Transporters Through the Looking Glass: An Insight into the Mechanisms of Ion‑Coupled Transport and Methods That Help Reveal Them

Abstract

Cell membranes, despite providing a barrier to protect intra‑ cellular constituents, require selective gating for the infux of important metabolites including ions, sugars, amino acids, neurotransmitters and effux of toxins and metabolic end-products. The machinery involved in carrying out this gating process comprises of integral membrane pro‑ teins that use ionic electrochemical gradients or ATP hydrolysis, to drive concentrative uptake or effux. The mechanism through which ion-cou‑ pled transporters function is referred to as alternating-access. In the recent past, discrete modes of alternating-access have been described with the elucidation of new transporter structures and their snapshots in altered conformational states. Despite X-ray structures being the primary sources of mechanistic information, other biophysical methods provide information related to the structural dynamics of these transporters. Methods including EPR and smFRET, have extensively helped validate or clarify ion-coupled transport mechanisms, in a near-native environ‑ ment. This review seeks to highlight the mechanistic details of ion-cou‑ pled transport and delve into the biophysical tools and methods that help in understanding these fascinating molecules.