9.A.14.22.4
TMEM87A of 555 aas and 7 TMSs. It may be involved in retrograde protein transport from endosomes to the trans-Golgi network (TGN) (Hirata et al. 2015). TMEM87s are eukaryotic transmembrane proteins with two members (TMEM87A and TMEM87B) in humans. TMEM87s have proposed roles in protein transport to and from the Golgi, as mechanosensitive ion channels, and in developmental signaling. TMEM87 disruption has been implicated in cancers and developmental disorders. Hoel et al. 2022 determined a cryo-EM structure of human TMEM87A in lipid nanodiscs. TMEM87A consists of a Golgi-dynamics (GOLD) domain atop a membrane-spanning seven-TMS domain with a large cavity open to solution and the membrane outer leaflet. Structural and functional analyses suggested that TMEM87A may not function as an ion channel or G-protein coupled receptor. They found that TMEM87A shares its characteristic domain arrangement with seven other proteins in humans; three that had been identified as evolutionary related (TMEM87B, GPR107, and GPR108) and four previously unrecognized homologs (GPR180, TMEM145, TMEM181, and WLS). Among these structurally related GOLD domain seven-TMS (GOST) proteins, WLS is best characterized as a membrane trafficking and secretion chaperone for lipidated Wnt signaling proteins. Key structural determinants for WLS function are conserved in TMEM87A. Hoel et al. 2022 proposed that TMEM87A and structurally homologous GOST proteins could serve a common role in trafficking membrane-associated cargo. According to Chakrabarti et al. 2024, touch sensation requires the mechanically gated ion channel ELKIN1.  GolpHCat (TMEM87A), a unique voltage-dependent cation channel in the Golgi apparatus, contributes to Golgi-pH maintenance and hippocampus-dependent memory (Kang et al. 2024).