To investigate the effect of sec61 mutants on protein homeostasis in the ER directly, we asked whether sec61L7 or sec61Y345H elicited the UPR. We trans most formed wildtype and mutant strains with a plasmid in which LacZ was expressed under control of a UPR elem ent, or without the UPRE as negative control, lysed the cells, and analyzed beta galactosidase activity. As shown in Figure 2C, sec61L7 elicited a very strong UPR, which was almost as strong as the UPR caused by tunicamycin treatment of wildtype cells. UPR induction in sec61L7 was substantially stronger than in sec61 3 expressing cells, although this mutation had been identified in a screen for UPR inducing sec61 mu tants. UPR induction in sec61Y345H cells was modest, but there was a significant difference between cells expressing UPRE LacZ and the control plasmid without the UPRE.

We conclude that L7 of Sec61p is important for maintenance of ER protein homeostasis. The ER is a repository for Ca2 which is an essential co factor for chaperones in the ER lumen. In mam malian cells the Sec61 channel is responsible for a Ca2 leak from the ER, and sec61Y344H leads to defects in ER Ca2 homeostasis. Therefore we investigated whether in yeast sec61L7 or sec61Y345H were defective in Ca2 sealing of the ER by analysing their growth in the presence of the Ca2 chelator EGTA. We detected no effect on growth of either mutant on EGTA, while growth of a strain deleted for the Ca2 pump Pmr1p was inhibited by 5 mM EGTA. We conclude that in yeast neither sec61Y345H nor sec61L7 cause gross defects in Ca2 sealing of the ER.

Deletion of L7 affects soluble protein import into the ER L7 is important for Sec61 channel function in protein transport across the ER membrane. We therefore asked whether we were able to detect secretory precursors in lysates of sec61L7 cells. Soluble prepro alpha factor is posttranslationally trans ported across the ER membrane and highly sensitive for defects in translocation. We analysed the accumulation of ppF in sec61L7 cells after incubation at 37 C, 30 C and 20 C for 3 h compared to SEC61, and sec61 32 yeast which are cold sensitive and defective in protein import into the ER. Cytosolic accumulation of ppF was increased in sec61L7 cells compared to wildtype at all temperatures, and similar to the accumulation in sec61 32 mutants. In contrast, cotranslational ER membrane integration of DPAPB was barely affected in sec61L7 cells. We next asked whether expression levels of the Sec61p homolog Ssh1p were altered in sec61L7 cells. Ssh1p forms a heterotri meric complex with Sbh2p and Sss1p which mediates ex clusively cotranslational import into the ER, and elevation of Ssh1p expression may Anacetrapib therefore be able to compensate a cotranslational import defect in sec61L7 cells.