IP3 receptors: Take four IP3 to open

Abstract

Inositol 1,4,5-trisphosphate receptors (IP3Rs) and ryanodine receptors are the channels responsible for Ca(2+)release from the endoplasmic and sarcoplasmic reticulum. Research inScience Signalingby Alzayadyet al show that all four IP3-binding sites within the tetrameric IP3R must bind IP3before the channel can open, which has important consequences for the distribution of both IP3and IP3R activity within cells.

Fig. 1. Structure of the IP₃R

(A) The essential 4- and 5-phosphate groups of IP₃ bind to opposite sides (β- and α-domains) of the clam-like IP₃-binding core (IBC) causing the clam to close and re-position the suppressor domain (SD), which moves with the α-domain of the IBC (Protein Data Bank, PDB, 3U4J) (5). Locations of key domains within the primary sequence are shown below. (B) View from the cytosol showing the four N-terminal regions (SD and IBC) and the C-terminal domains (CTD) from each subunit (the central splayed α-helices). The coloured subunits (orange and green) show how the CTD from one subunit contacts the N-terminal region (the β-IBC) of another. The N- (blue) and C-termini (red) of each subunit, through which Alzayady et al. linked subunits to form concatamers (16), are highlighted; the termini of one subunit are enclosed in the yellow box. Within each IBC, the position where IP₃ would bind is shown by a yellow circle (PDB, 3JAV) (11). (C) View across the ER membrane, with 2 subunits highlighted in shades of orange and green. The transmembrane region with its 24 α-helices, six from each subunit, are shown (PDB, 3JAV) (11). (D) One of the likely inter-subunit interactions through which IP₃R binding to the IBC is communicated to the pore. The β-domain of the IBC of one subunit contacts the C-terminal domain (CTD) of another. The CTD extends as an almost unbroken α-helix through the centre of the protein to helix S6, which lines the pore. For the pore to open and allow Ca²⁺ to pass, S6 must move to disrupt the hydrophobic constriction that occludes the closed pore (red arrow).

Fig. 2. IP₃Rs bind and buffer IP₃

(A) Within a typical cell, the concentration of IP₃-binding sites and their affinity (K_D) for IP₃ are similar (~100nM). An inescapable consequence is that within the range of IP₃ concentrations that achieve graded occupancy of the binding sites, a substantial fraction of the IP₃ is bound, thereby depleting the cytosol of free IP₃ (18). (B) Buffering of cytosolic IP₃ by IP₃R shifts the relationship between IP₃ concentration and IP₃ binding to higher concentrations of IP₃: a total concentration that exceeds the K_D is now required to occupy 50% of the binding sites. Because 4 sites must be occupied to activate an IP₃R, the IP₃ concentrations needed to activate 50% of IP₃Rs are much higher than those needed to occupy 50% of the binding sites. In this simple scheme, a concentration of IP₃ that occupied 50% of all binding sites, would allow only 6% of all IP₃Rs, about 450 IP₃Rs in a typical cell, to bind the 4 IP₃ needed for activity. (C) IP₃ produced by PLC is captured by IP₃Rs as it diffuses into the cytosol creating a radial concentration gradient. Within the gradient, IP₃Rs very close to PLC may acquire the 4 IP₃ molecules needed for activity (green), but beyond that there is an extensive penumbra of IP₃Rs that bind IP₃ to fewer of their subunits.