Genetic screens with barcoded PlasmoGEM vectors have identified thousands of Plasmodium berghei gene functions in haploid blood stages, gametocytes and liver stages. However, the formation of diploid cells by fertilisation has hindered similar research on the parasites’ mosquito stages. In this study, we develop a scalable genetic system that uses barcoded gene targeting vectors equipped with a CRISPR-mediated homing mechanism to generate homozygous loss-of-function mutants after one parent introduces a modified allele into the zygote. To achieve this, we use vectors additionally expressing a target gene specific gRNA. When integrated into one of the parental alleles it directs Cas9 to the intact allele after fertilisation, leading to its disruption. This homing strategy is 90% effective at generating homozygous gene editing of a fluorescence-tagged reporter locus in the oocyst. A pilot screen identifies PBANKA_0916000 as a chloroquine resistance transporter-like protein (CRTL) essential for oocyst growth and sporogony, pointing to an unexpected importance for malaria transmission of the poorly understood digestive vacuole of the oocyst that contains hemozoin granules. Homing screens provide a method for the systematic discovery of malaria transmission genes whose first essential functions are after fertilisation in the bloodmeal, enabling their potential as targets for transmission-blocking interventions to be assessed.

The strict exchange of Na⁺ for H⁺ ions across cell membranes is a reaction carried out in almost every cell. Na⁺/H⁺ exchangers that perform this task are physiological homodimers, and whilst the ion transporting domain is highly conserved, their dimerization differs. The Na⁺/H⁺ exchanger NhaA from Escherichia coli has a weak dimerization interface mediated by a β-hairpin domain and with dimer retention dependent on cardiolipin. Similarly, organellar Na⁺/H⁺ exchangers NHE6, NHE7 and NHE9 also contain β-hairpin domains and recent analysis of Equus caballus NHE9 indicated PIP₂ lipids could bind at the dimer interface. However, structural validation of the predicted lipid-mediated oligomerization has been lacking. Here, we report cryo-EM structures of E. coli NhaA and E. caballus NHE9 in complex with cardiolipin and phosphatidylinositol-3,5-bisphosphate PI(3,5)P₂ lipids binding at their respective dimer interfaces. We further show how the endosomal specific PI(3,5)P₂ lipid stabilizes the NHE9 homodimer and enhances transport activity. Indeed, we show that NHE9 is active in endosomes, but not at the plasma membrane where the PI(3,5)P₂ lipid is absent. Thus, specific lipids can regulate Na⁺/H⁺ exchange activity by stabilizing dimerization in response to either cell specific cues or upon trafficking to their correct membrane location.

ATP generated in the mitochondria is exported by an ADP/ATP carrier of the SLC25 family. The endoplasmic reticulum (ER) cannot synthesize ATP but must import cytoplasmic ATP to energize protein folding, quality control and trafficking. It was recently proposed that a member of the nucleotide sugar transporter family, termed SLC35B1 (also known as AXER), is not a nucleotide sugar transporter but a long-sought-after ER importer of ATP. Here we report that human SLC35B1 does not bind nucleotide sugars but indeed executes strict ATP/ADP exchange with uptake kinetics consistent with the import of ATP into crude ER microsomes. A CRISPR–Cas9 cell-line knockout demonstrated that SLC35B1 clusters with the most essential SLC transporters for cell growth, consistent with its proposed physiological function. We have further determined seven cryogenic electron microscopy structures of human SLC35B1 in complex with an Fv fragment and either bound to an ATP analogue or ADP in all major conformations of the transport cycle. We observed that nucleotides were vertically repositioned up to approximately 6.5 Å during translocation while retaining key interactions with a flexible substrate-binding site. We conclude that SLC35B1 operates by a stepwise ATP translocation mechanism, which is a previously undescribed model for substrate translocation by an SLC transporter.

