title: IRBIT, inositol 1,4,5-triphosphate (IP3) receptor-binding protein released with IP3, binds Na+/H+ exchanger NHE3 and activates NHE3 activity in response to calcium.
abstract: Calcium (Ca2+) is a highly versatile second messenger that regulates various cellular processes. Previous studies showed that elevation of intracellular Ca2+ regulates the activity of Na+/H+ exchanger 3 (NHE3). However, the effect of Ca2+-dependent signaling on NHE3 activity varies depending on cell types. In this study, we report the identification of IP3 receptor-binding protein released with IP3 (IRBIT) as a NHE3 interacting protein and its role in regulation of NHE3 activity. IRBIT bound to the carboxyl-terminal domain of NHE3, which is necessary for acute regulation of NHE3. Ectopic expression of IRBIT resulted in Ca2+-dependent activation of NHE3 activity, whereas silencing of endogenous IRBIT resulted in inhibition of NHE3 activity. Ca2+-dependent stimulation of NHE3 activity was dependent on the binding of IRBIT to NHE3. Previously Ca2+-dependent inhibition of NHE3 was demonstrated in the presence of NHERF2. Co-expression of IRBIT was able to reverse the NHERF2-dependent inhibition of NHE3. We also showed that IRBIT-dependent activation of NHE3 involves exocytic trafficking of NHE3 to the plasma membrane and this activation was blocked by inhibition of calmodulin (CaM) or CaM-dependent kinase II. These results suggest that the overall effect of Ca2+ on NHE3 activity is balanced by IRBIT-dependent activation and NHERF2-dependent inhibition.
authors.affiliation: Division of Digestive Diseases, Department of Medicine, Emory University School of Medicine, Atlanta, Georgia 30322, USA.
authors.forename: Peijian
authors.initials: P
authors.lastname: He
authors.forename: Huanchun
authors.initials: H
authors.lastname: Zhang
authors.forename: C Chris
authors.initials: CC
authors.lastname: Yun
mesh_terms: D000818:Animals; D018938:Caco-2 Cells; D002118:Calcium / Q000378:metabolism*; D020013:Calcium Signaling / Q000502:physiology*; D054732:Calcium-Calmodulin-Dependent Protein Kinase Type 2 / Q000235:genetics / Q000378:metabolism; D002147:Calmodulin / Q000235:genetics / Q000378:metabolism; D002462:Cell Membrane / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D037181:Lectins, C-Type / Q000235:genetics / Q000378:metabolism*; D011485:Protein Binding / Q000502:physiology; D017434:Protein Structure, Tertiary / Q000502:physiology; D011817:Rabbits; D017923:Sodium-Hydrogen Exchangers / Q000235:genetics / Q000378:metabolism*
chemical_list: D002147:Calmodulin; D037181:Lectins, C-Type; D017923:Sodium-Hydrogen Exchangers; D054732:Calcium-Calmodulin-Dependent Protein Kinase Type 2; D002118:Calcium