Programmed cell-death involves a set of Bcl-2 family proteins, some of which inhibit apoptosis (Bcl-2 and Bcl-XL) and some of which promote it (Bax and Bak). Human Bax inhibitor, BI-1, is an evolutionarily conserved integral membrane protein containing multiple membrane-spanning segments predominantly localised to intracellular membranes. It has 6-7 membrane-spanning domains. The C termini of the mammalian BI-1 proteins are comprised of basic amino acids resembling some nuclear targeting sequences, but otherwise the predicted proteins lack motifs that suggest a function. As plant BI-1 appears to localise predominantly to the ER, we hypothesized that plant BI-1 could also regulate cell death triggered by ER stress [2]. BI-1 appears to exert its effect through an interaction with calmodulin [3]. The budding yeast member of this family has been found unexpectedly to encode a BH3 domain-containing protein (Ybh3p) that regulates the mitochondrial pathway of apoptosis in a phylogenetically conserved manner [4]. Examination of the crystal structure of a bacterial member of this family shows that these proteins mediate a calcium leak across the membrane that is pH-dependent. Calcium homoeostasis balances passive calcium leak with active calcium uptake. The structure exists in a pore-closed and pore-open conformation, at pHs of 8 and 6 respectively, and the pore can be opened by intracrystalline transition; together these findings suggest that pH controls the conformational transition [5].

1: Dissection of Arabidopsis Bax inhibitor-1 suppressing Bax-, hydrogen peroxide-, and salicylic acid-induced cell death. Kawai-Yamada M, Ohori Y, Uchimiya H; Plant Cell. 2004;16:21-32. PMID:14671021

2: Arabidopsis Bax inhibitor-1: A rheostat for ER stress-induced programmed cell death. Watanabe N, Lam E; Plant Signal Behav. 2008;3:564-566. PMID:19704470

3: Bax inhibitor-1, a conserved cell death suppressor, is a key molecular switch downstream from a variety of biotic and abiotic stress signals in plants. Watanabe N, Lam E; Int J Mol Sci. 2009;10:3149-3167. PMID:19742129

4: A yeast BH3-only protein mediates the mitochondrial pathway of apoptosis. Buttner S, Ruli D, Vogtle FN, Galluzzi L, Moitzi B, Eisenberg T, Kepp O, Habernig L, Carmona-Gutierrez D, Rockenfeller P, Laun P, Breitenbach M, Khoury C, Frohlich KU, Rechberger G, Meisinger C, Kroemer G, Madeo F; EMBO J. 2011;30:2779-2792. PMID:21673659

5: Structural basis for a pH-sensitive calcium leak across membranes. Chang Y, Bruni R, Kloss B, Assur Z, Kloppmann E, Rost B, Hendrickson WA, Liu Q; Science. 2014;344:1131-1135. PMID:24904158