Bax Inhibitor-1 Is a pH-dependent regulator of Ca2+ channel activity in the endoplasmic reticulum.
In this study, Bax inhibitor-1 (BI-1) overexpression reduces the ER pool of Ca(2+) released by thapsigargin. Cells overexpressing BI-1 also showed lower intracellular Ca(2+) release induced by the Ca(2+) ionophore ionomycin as well as agonists of ryanodine receptors and inositol trisphosphate receptors. In contrast, cells expressing carboxyl-terminal deleted BI-1 (CDelta-BI-1 cells) displayed normal intracellular Ca(2+) mobilization. Basal Ca(2+) release rates from the ER were higher in BI-1-overexpressing cells than in control or CDelta-BI-1 cells. We determined that the carboxyl-terminal cytosolic region of BI-1 contains a lysine-rich motif (EKDKKKEKK) resembling the pH-sensing domains of ion channels. Acidic conditions triggered more extensive Ca(2+) release from ER microsomes from BI-1-overexpressing cells and BI-1-reconstituted liposomes. Acidic conditions also induced BI-1 protein oligomerization. Interestingly subjecting BI-1-overexpressing cells to acidic conditions induced more Bax recruitment to mitochondria, more cytochrome c release from mitochondria, and more cell death. These findings suggest that BI-1 increases Ca(2+) leak rates from the ER through a mechanism that is dependent on pH and on the carboxyl-terminal cytosolic region of the BI-1 protein. The findings also reveal a cell death-promoting phenotype for BI-1 that is manifested under low pH conditions.
Kim
Hyung-Ryong
HR
Department of Dental Pharmacology and Wonkwang Biomaterial Implant Research Institute, School of Dentistry, Wonkwang University, Iksan, Chonbuk 570-749, Republic of Korea.
Lee
Geum-Hwa
GH
Ha
Ki-Chan
KC
Ahn
Taeho
T
Moon
Ji-Yong
JY
Lee
Bong-Jin
BJ
Cho
Ssang-Goo
SG
Kim
Sanguk
S
Seo
Young-Rok
YR
Shin
Yong-Joo
YJ
Chae
Soo-Wan
SW
Reed
John C
JC
Chae
Han-Jung
HJ
D020816:Amino Acid Motifs / Q000502:physiology; D000818:Animals; D051017:Apoptosis Regulatory Proteins / Q000096:biosynthesis* / Q000235:genetics; D002118:Calcium / Q000378:metabolism*; D002417:Cattle; D045304:Cytochromes c / Q000235:genetics / Q000378:metabolism; D004721:Endoplasmic Reticulum / Q000235:genetics / Q000378:metabolism*; D004791:Enzyme Inhibitors / Q000494:pharmacology; D015870:Gene Expression; D006367:HeLa Cells; D006801:Humans; D006863:Hydrogen-Ion Concentration; D053496:Inositol 1,4,5-Trisphosphate Receptors / Q000235:genetics / Q000378:metabolism*; D008565:Membrane Proteins / Q000096:biosynthesis* / Q000235:genetics; D008861:Microsomes / Q000378:metabolism; D008928:Mitochondria / Q000235:genetics / Q000378:metabolism; D017434:Protein Structure, Tertiary / Q000502:physiology; D019837:Ryanodine Receptor Calcium Release Channel / Q000235:genetics / Q000378:metabolism*; D019284:Thapsigargin / Q000494:pharmacology
D051017:Apoptosis Regulatory Proteins; D004791:Enzyme Inhibitors; D053496:Inositol 1,4,5-Trisphosphate Receptors; D008565:Membrane Proteins; D019837:Ryanodine Receptor Calcium Release Channel; C484209:TMBIM6 protein, human; D019284:Thapsigargin; D045304:Cytochromes c; D002118:Calcium