GRP78/BiP by about 50%. Furthermore, heat shock at the late-log
glucose growth phase also improved endogenous
invertase yield by approximately 2.7-fold. In contrast, a transfer of yeast culture to lower temperature at diauxic shift followed by
protein expression at 20°C almost totally inhibited translocation of MeH precursors. The difference in amounts of MeH
glycoprotein under expression at 37°C and 20°C was about 80-fold, while amounts of unglycosylated MeH
polypeptides were similar under both conditions. Comparative proteomic analysis revealed that besides over-expressed ER-resident chaperone Kar2, an increased expression of several cytosolic
proteins (such as Hsp104, Hsp90 and eEF1A) may contribute to improved translocation of MeH.