The lysosomal transporter TAPL has a dual role as peptide translocator and phosphatidylserine floppase

Structural insights into human ABCA7-mediated lipid transport

The human ATP-binding cassette (ABC) transporter ABCA7 participates in the lipidation of apolipoprotein ApoE, a commonly recognized risk factor for Alzheimer's disease (AD). How ABCA7 is involved in the molecular pathogenesis of AD remains poorly understood. Using cryoelectron microscopy (cryo-EM), we determined ABCA7 structures in the apo and substrate-bound forms, respectively. Combined with activity assays, we assigned the residues that specifically bind two molecules of phosphatidylserine (PS) that are arranged in a "tail-to-tail" manner. Pull-down assays confirmed that ApoE directly interacts with ABCA7; and moreover, both ATPase and lipid transport activities of ABCA7 were significantly enhanced in the presence of ApoE. We also measured the activities of a familial AD variant and a protective clinically reported variant in the ABCA7 gene. Our findings not only give structural insights into ABCA7-mediated PS translocation, but we also provide first biochemical evidence for its link to AD by forwarding lipids to ApoE.

Successful strategies for expression and purification of ABC transporters

ATP-binding cassette (ABC) transporters are proteins responsible for active transport of various compounds, from small ions to macromolecules, across membranes. Proteins from this superfamily also pump drugs out of the cell resulting in multidrug resistance. Based on the cellular functions of ABC-transporters they are commonly associated with diseases like cancer and cystic fibrosis. To understand the molecular mechanism of this critical family of integral membrane proteins, structural characterization is a powerful tool which in turn requires successful recombinant production of stable and functional protein in good yields. In this review we have used high resolution structures of ABC transporters as a measure of successful protein production and summarized strategies for prokaryotic and eukaryotic proteins, respectively. In general, Escherichia coli is the most frequently used host for production of prokaryotic ABC transporters while human embryonic kidney 293 (HEK293) cells are the preferred host system for eukaryotic proteins. Independent of origin, at least two-steps of purification were required after solubilization in the most used detergent DDM. The purification tag was frequently cleaved off before structural characterization using cryogenic electron microscopy, or crystallization and X-ray analysis for prokaryotic proteins.

Structure of a dimeric full-length ABC transporter

Activities of ATP binding cassette (ABC) proteins are regulated by multiple mechanisms, including protein interactions, phosphorylation, proteolytic processing, and/or oligomerization of the ABC protein itself. Here we present the structure of yeast cadmium factor 1 (Ycf1p) in its mature form following cleavage by Pep4p protease. Ycf1p, a C subfamily ABC protein (ABCC), is homologue of human multidrug resistance protein 1. Remarkably, a portion of cleaved Ycf1p forms a well-ordered dimer, alongside monomeric particles also present in solution. While numerous other ABC proteins have been proposed to dimerize, no high-resolution structures have been reported. Both phosphorylation of the regulatory (R) region and ATPase activity are lower in the Ycf1p dimer compared to the monomer, indicating that dimerization affects Ycf1p function. The interface between Ycf1p protomers features protein-protein interactions and contains bound lipids, suggesting that lipids stabilize the dimer. The Ycf1p dimer structure may inform the dimerization interfaces of other ABCC dimers.

Interdomain communication in a homodimeric ABC transporter

ABC transporters are found in all organisms and almost every cellular compartment. They mediate the transport of various solutes across membranes, energized by ATP binding and hydrolysis. Dysfunctions can result in severe diseases, such as cystic fibrosis or antibiotic resistance. In type IV ABC transporters, each of the two nucleotide-binding domains is connected to a transmembrane domain by two coupling helices, which are part of cytosolic loops. Although there are many structural snapshots of different conformations, the interdomain communication is still enigmatic. Therefore, we analyzed the function of three conserved charged residues in the intracytosolic loop 1 of the human homodimeric, lysosomal peptide transporter TAPL (transporter associated with antigen processing-like). Substitution of D278 in coupling helix 1 by alanine interrupted peptide transport by impeding ATP hydrolysis. Alanine substitution of R288 and D292, both localized next to the coupling helix 1 extending to transmembrane helix 3, reduced peptide transport but increased basal ATPase activity. Surprisingly, the ATPase activity of the R288A variant dropped in a peptide-dependent manner, whereas ATPase activity of wildtype and D292A was unaffected. Interestingly, R288A and D292A mutants did not differentiate between ATP and GTP in respect of hydrolysis. However, in contrast to wildtye TAPL, only ATP energized peptide transport. In sum, D278 seems to be involved in bidirectional interdomain communication mediated by network of polar interactions, whereas the two residues in the cytosolic extension of transmembrane helix 3 are involved in regulation of ATP hydrolysis, most likely by stabilization of the outward-facing conformation.

Functional Characterization of the Lysosomal Peptide/Histidine Transporter PHT1 (SLC15A4) by Solid Supported Membrane Electrophysiology (SSME)

The peptide/histidine transporter PHT1 (SLC15A4) is expressed in the lysosomal membranes of immune cells where it plays an important role in metabolic and inflammatory signaling. PHT1 is an H+-coupled/histidine symporter that can transport a wide range of oligopeptides, including a variety of bacterial-derived peptides. Moreover, it enables the scaffolding of various metabolic signaling molecules and interacts with key regulatory elements of the immune response. Not surprisingly, PHT1 has been implicated in the pathogenesis of autoimmune diseases such as systemic lupus erythematosus (SLE). Unfortunately, the pharmacological development of PHT1 modulators has been hampered by the lack of suitable transport assays. To address this shortcoming, a novel transport assay based on solid-supported membrane-based electrophysiology (SSME) is presented. Key findings of the present SSME studies include the first recordings of electrophysiological properties, a pH dependence analysis, an assessment of PHT1 substrate selectivity, as well as the transport kinetics of the identified substrates. In contrast to previous work, PHT1 is studied in its native lysosomal environment. Moreover, observed substrate selectivity is validated by molecular docking. Overall, this new SSME-based assay is expected to contribute to unlocking the pharmacological potential of PHT1 and to deepen the understanding of its functional properties.

