CONSTITUTIVE OVEREXPRESSION OF SECRETED HETEROLOGOUS PROTEINS DECREASES EXTRACTABLE BIP AND PROTEIN DISULFIDE-ISOMERASE LEVELS IN SACCHAROMYCES-CEREVISIAE

High-level gene expression does not always lead to corresponding high- level secretion of heterologous proteins in yeast. The rate-limiting s tep in many cases has been shown to exit from the endoplasmic reticulu m (ER)I Within the ER, the correct folding of secreted proteins is req uired for export competence; hence, the cellular proteins involved in these events are likely to be important for efficient secretion. We ha ve found that the extractable levels of two ER-resident proteins invol ved in folding-heavy chain binding protein (BiP) and protein disulfide isomerase (PDI)-are significantly reduced by prolonged constitutive o verexpression of human granulocyte colony stimulating factor (GCSF), h uman erythropoietin, or Schizosaccharomyces pombe acid phosphatase. Ho wever, the rate of BiP synthesis measured in pulse-chase radiolabeling experiments is not reduced by GCSF overexpression, and galactose-dire cted transcription of the BiP gene does not restore normal BiP protein levels once they have been depleted. The observed loss of lumenal res ident proteins, either by proteolysis or irreversible aggregation, is expected to contribute significantly to the inefficiency of foreign pr otein secretion in yeast.