title: IRBIT, an inositol 1,4,5-trisphosphate receptor-binding protein, specifically binds to and activates pancreas-type Na+/HCO3- cotransporter 1 (pNBC1).
abstract: Inositol 1,4,5-trisphosphate (IP(3)) receptors (IP(3)Rs) are IP(3)-gated Ca(2+) channels that are located on intracellular Ca(2+) stores. We previously identified an IP(3)R binding protein, termed IP(3)R binding protein released with IP(3) (IRBIT). Because IRBIT is released from IP(3)R by physiological concentrations of IP(3), we hypothesized that IRBIT is a signaling molecule that is released from IP(3)R and regulates downstream target molecules in response to the production of IP(3). Therefore, in this study, we attempted to identify the target molecules of IRBIT, and we succeeded in identifying Na(+)/HCO(3)(-) cotransporter 1 (NBC1) as an IRBIT binding protein. Of the two major splicing variants of NBC1, pancreas-type NBC1 (pNBC1) and kidney-type NBC1 (kNBC1), IRBIT was found to bind specifically to pNBC1 and not to bind to kNBC1. IRBIT binds to the N-terminal pNBC1-specific domain, and its binding depends on the phosphorylation of multiple serine residues of IRBIT. Also, an electrophysiological analysis in Xenopus oocytes revealed that pNBC1 requires coexpression of IRBIT to manifest substantial activity comparable with that of kNBC1, which displays substantial activity independently of IRBIT. These results strongly suggest that pNBC1 is the target molecule of IRBIT and that IRBIT has an important role in pH regulation through pNBC1. Also, our findings raise the possibility that the regulation through IRBIT enables NBC1 variants to have different physiological roles.
authors.affiliation: Division of Neural Signal Information NTT-IMSUT, Institute of Medical Science, University of Tokyo, 4-6-1 Shirokanedai, Minato-ku, Tokyo 108-8639, Japan.
authors.forename: Kyoko
authors.initials: K
authors.lastname: Shirakabe
authors.forename: Giuseppina
authors.initials: G
authors.lastname: Priori
authors.forename: Hideomi
authors.initials: H
authors.lastname: Yamada
authors.forename: Hideaki
authors.initials: H
authors.lastname: Ando
authors.forename: Shoko
authors.initials: S
authors.lastname: Horita
authors.forename: Toshiro
authors.initials: T
authors.lastname: Fujita
authors.forename: Ichiro
authors.initials: I
authors.lastname: Fujimoto
authors.forename: Akihiro
authors.initials: A
authors.lastname: Mizutani
authors.forename: George
authors.initials: G
authors.lastname: Seki
authors.forename: Katsuhiko
authors.initials: K
authors.lastname: Mikoshiba
mesh_terms: D043383:Adenosylhomocysteinase / Q000737:chemistry / Q000378:metabolism*; D000595:Amino Acid Sequence; D000818:Animals; D002522:Chlorocebus aethiops; D015870:Gene Expression; D051379:Mice; D008969:Molecular Sequence Data; D009865:Oocytes; D010766:Phosphorylation; D011485:Protein Binding; D027982:Sodium-Bicarbonate Symporters / Q000235:genetics / Q000378:metabolism*; D014982:Xenopus laevis
chemical_list: C498899:Slc4a4 protein, mouse; D027982:Sodium-Bicarbonate Symporters; D043383:Adenosylhomocysteinase; C510392:IRBIT protein, mouse