title: Bax Inhibitor-1-mediated Ca2+ leak is decreased by cytosolic acidosis.
abstract: 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.
authors.affiliation: Laboratory of Molecular and Cellular Signaling, Department of Cellular and Molecular Medicine, KU Leuven, Campus Gasthuisberg O/N-I, Herestraat 49--box 802, BE-3000 Leuven, Belgium.
authors.forename: Santeri
authors.initials: S
authors.lastname: Kiviluoto
authors.forename: Tomas
authors.initials: T
authors.lastname: Luyten
authors.forename: Lars
authors.initials: L
authors.lastname: Schneider
authors.forename: Dmitrij
authors.initials: D
authors.lastname: Lisak
authors.forename: Diego
authors.initials: D
authors.lastname: Rojas-Rivera
authors.forename: Kirsten
authors.initials: K
authors.lastname: Welkenhuyzen
authors.forename: Ludwig
authors.initials: L
authors.lastname: Missaen
authors.forename: Humbert
authors.initials: H
authors.lastname: De Smedt
authors.forename: Jan B
authors.initials: JB
authors.lastname: Parys
authors.forename: Claudio
authors.initials: C
authors.lastname: Hetz
authors.forename: Axel
authors.initials: A
authors.lastname: Methner
authors.forename: Geert
authors.initials: G
authors.lastname: Bultynck
mesh_terms: D000138:Acidosis / Q000378:metabolism / Q000473:pathology; D000818:Animals; D017209:Apoptosis; D051017:Apoptosis Regulatory Proteins / Q000172:deficiency / Q000235:genetics / Q000378:metabolism*; D000001:Calcimycin / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D002460:Cell Line; D004721:Endoplasmic Reticulum / Q000187:drug effects / Q000378:metabolism; D006367:HeLa Cells; D006801:Humans; D006863:Hydrogen-Ion Concentration; D008565:Membrane Proteins / Q000172:deficiency / Q000235:genetics / Q000378:metabolism*; D051379:Mice; D010455:Peptides / Q000494:pharmacology; D017434:Protein Structure, Tertiary
chemical_list: D051017:Apoptosis Regulatory Proteins; D008565:Membrane Proteins; D010455:Peptides; C484209:TMBIM6 protein, human; D000001:Calcimycin; D002118:Calcium
keywords: Acidification; Bax Inhibitor-1; Ca(2+) signaling; Endoplasmic reticulum