Bax Inhibitor-1-mediated Ca2+ leak is decreased by cytosolic acidosis

Bax Inhibitor-1 (BI-1) is an evolutionarily conserved six-transmembrane domain endoplasmic reticulum (ER)-localized protein that protects against ER stress-induced apoptotic cell death. This function is closely connected to its ability to lower steady-state ER Ca2+ levels. Recently, we elucidated BI-1's Ca(2+)-channel pore in the C-terminal part of the protein and identified the critical amino acids of its pore. Based on these insights, a Ca(2+)-channel pore-dead mutant BI-1 (BI-1(D213R)) was developed. We determined whether BI-1 behaves as a bona fide H+/Ca2+ antiporter or as an ER Ca(2+)-leak channel by investigating the effect of pH on unidirectional Ca(2+)-efflux rates. At pH 6.8, wild-type BI-1 expression in BI-1(-/-) cells increased the ER Ca(2+)-leak rate, correlating with its localization in the ER compartment. In contrast, BI-1(D231R) expression in BI-1(-/-), despite its ER localization, did not increase the ER Ca(2+)-leak rate. However, at pH < 6.8, the BI-1-mediated ER Ca2+ leak was blocked. Finally, a peptide representing the Ca(2+)-channel pore of BI-1 promoting Ca2+ flux from the ER was used. Lowering the pH from 6.8 to 6.0 completely abolished the ability of the BI-1 peptide to mediate Ca2+ flux from the ER. We propose that this pH dependence is due to two aspartic acid residues critical for the function of the Ca(2+)-channel pore and located in the ER membrane-dipping domain, which facilitates the protonation of these residues.