Structural insights into ion conduction by novel cation channel, TMEM87A, in Golgi apparatus

Abstract

TMEM87 family is evolutionarily conserved eukaryotic transmembrane proteins residing in the Golgi1. TMEM87 members play a role in retrograde transport in Golgi and are also proposed mechanosensitive ion channel implicated in cancer and heart disease2-7. In an accompanying study, TMEM87A is described as a voltage-gated, pH-sensitive, non-selective cation channel whose genetic ablation in mice disrupts Golgi morphology, alters glycosylation and protein trafficking, and impairs hippocampal memory. Despite the pivotal functions of TMEM87s in Golgi, underlying molecular mechanisms of channel gating and ion conduction have remained unknown. Here, we present a high-resolution cryo-electron microscopy structure of human TMEM87A (hTMEM87A). Compared with typical ion channels, the architecture of hTMEM87A is unique: a monomeric cation channel consisting of a globular extracellular/luminal domain and a seven-transmembrane domain (TMD) with close structural homology to channelrhodopsin. The central cavity within TMD is occupied by endogenous phosphatidylethanolamine, which seals a lateral gap between two TMs exposed to the lipid bilayer. By combining electrophysiology and molecular dynamics analysis, we identify a funnel-shaped electro-negative luminal vestibule that effectively attracts cations, and phosphatidylethanolamine occludes ion conduction. Our findings suggest that a conformational switch of highly conserved positively-charged residues on TM3 and displacement of phosphatidylethanolamine are opening mechanisms for hTMEM87A, providing an unprecedented insight into the molecular basis for voltage-gated ion conduction in Golgi.

Touch sensation requires the mechanically gated ion channel ELKIN1

“…S1B). Additionally, in agreement with recent reports ( 11 , 15 ), we found that cells expressing mouse Elkin1 display prominent leak currents at very positive (>+60 mV) and very negative potentials (<−100 mV) (Fig. 1G and fig.…”

“…Cryo–electron microscopy (cryo-EM) structures of human ELKIN1 recently revealed a monomeric seven-transmembrane protein with an N-terminal extracellular Golgi-dynamics domain fold (GOLD domain) ( 10 ). A second, higher-resolution structure recently identified a cation-conduction pathway through the protein ( 11 ). We overexpressed Elkin1 in human embryonic kidney (HEK) 293T cells lacking PIEZO1 channels (HEK293T Piezo1−/− cells) ( 12 ) and found large indentation-induced mechanically activated currents (MA currents) in a majority of transfected cells (Fig.…”

“…We generated a CRISPR-Cas9–mediated genomic deletion of the mouse Tmem87a/Elkin1 gene locus spanning sequences coding for transmembrane domains 2 to 6, which includes the proposed ion-conduction pathway (fig. S3A) ( 11 ). Mice homozygous for the genomic deletion ( Elkin1 −/− mice) were viable and born at the expected Mendelian ratios [wild type (WT), 25.7%; Elkin1 +/− , 46.2%; Elkin1 −/− , 28%; n = 132] (table S2).…”

Touch sensation requires the mechanically-gated ion channel Elkin1

“…Therefore, the GOLD-domain is not necessary for mechanical activation of Elkin1. Elkin1 currents showed a distinctive pharmacological profile, being highly sensitive to the non-specific pore-blocker Gd 3+ Additionally, in agreement with a recent report 14 Since Elkin1 is a mechanically-gated channel with properties similar to Piezo2 channels, we hypothesized that this protein could also be involved in mammalian touch sensation. We generated a CRISPR/Cas9 mediated genomic deletion of the mouse Tmem87a/Elkin1 gene locus spanning sequences coding for transmembrane domains 2-6, that includes the proposed ion conduction pathway (Extended Data Fig.…”

“…Cryo-EM structures of human Elkin1 recently revealed a monomeric 7 transmembrane protein with an N-terminal extracellular Golgi-dynamics domain fold (GOLD-domain) 13 . A second higher resolution structure recently identified a cation conduction pathway through the protein 14 . We overexpressed Elkin1 in HEK293T cells lacking Piezo1 channels (HEK293T Piezo1-/cells) 15 and found large pokinginduced mechanically-activated currents (MA-currents) in a majority of transfected cells (Fig.…”