Unfolded protein response-induced Bip/Kar2p production protects cell growth against accumulation of misfolded protein aggregates in the yeast endoplasmic reticulum

Overproduction of Delta pro, a mutated secretory proteinase derived from a filamentous fungus Rhizopus niveus, results in formation of gross aggregates (Delta pro aggregates) in the yeast endoplasmic reticulum (ER) lumen, activation of the unfolded protein response (UPR) and ER membrane proliferation. To investigate the roles of the UPR against the Delta pro aggregates, we constructed an IRE1-deleted (Delta ire1) strain. In contrast to wild-type cells, Delta ire1 cells ceased to grow several hours after the overproduction of Delta pro. Two lines of evidence argued against the possibility that the growth defect was due to the inability to make extra ER membrane which accommodates the Delta pro aggregates. First, by electron microscopy, ER membrane proliferation was observed in Delta ire1 cells overproducing Delta pro. Second, disruption of the OPI1 gene in the Delta ire1 mutant, which is considered to derepress the activities of phospholipid-synthesizing enzymes, did not restore the growth upon the overproduction of Delta pro. Instead, the growth was restored when an extra copy of the KAR2 gene, which encodes yeast BiP, was introduced, indicating that an increase in the amount of BiP is essential for cell growth when the Delta pro aggregates accumulate in the ER. Since BiP is included in the insoluble Delta pro aggregates, it is likely that the amount of free BiP in the ER lumen is insufficient without the UPR to fully exert its functions. Consistently, overproduction of Delta pro impaired protein translocation and folding in Delta ire1 cells but not in wild-type cells. The tunicamycin sensitivity of Delta ire1 cells was also suppressed by extra expression of KAR2, suggesting that BiP plays a principal role in protecting cell growth against misfolded proteins accumulated in the ER.