Cryo-EM structures of calcium homeostasis modulator (CALHM) channels
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ABSTRACT:
SUBMITTER: Osamu Nureki
PROVIDER: EMPIAR-10444 | biostudies-other |
REPOSITORIES: biostudies-other
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Project description:Calcium homeostasis modulator (CALHM) family proteins are Ca2+-regulated adenosine triphosphate (ATP)-release channels involved in neural functions including neurotransmission in gustation. Here, we present the cryo-electron microscopy (EM) structures of killifish CALHM1, human CALHM2, and Caenorhabditis elegans CLHM-1 at resolutions of 2.66, 3.4, and 3.6 Å, respectively. The CALHM1 octamer structure reveals that the N-terminal helix forms the constriction site at the channel pore in the open state and modulates the ATP conductance. The CALHM2 undecamer and CLHM-1 nonamer structures show the different oligomeric stoichiometries among CALHM homologs. We further report the cryo-EM structures of the chimeric construct, revealing that the intersubunit interactions at the transmembrane domain (TMD) and the TMD-intracellular domain linker define the oligomeric stoichiometry. These findings advance our understanding of the ATP conduction and oligomerization mechanisms of CALHM channels.
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Project description:Calcium homeostasis modulator 1 (CALHM1) is a voltage-gated ATP release channel that plays an important role in neural gustatory signaling and the pathogenesis of Alzheimer's disease. Here, we present a cryo-electron microscopy structure of full-length Ca2+-free CALHM1 from Danio rerio at an overall resolution of 3.1 Å. Our structure reveals an octameric architecture with a wide pore diameter of ~20 Å, presumably representing the active conformation. The overall structure is substantially different from that of the isoform CALHM2, which forms both undecameric hemichannels and gap junctions. The N-terminal small helix folds back to the pore and forms an antiparallel interaction with transmembrane helix 1. Structural analysis revealed that the extracellular loop 1 region within the dimer interface may contribute to oligomeric assembly. A positive potential belt inside the pore was identified that may modulate ion permeation. Our structure offers insights into the assembly and gating mechanism of the CALHM1 channel.
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Project description:Calcium homeostasis modulator 1 (CALHM1) is a voltage- and Ca2+-gated ATP channel that plays an important role in neuronal signaling. However, as the previously reported CALHM structures are all in the ATP-conducting state, the gating mechanism of ATP permeation is still elusive. Here, we report cryo-EM reconstructions of two Danio rerio CALHM1 heptamers with ordered or flexible long C-terminal helices at resolutions of 3.2 Å and 2.9 Å, respectively, and one D. rerio CALHM1 octamer with flexible long C-terminal helices at a resolution of 3.5 Å. Structural analysis shows that the heptameric CALHM1s are in an ATP-nonconducting state with a central pore diameter of approximately 6.6 Å. Compared with those inside the octameric CALHM1, the N-helix inside the heptameric CALHM1 is in the "down" position to avoid steric clashing with the adjacent TM1 helix. Molecular dynamics simulations show that as the N-helix moves from the "down" position to the "up" position, the pore size of ATP molecule permeation increases significantly. Our results provide important information for elucidating the mechanism of ATP molecule permeation in the CALHM1 channel.
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Project description:Opposite effects of 1,4-dihydropyridine (DHP) agonists and antagonists on the L-type calcium channels are a challenging problem. Cryo-EM structures visualized DHPs between the pore-lining helices S6III and S6IV in agreement with published mutational data. However, the channel conformations in the presence of DHP agonists and antagonists are virtually the same, and the mechanisms of the ligands' action remain unclear. We docked the DHP agonist S-Bay k 8644 and antagonist R-Bay k 8644 in Cav1.1 channel models with or without π-bulges in helices S6III and S6IV. Cryo-EM structures of the DHP-bound Cav1.1 channel show a π-bulge in helix S6III but not in S6IV. The antagonist's hydrophobic group fits into the hydrophobic pocket formed by residues in S6IV. The agonists' polar NO2 group is too small to fill up the pocket. A water molecule could sterically fit into the void space, but its contacts with isoleucine in helix S6IV (motif INLF) would be unfavorable. In a model with π-bulged S6IV, this isoleucine turns away from the DHP molecule and its position is occupied by the asparagine from the same motif INLF. The asparagine provides favorable contacts for the water molecule at the agonist's NO2 group but unfavorable contacts for the antagonist's methoxy group. In our models, the DHP antagonist stabilizes entirely α-helical S6IV. In contrast, the DHP agonist stabilizes π-bulged helix S6IV whose C-terminal part turned and rearranged the activation-gate region. This would stabilize the open channel. Thus, agonists, but not antagonists, would promote channel opening by stabilizing π-bulged helix S6IV.
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Project description:The transport of substances across the placenta is essential for the development of the fetus. Here, we were interested in the role of channels of the calcium homeostasis modulator (CALHM) family in the human placenta. By transcript analysis, we found the paralogs CALHM2, 4, and 6 to be highly expressed in this organ and upregulated during trophoblast differentiation. Based on electrophysiology, we observed that activation of these paralogs differs from the voltage- and calcium-gated channel CALHM1. Cryo-EM structures of CALHM4 display decameric and undecameric assemblies with large cylindrical pore, while in CALHM6 a conformational change has converted the pore shape into a conus that narrows at the intracellular side, thus describing distinct functional states of the channel. The pore geometry alters the distribution of lipids, which occupy the cylindrical pore of CALHM4 in a bilayer-like arrangement whereas they have redistributed in the conical pore of CALHM6 with potential functional consequences.
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