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Energy-coupling factor transporters mediate the uptake of essential micronutrients in prokaryotes. On the basis of recent structural studies, Dirk Slotboom discusses a testable model for the unusual mechanism of transport that is involved and considers the implications for our understanding of membrane transporters.

In neurons and glia, glutamate transporters catalyse the reuptake of this neurotransmitter by coupling it with cation transport. Here the authors combine X-ray crystallography and molecular dynamics simulations of the archeal glutamate transporter Glt_Tkto get insight into the coupled transport mechanism.

GltPh is a homotrimeric Na+-coupled aspartate transporter that belongs to the glutamate transporter family. The conformational changes that occur during GltPh transport are now directly observed using EPR spectroscopy, revealing that the transporting domains sample multiple states, regardless of the presence of substrate or ions.

Prokaryotes use energy-coupling factor transporters to uptake required micronutrients and an unusual toppling mechanism has been proposed for their function. Here, the authors provide structural support for this mechanism, allowing direct visualization of the toppled state.

The association and dissociation dynamics of the ECF transporter complex for vitamin B12 are visualized by single-molecule FRET, highlighting the original transport mechanism of this group of ABC transporters.

Archaeal glutamate transporter homologs catalyze the coupled uptake of aspartate and sodium ions. A new crystal structure of Glt_Tk from Thermococcus kodakarensis shows the empty transporter oriented in the outward-facing conformation after substrate delivery, revealing how it is reset in preparation for another translocation cycle.

ABC transporters are generally considered to be unidirectional. Here, the authors develop a fluorescence-based transport assay and show that the mycobacterial ABC transporter BacA instead acts as a bidirectional transporter for cobalamin.

ASCT2 is a Na⁺-dependent obligatory amino acid exchanger. Here, the authors untangle the structural basis of the exchange mechanism in ASCT2, revealing that structural rigidity and a high-affinity Na⁺ binding site effectively confine ASCT2 to an exchange mode.

Episodic ataxias (EAs) are rare neurological conditions that cause motor impairment. Here the authors investigated the molecular mechanism of such a pathological condition caused by a mutation of a conserved proline into arginine residue.

The final step in an ECF transporters transport cycle involves the expulsion of the membrane embedded substrate binding protein (the S-component) from the motor (the ECF module). Here the authors show how the motor uses ATP binding to load a molecular spring, and adjusts the shape of the membrane, to achieve this step.

Glutamate transporters are membrane transporters that clear the neurotransmitter L-glutamate from the synaptic cleft via a so-called elevator mechanism. Here the authors present five cryo-EM structures of the transporter homologue Glt_Tk, which explain how substrate leakage is prevented.

Energy coupling factor (ECF) transporters consist of an integral membrane protein that confers substrate specificity (S-component) and an energizing module related to ATP-cassette (ABC) transporters. Structural studies of the thiamin-specific S-component ThiT from Lactococcus lactis reveal an interaction with the energizing module through a conserved motif on the membrane-embedded surface of ThiT, thereby providing insight into the substrate translocation mechanism.

How the human Alanine Serine Cysteine Transporter 2 (ASCT2) binds its substrates, neutral amino acids, and releases them on the cytoplasmic side remains unclear. Here authors present an inward-open structure of the human ASCT2 which shows that a hairpin serves as a gate in the inward-facing state.

A cryo-EM structure of the human SLC1 transporter ASCT2 in the inward-facing conformation reveals the retrovirus-docking site and helps to elucidate the transport cycle. The transport domain is more solvent exposed than in most of the homolog structures.

An X-ray crystal structure of substrate-bound Neisseria PnuC, a bacterial member of the SWEET family of transporters, provides key insights into the translocation mechanism and potential evolution of these membrane proteins.

Replicating natural processes in synthetic cells is key to further development and understanding. Here, the authors develop a synthetic reaction network for the generation of metabolic energy in the form of proton motive force, used to drive the accumulation of nutrients and enable internal metabolism in cell-like vesicles.

The vitamin B₁₂ importer BtuCD-F is a type II ATP-binding cassette transporter. Here the authors use single-molecule fluorescence techniques to analyse ATP hydrolysis and substrate transport in individual BtuCD-F complexes embedded in liposomes, and propose a transport model.

Energy coupling factor (ECF) transporters are responsible for the uptake of micronutrients and consist of an integral membrane unit, the S-component, which confers substrate specificity. Here, authors present multi-scale molecular dynamics simulations and in vitro experiments to study the molecular toppling mechanism of the S-component of a folate-specific ECF transporter.

AlphaFold structure predictions uncover an unexpected connectedness between prokaryotic membrane transport proteins and two-component sensory kinases, thereby defining a large “transceptor” class of membrane proteins that have crossed the border between transporters and receptors.

The bacterial zinc transporter ZntB is important for maintaining zinc homeostasis and is mechanistically not well understood. Here, the authors present the cryo-EM structure of ZntB at 4.2 Å resolution, perform transport assays and propose a model for its Zn²⁺ transport mechanism.

Glutamate transporters are integral membrane proteins that facilitate neurotransmitter uptake from the synaptic cleft into the cytoplasm of glial cells and neurons, the mechanism of transport involves transitions between extracellular- and intracellular-facing conformations; here the authors used single-molecule fluorescence resonance energy transfer imaging to directly observe conformational dynamics in trimers of a bacterial homologue of glutamate transporters that was embedded in the membrane.

Trinco et al. measure aspartate uptake rates in proteoliposomes containing purified prokaryotic Na⁺-coupled aspartate transporter GltTk. To overcome limitation of protein orientation, they use synthetic nanobody that blocks transporters from outside and reveal mechanistic features of Na⁺-aspartate symport that cannot be observed in detergent solution.

Virtual screening, biochemical assays, and coarse-grained simulations identify an inhibitor of energy-coupling factor transporters and its potential mechanism of action.