title: BI-1 regulates endoplasmic reticulum Ca2+ homeostasis downstream of Bcl-2 family proteins.
abstract: BI-1 (Bax inhibitor-1) is an evolutionarily conserved multitransmembrane protein that resides in the endoplasmic reticulum (ER) and that has documented cytoprotective functions in both animals and plants. Recent studies indicate that BI-1 shares in common with Bcl-2/Bax family proteins the ability to regulate the amounts of Ca(2+) that can be released from the ER by agents, such as the ER-Ca(2+)-ATPase (SERCA) inhibitor thapsigargin (TG). Using an ER-targeted, Ca(2+) indicator (cameleon), with characteristics optimized for measuring ER Ca(2+) ([Ca(2+)](er)), we studied the effects of BI-1 on [Ca(2+)](er) in resting and TG-treated cells. Similar to cells overexpressing antiapoptotic Bcl-2 or Bcl-X(L), overexpression of BI-1 resulted in lower resting [Ca(2+)](er), with concomitantly less Ca(2+) released into the cytosol upon stimulation by TG and with a higher rate of Ca(2+) leakage from the ER. Co-expression of SERCA restored levels of [Ca(2+)](er) to normal, showing opposing actions of the ER-Ca(2+)ATPase and BI-1 on ER Ca(2+) homeostasis. Conversely, cells with deficient BI-1 have increased [Ca(2+)](er), and release more Ca(2+) into the cytosol when challenged with TG. In BI-1-deficient cells, Bcl-X(L) fails to reduce [Ca(2+)](er), indicating that BI-1 functions downstream of Bcl-X(L). In bax(-/-)bak(-/-) double knock-out cells, both BI-1 and Bcl-X(L) retained their ability to reduce [Ca(2+)](er), suggesting that BI-1 and Bcl-X(L) operate downstream of or parallel to Bax/Bak. The findings reveal a hierarchy of functional interactions of BI-1 with Bcl-2/Bax family proteins in regulating ER Ca(2+) homeostasis.
authors.affiliation: Program on Apoptosis and Cell Death Research, Burnham Institute for Medical Research, La Jolla, California 92037, USA.
authors.forename: Chunyan
authors.initials: C
authors.lastname: Xu
authors.forename: Wenjie
authors.initials: W
authors.lastname: Xu
authors.forename: Amy E
authors.initials: AE
authors.lastname: Palmer
authors.forename: John C
authors.initials: JC
authors.lastname: Reed
mesh_terms: D000818:Animals; D017209:Apoptosis; D051017:Apoptosis Regulatory Proteins / Q000096:biosynthesis* / Q000502:physiology*; D002118:Calcium / Q000378:metabolism*; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D005347:Fibroblasts / Q000378:metabolism; D005786:Gene Expression Regulation; D006367:HeLa Cells; D006801:Humans; D008565:Membrane Proteins / Q000096:biosynthesis* / Q000502:physiology*; D051379:Mice; D019253:Proto-Oncogene Proteins c-bcl-2 / Q000378:metabolism*; D050998:bcl-2 Homologous Antagonist-Killer Protein / Q000378:metabolism*; D051028:bcl-2-Associated X Protein / Q000378:metabolism*; D051020:bcl-X Protein / Q000378:metabolism
chemical_list: D051017:Apoptosis Regulatory Proteins; D008565:Membrane Proteins; D019253:Proto-Oncogene Proteins c-bcl-2; C484209:TMBIM6 protein, human; C490015:Tmbim6 protein, mouse; D050998:bcl-2 Homologous Antagonist-Killer Protein; D051028:bcl-2-Associated X Protein; D051020:bcl-X Protein; D002118:Calcium