The sodium/proton exchanger NHA2, also known as SLC9B2, is important for insulin secretion, renal blood pressure regulation, and electrolyte retention. Recent structures of bison NHA2 has revealed its unique 14-transmembrane helix architecture, which is different from SLC9A/NHE members made up from 13-TM helices. Sodium/proton exchangers are functional homodimers, and the additional N-terminal helix in NHA2 was found to alter homodimer assembly. Here, we present the cryo-electron microscopy structures of apo human NHA2 in complex with a Fab fragment and also with the inhibitor phloretin bound at 2.8 and 2.9 Å resolution, respectively. We show how phosphatidic acid (PA) lipids bind to the homodimer interface of NHA2 on the extracellular side, which we propose has a regulatory role linked to cell volume regulation. The ion binding site of human NHA2 has a salt bridge interaction between the ion binding aspartate D278 and R432, an interaction previously broken in the bison NHA2 structure, and these differences suggest a possible ion coupling mechanism. Lastly, the human NHA2 structure in complex with phloretin offers a template for structure-guided drug design, potentially leading to the development of more selective and potent NHA2 inhibitors.

Transporting small molecules across cell membranes is an essential process in cell physiology. Many structurally diverse, secondary active transporters harness transmembrane electrochemical gradients of ions to power the uptake or efflux of nutrients, signalling molecules, drugs and other ions across cell membranes. Transporters reside in lipid bilayers on the interface between two aqueous compartments, where they are energized and regulated by symported, antiported and allosteric ions on both sides of the membrane and the membrane bilayer itself. Here we outline the mechanisms by which transporters couple ion and solute fluxes and discuss how structural and mechanistic variations enable them to meet specific physiological needs and adapt to environmental conditions. We then consider how general bilayer properties and specific lipid binding modulate transporter activity. Together, ion gradients and lipid properties ensure the effective transport, regulation and distribution of small molecules across cell membranes.

Tripartite ATP-independent periplasmic (TRAP) transporters are secondary-active transporters that receive their substrates via a soluble-binding protein to move bioorganic acids across bacterial or archaeal cell membranes. Recent cryo-electron microscopy (cryo-EM) structures of TRAP transporters provide a broad framework to understand how they work, but the mechanistic details of transport are not yet defined. Here we report the cryo-EM structure of the Haemophilus influenzae N-acetylneuraminate TRAP transporter (HiSiaQM) at 2.99 Å resolution (extending to 2.2 Å at the core), revealing new features. The improved resolution (the previous HiSiaQM structure is 4.7 Å resolution) permits accurate assignment of two Na⁺ sites and the architecture of the substrate-binding site, consistent with mutagenic and functional data. Moreover, rather than a monomer, the HiSiaQM structure is a homodimer. We observe lipids at the dimer interface, as well as a lipid trapped within the fusion that links the SiaQ and SiaM subunits. We show that the affinity (K_D) for the complex between the soluble HiSiaP protein and HiSiaQM is in the micromolar range and that a related SiaP can bind HiSiaQM. This work provides key data that enhances our understanding of the ‘elevator-with-an-operator’ mechanism of TRAP transporters.

Multidrug resistance-associated protein 2 (MRP2/ABCC2) is a polyspecific efflux transporter of organic anions expressed in hepatocyte canalicular membranes. MRP2 dysfunction, in Dubin-Johnson syndrome or by off-target inhibition, for example by the uricosuric drug probenecid, elevates circulating bilirubin glucuronide and is a cause of jaundice. Here, we determine the cryo-EM structure of rat Mrp2 (rMrp2) in an autoinhibited state and in complex with probenecid. The autoinhibited state exhibits an unusual conformation for this class of transporter in which the regulatory domain is folded within the transmembrane domain cavity. In vitro phosphorylation, mass spectrometry and transport assays show that phosphorylation of the regulatory domain relieves this autoinhibition and enhances rMrp2 transport activity. The in vitro data is confirmed in human hepatocyte-like cells, in which inhibition of endogenous kinases also reduces human MRP2 transport activity. The drug-bound state reveals two probenecid binding sites that suggest a dynamic interplay with autoinhibition. Mapping of the Dubin-Johnson mutations onto the rodent structure indicates that many may interfere with the transition between conformational states. The ABC transporter MRP2/ABCC2 is a polyspecific efflux transporter of organic anions expressed in hepatocyte canalicular membranes. Dysfunction leads to Dubin-Johnson syndrome. Here the authors provide structural and biochemical evidence on the modulation of MRP2 by intracellular kinases and inhibition by therapeutic drugs.

