To be or not to be an ion channel: cryo-EM structures have a say

Identification of previously unknown ion channels is always a challenging work though the transmembrane protein-coding genes in human genome has been comprehensively annotated for years. Despite being the gold standard of functional assay for ion channels, electrophysiological recordings are often accompanied by electrical noise, leak conductance and background currents of the membrane system. These unwanted signals, if not treated properly, lead to mischaracterization of proteins with seemingly unusual ion-conducting properties. In recent ten years, the technical revolution of cryo-electron microscopy (cryo-EM) have greatly advanced our understanding on the structures and gating mechanisms of various ion channels, and also raised concerns about the pore-forming ability of some previously identified channel proteins. In this review, we summarize cryo-EM findings on ion channels with molecular identities recognized or disputed in recent ten years, and discuss current knowledge of proposed channel proteins awaiting cryo-EM analyses. We also present a classification of ion channels according to their architectures and evolutionary relationships, and discuss the possibility and strategy of identifying more ion channels by analyzing structures of transmembrane proteins of unknown function.