No 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.

The Golgi apparatus is a critical intracellular organelle that is responsible for modifying, packaging, and transporting proteins to their destinations. Golgi homeostasis involving the acidic pH, ion concentration, and membrane potential, is critical for proper functions and morphology of the Golgi. Although transporters and anion channels that contribute to Golgi homeostasis have been identified, the molecular identity of cation channels remains unknown. Here we identify TMEM87A as a novel Golgi-resident cation channel that contributes to pH homeostasis and rename it as GolpHCat (Golgi pH-sensitive Cation channel). The genetic ablation of GolpHCat exhibits an impaired resting pH in the Golgi. Heterologously expressed GolpHCat displays voltage- and pH-dependent, non-selective cationic, and inwardly rectifying currents, with potent inhibition by gluconate. Furthermore, reconstitution of purified GolpHCat in liposomes generates functional channel activities with unique voltage-dependent gating and ion permeation. GolpHCat is expressed in various cell types such as neurons and astrocytes in the brain. In the hippocampus, GolpHCat-knockout mice show dilated Golgi morphology and altered glycosylation and protein trafficking, leading to impaired spatial memory with significantly reduced long-term potentiation. We elucidate that GolpHCat, by maintaining Golgi membrane potential, regulates ionic and osmotic homeostasis, protein glycosylation/trafficking, and brain functions. Our results propose a new molecular target for Golgi-related diseases and cognitive impairment.