Principles of peptide selection by the transporter associated with antigen processing

Our ability to fight pathogens relies on major histocompatibility complex class I (MHC-I) molecules presenting diverse antigens on the surface of diseased cells. The transporter associated with antigen processing (TAP) transports nearly the entire repertoire of antigenic peptides into the endoplasmic reticulum for MHC-I loading. How TAP transports peptides specific for MHC-I is unclear. In this study, we used cryo-EM to determine a series of structures of human TAP, both in the absence and presence of peptides with various sequences and lengths. The structures revealed that peptides of eight or nine residues in length bind in a similarly extended conformation, despite having little sequence overlap. We also identified two peptide-anchoring pockets on either side of the transmembrane cavity, each engaging one end of a peptide with primarily main chain atoms. Occupation of both pockets results in a global conformational change in TAP, bringing the two halves of the transporter closer together to prime it for isomerization and ATP hydrolysis. Shorter peptides are able to bind to each pocket separately but are not long enough to bridge the cavity to bind to both simultaneously. Mutations that disrupt hydrogen bonds with the N and C termini of peptides almost abolish MHC-I surface expression. Our findings reveal that TAP functions as a molecular caliper that selects peptides according to length rather than sequence, providing antigen diversity for MHC-I presentation.

Transport mechanism of human bilirubin transporter ABCC2 tuned by the inter-module regulatory domain

Bilirubin is mainly generated from the breakdown of heme when red blood cells reach the end of their lifespan. Accumulation of bilirubin in human body usually leads to various disorders, including jaundice and liver disease. Bilirubin is conjugated in hepatocytes and excreted to bile duct via the ATP-binding cassette transporter ABCC2, dysfunction of which would lead to Dubin-Johnson syndrome. Here we determine the structures of ABCC2 in the apo, substrate-bound and ATP/ADP-bound forms using the cryo-electron microscopy, exhibiting a full transporter with a regulatory (R) domain inserted between the two half modules. Combined with substrate-stimulated ATPase and transport activity assays, structural analysis enables us to figure out transport cycle of ABCC2 with the R domain adopting various conformations. At the rest state, the R domain binding to the translocation cavity functions as an affinity filter that allows the substrates of high affinity to be transported in priority. Upon substrate binding, the R domain is expelled from the cavity and docks to the lateral of transmembrane domain following ATP hydrolysis. Our findings provide structural insights into a transport mechanism of ABC transporters finely tuned by the R domain.

Reconstitution of ATP-dependent lipid transporters: gaining insight into molecular characteristics, regulation, and mechanisms

Lipid transporters play a crucial role in supporting essential cellular processes such as organelle assembly, vesicular trafficking, and lipid homeostasis by driving lipid transport across membranes. Cryo-electron microscopy has recently resolved the structures of several ATP-dependent lipid transporters, but functional characterization remains a major challenge. Although studies of detergent-purified proteins have advanced our understanding of these transporters, in vitro evidence for lipid transport is still limited to a few ATP-dependent lipid transporters. Reconstitution into model membranes, such as liposomes, is a suitable approach to study lipid transporters in vitro and to investigate their key molecular features. In this review, we discuss the current approaches for reconstituting ATP-driven lipid transporters into large liposomes and common techniques used to study lipid transport in proteoliposomes. We also highlight the existing knowledge on the regulatory mechanisms that modulate the activity of lipid transporters, and finally, we address the limitations of the current approaches and future perspectives in this field.

Differential Expression of ABC Transporter Genes in Brain Vessels vs. Peripheral Tissues and Vessels from Human, Mouse and Rat

BACKGROUND

ATP-binding cassette (ABC) transporters comprise a superfamily of genes encoding membrane proteins with nucleotide-binding domains (NBD). These transporters, including drug efflux across the blood-brain barrier (BBB), carry a variety of substrates through plasma membranes against substrate gradients, fueled by hydrolyzing ATP. The expression patterns/enrichment of ABC transporter genes in brain microvessels compared to peripheral vessels and tissues are largely uncharacterized.

METHODS

In this study, the expression patterns of ABC transporter genes in brain microvessels, peripheral tissues (lung, liver and spleen) and lung vessels were investigated using RNA-seq and Wes^TM analyses in three species: human, mouse and rat.

RESULTS

The study demonstrated that ABC drug efflux transporter genes (including ABCB1, ABCG2, ABCC4 and ABCC5) were highly expressed in isolated brain microvessels in all three species studied; the expression of ABCB1, ABCG2, ABCC1, ABCC4 and ABCC5 was generally higher in rodent brain microvessels compared to those of humans. In contrast, ABCC2 and ABCC3 expression was low in brain microvessels, but high in rodent liver and lung vessels. Overall, most ABC transporters (with the exception of drug efflux transporters) were enriched in peripheral tissues compared to brain microvessels in humans, while in rodent species, additional ABC transporters were found to be enriched in brain microvessels.

CONCLUSIONS

This study furthers the understanding of species similarities and differences in the expression patterns of ABC transporter genes; this is important for translational studies in drug development. In particular, CNS drug delivery and toxicity may vary among species depending on their unique profiles of ABC transporter expression in brain microvessels and BBB.