Targeted expression of the inositol 1,4,5-triphosphate receptor (IP3R) ligand-binding domain releases Ca2+ via endogenous IP3R channels.
Virtually all functions of a cell are influenced by cytoplasmic [Ca(2+)] increases. Inositol 1,4,5-trisphosphate receptor (IP(3)R) channels, located in the endoplasmic reticulum (ER), release Ca(2+) in response to binding of the second messenger, IP(3).IP(3)Rs thus are part of the information chain interpreting external signals and transforming them into cytoplasmic Ca(2+) transients. IP(3)Rs function as tetramers, each unit comprising an N-terminal ligand-binding domain (LBD) and a C-terminal channel domain linked by a long regulatory region. It is not yet understood how the binding of IP(3) to the LBD regulates the gating properties of the channel. Here, we use the expression of IP(3) binding protein domains tethered to the surface of the endoplasmic reticulum (ER) to show that the all-helical domain of the IP(3)R LBD is capable of depleting the ER Ca(2+) pools by opening the endogenous IP(3)Rs, even without IP(3) binding. This effect requires the domain to be within 50 A of the ER membrane and is impaired by the presence of the N-terminal inhibitory segment on the LBD. These findings raise the possibility that the helical domain of the LBD functions as an effector module possibly interacting with the channel domain, thereby being part of the gating mechanisms by which the IP(3)-induced conformational change within the LBD regulates Ca(2+) release.
Várnai
Péter
P
Endocrinology and Reproduction Research Branch, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD 20892, USA.
Balla
András
A
Hunyady
László
L
Balla
Tamas
T
D000818:Animals; D019556:COS Cells; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000378:metabolism* / Q000502:physiology; D002522:Chlorocebus aethiops; D003593:Cytoplasm / Q000378:metabolism; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D005822:Genetic Vectors / Q000235:genetics; D015544:Inositol 1,4,5-Trisphosphate / Q000378:metabolism; D053496:Inositol 1,4,5-Trisphosphate Receptors; D015640:Ion Channel Gating / Q000502:physiology*; D008024:Ligands; D008164:Luminescent Proteins; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D018160:Receptors, Cytoplasmic and Nuclear / Q000378:metabolism* / Q000502:physiology; D000097573:Red Fluorescent Protein
D002118:Calcium; D015220:Calcium Channels; D015544:Inositol 1,4,5-Trisphosphate; D053496:Inositol 1,4,5-Trisphosphate Receptors; D008024:Ligands; D008164:Luminescent Proteins; D018160:Receptors, Cytoplasmic and Nuclear; D000097573:Red Fluorescent Protein