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

Abstract

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 Sec61α. 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 Sec61α with mutant Sec61αY344H 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 Sec61α in the vicinity of tyrosine 344.