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