Mutant invertase proteins accumulate in the yeast endoplasmic reticulum

Intercompartmental transport of secreted proteins in yeast was analysed using invertase mutants. Deletions and insertions at the BamHI (position + 787) or the Asp718 (position + 1159) sites of the SUC2 gene led to mutant proteins with different behaviour regarding secretion, localization and enzyme activity. The deletion mutants showed accumulation of core glycosylated material in the endoplasmic reticulum (ER) a decrease of secreted protein by 5%-30% and loss of enzyme activity. The secreted material was localized in the culture medium and not - as is normal for invertase - in the cell wall. No delay in transport from the Golgi to the cell surface was observed, indicating that the rate-limiting step for secretion is at the ER-Golgi stage. Two insertion mutants, pIPA and pIPB, retained enzyme activity. Mutant pIPB showed 10% secretion, while 60%-70% secretion was observed for pIPA. While the non-secreted material accumulated in the ER, the secreted material was present in the cell wall. The results suggest that the presence of structures incompatible with secretion leads to ER accumulation of mutated invertase.