BiP-mediated closing of the Sec61 channel limits Ca2+ leakage from the ER

In mammalian cells, signal peptide-dependent protein transport into the endoplasmic reticulum (ER) is mediated by a dynamic protein-conducting channel, the Sec61 complex. Previous work has characterized the Sec61 channel as a potential ER Ca2+ leak channel and identified calmodulin as limiting Ca2+ leakage in a Ca2+-dependent manner by binding to an IQ motif in the cytosolic aminoterminus of Sec61 alpha. Here, we manipulated the concentration of the ER lumenal chaperone BiP in cells in different ways and used live cell Ca2+ imaging to monitor the effects of reduced levels of BiP on ER Ca2+ leakage. Regardless of how the BiP concentration was lowered, the absence of available BiP led to increased Ca2+ leakage via the Sec61 complex. When we replaced wild-type Sec61 alpha with mutant Sec61 alpha Y344H in the same model cell, however, Ca2+ leakage from the ER increased and was no longer affected by manipulation of the BiP concentration. Thus, BiP limits ER Ca2+ leakage through the Sec61 complex by binding to the ER lumenal loop 7 of Sec61 alpha in the vicinity of tyrosine 344. The EMBO Journal (2012) 31, 3282-3296. doi:10.1038/emboj.2012.189; Published online 13 July 2012