Hepatitis B virus (HBV), a leading cause of developing hepatocellular carcinoma affecting more than 290 million people worldwide, is an enveloped DNA virus specifically infecting hepatocytes. Myristoylated preS1 domain of the HBV large surface protein binds to the host receptor sodium-taurocholate cotransporting polypeptide (NTCP), a hepatocellular bile acid transporter, to initiate viral entry. Here, we report the cryogenic-electron microscopy structure of the myristoylated preS1 (residues 2–48) peptide bound to human NTCP. The unexpectedly folded N-terminal half of the peptide embeds deeply into the outward-facing tunnel of NTCP, whereas the C-terminal half formed extensive contacts on the extracellular surface. Our findings reveal an unprecedented induced-fit mechanism for establishing high-affinity virus–host attachment and provide a blueprint for the rational design of anti-HBV drugs targeting virus entry. 

Intracellular potassium (K⁺) homeostasis is fundamental to cell viability. In addition to channels, K⁺ levels are maintained by various ion transporters. One major family is the proton-driven K⁺ efflux transporters, which in gram-negative bacteria is important for detoxification and in plants is critical for efficient photosynthesis and growth. Despite their importance, the structure and molecular basis for K⁺-selectivity is poorly understood. Here, we report ~3.1 Å resolution cryo-EM structures of the Escherichia coli glutathione (GSH)-gated K⁺ efflux transporter KefC in complex with AMP, AMP/GSH and an ion-binding variant. KefC forms a homodimer similar to the inward-facing conformation of Na⁺/H⁺ antiporter NapA. By structural assignment of a coordinated K⁺ ion, MD simulations, and SSM-based electrophysiology, we demonstrate how ion-binding in KefC is adapted for binding a dehydrated K⁺ ion. KefC harbors C-terminal regulator of K⁺ conductance (RCK) domains, as present in some bacterial K⁺-ion channels. The domain-swapped helices in the RCK domains bind AMP and GSH and they inhibit transport by directly interacting with the ion-transporter module. Taken together, we propose that KefC is activated by detachment of the RCK domains and that ion selectivity exploits the biophysical properties likewise adapted by K⁺-ion-channels.

In mammals, glucose transporters (GLUT) control organism-wide blood-glucose homeostasis. In human, this is accomplished by 14 different GLUT isoforms, that transport glucose and other monosaccharides with varying substrate preferences and kinetics. Nevertheless, there is little difference between the sugar-coordinating residues in the GLUT proteins and even the malarial Plasmodium falciparum transporter PfHT1, which is uniquely able to transport a wide range of different sugars. PfHT1 was captured in an intermediate 'occluded' state, revealing how the extracellular gating helix TM7b has moved to break and occlude the sugar-binding site. Sequence difference and kinetics indicated that the TM7b gating helix dynamics and interactions likely evolved to enable substrate promiscuity in PfHT1, rather than the sugar-binding site itself. It was unclear, however, if the TM7b structural transitions observed in PfHT1 would be similar in the other GLUT proteins. Here, using enhanced sampling molecular dynamics simulations, we show that the fructose transporter GLUT5 spontaneously transitions through an occluded state that closely resembles PfHT1. The coordination of D-fructose lowers the energetic barriers between the outward- and inward-facing states, and the observed binding mode for D-fructose is consistent with biochemical analysis. Rather than a substrate-binding site that achieves strict specificity by having a high affinity for the substrate, we conclude GLUT proteins have allosterically coupled sugar binding with an extracellular gate that forms the high-affinity transition-state instead. This substrate-coupling pathway presumably enables the catalysis of fast sugar flux at physiological relevant blood-glucose concentrations.