Abstract
Inositol-1,4,5-trisphosphate receptors (InsP3Rs) are cation channels that mobilize Ca2+ from intracellular stores in response to a wide range of cellular stimuli. The paradigm of InsP3R activation is the coupled interplay between binding of InsP3 and Ca2+ that switches the ion conduction pathway between closed and open states to enable the passage of Ca2+ through the channel. However, the molecular mechanism of how the receptor senses and decodes ligand-binding signals into gating motion remains unknown. Here, we present the electron cryo-microscopy structure of InsP3R1 from rat cerebellum determined to 4.1 Å resolution in the presence of activating concentrations of Ca2+ and adenophostin A (AdA), a structural mimetic of InsP3 and the most potent known agonist of the channel. Comparison with the 3.9 Å-resolution structure of InsP3R1 in the Apo-state, also reported herein, reveals the binding arrangement of AdA in the tetrameric channel assembly and striking ligand-induced conformational rearrangements within cytoplasmic domains coupled to the dilation of a hydrophobic constriction at the gate. Together, our results provide critical insights into the mechanistic principles by which ligand-binding allosterically gates InsP3R channel.
Highlights
Inositol 1,4,5-trisphosphate receptors (InsP3Rs) constitute a functionally important class of intracellular Ca2+ channels that are capable of converting a wide variety of cellular signals to intracellular calcium signals, which trigger markedly different cellular actions ranging from gene transcription to secretion, from proliferation to cell death.[1,2,3,4]
Structure of adenophostin A (AdA)-InsP3R1 To understand how ligand-binding triggers a drastic change in the permeability of InsP3R channel to specific ions, we determined the structure of InsP3R1 in the presence of activating concentrations of AdA (100 nM) and Ca2+ (300 nM), which works as a co-agonist to promote channel opening, as demonstrated in numerous electrophysiological studies.[9,10,11,12,13]
These structures bring into focus molecular features of the ligand binding domains and ion conduction pathway, in which conformational changes are required for activation of the channel gate that enables passage of Ca2+ ions
Summary
Inositol 1,4,5-trisphosphate receptors (InsP3Rs) constitute a functionally important class of intracellular Ca2+ channels that are capable of converting a wide variety of cellular signals (e.g., hormones, neurotransmitters, growth factors, light, odorants, signaling proteins) to intracellular calcium signals, which trigger markedly different cellular actions ranging from gene transcription to secretion, from proliferation to cell death.[1,2,3,4] The cellular signals are transmitted to the receptor by the secondary messenger molecule inositol 1,4,5-trisphosphate (InsP3), the primary agonist of InsP3Rs, generated within an essential intracellular signaling pathway initiated by phospholipase C. There is a general consensus that activation of channel gating is associated with conformational rearrangements at the inner pore-lining helix bundle that are triggered by InsP3 binding within the first 600 residues of the InsP3R protein.[5,6] This functional coupling has been experimentally demonstrated through electrophysiological, ligand-binding and mutagenesis studies,[1,7] the precise molecular mechanism by which InsP3 exerts its effect on InsP3R function is still largely unknown. We have extended our structural analysis of InsP3R1 in an Apo-state to 3.9 Å resolution Together, these structures reveal how InsP3R1 channel performs its mechanical work through ligand-driven allostery that removes the molecular barrier within the ion permeation pathway and allows for Ca2+ translocation across the membrane
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