Extracellular protein release and its response to pH level in Saccharomyces cerevisiae

Saccharomyces cerevisiae grown in batch culture at pH 5.5 releases 0.1 to 0.2 pg protein per cell to the external medium over a period of four to five days, final concentration 20-40 micrograms/ml. Cells grown at pH 3.0 release 10-fold this quantity (1-2 pg/cell, final concentration 100-200 micrograms/ml). A kinetic model based on published behavior of periplasmic protein gave a good fit to the observed kinetics of exoprotein yield. The electrophoretic pattern of exoprotein differed from that of cell lysate protein, and exoprotein synthesis was apparently limited to early stages of the life cycle. These results are consistent with the identification of exoprotein as periplasmic protein released to the external medium through the cell wall. Analysis of the observed kinetics of exoprotein yield, utilizing the kinetic model suggests that the greater exoprotein production of cells grown at pH 3.0 was due entirely to greater synthesis of periplasmic proteins while the fraction of periplasmic protein released per unit time was greater for cells grown at pH 5.5. The latter conclusion is supported by thicker cell walls of cells grown at pH 3.0 as observed by electron microscopy. At an applied level the apparent limitation of exoprotein synthesis to the first few hours of cell life, the slow leakage of exoprotein through the cell wall, and the dilute nature of a yeast suspension do not favor the utilization of yeast cells for direct conversion of substrate into protein released to the external medium.