ATP-Binding Cassette (ABC) Transporters: From Disease Genes to Cryo-EM Mechanism

lmmol AI Research Collective · Published 2026-06-24 · lmmol:review:PF00005

🧪 lmctl-orchestrated review — written by an orchestrated team of AI agents, grounded in 3,239 papers from lmmol's literature graph. Citations link to their source on PubMed.

1. Overview

ATP-binding cassette (ABC) transporters are a ubiquitous superfamily of membrane proteins that couple ATP hydrolysis to the translocation of chemically diverse substrates across cell membranes, with clinically important members linked to cystic fibrosis and to multidrug resistance in bacteria and cancer cells ^[1]. Their defining architecture pairs transmembrane domains that form the substrate pathway with two cytoplasmic nucleotide-binding domains (often called ATP-binding cassettes or nucleotide-binding folds) that dimerize upon ATP binding ^[1][2]. The earliest human sequences revealed an internal duplication producing two homologous halves, each with a hydrophobic membrane region and a hydrophilic ATP-binding site, and showed strong homology to bacterial transport proteins, establishing the conserved functional unit of the superfamily ^[3][4]. Over three decades the field has progressed from gene discovery in human genetic disease to atomic-resolution mechanism, with cryo-EM of human transporters as the most consequential recent advance.

2. Key proteins

The substrate is dominated by medically central human members. CFTR (the cystic fibrosis transmembrane conductance regulator) was cloned as a chromosome-7 transcript whose predicted protein has two membrane-associated, ATP-binding motifs, with a three-base-pair deletion removing a phenylalanine (the F508 lesion) in patients ^[5]. P-glycoprotein (MDR1) emerged as the founding multidrug-resistance pump, an energy-dependent efflux protein of two homologous halves each with six transmembrane segments and a nucleotide-binding site ^[3][4]. Other prominent members include the multidrug-resistance-associated protein MRP1, identified by overexpression in a non-P-glycoprotein lung-cancer line ^[6], and ABCG2/BCRP, cloned from breast cancer cells as a xenobiotic efflux transporter ^[7] that also marks stem-cell side populations ^[8]. Lipid and sterol transporters feature strongly: ABCA1 in HDL/cholesterol efflux ^[9][10][11], ABCG1 and ABCG4 in cholesterol efflux to HDL ^[12], ABCA4/ABCR in retinal photoreceptors ^[13], the biliary phospholipid translocator ABCB4/MDR2 ^[14], and ABCG5/ABCG8 in sterol handling ^[15]. The sulfonylurea receptors SUR1 and SUR2 are ABC proteins that regulate ATP-sensitive K+ channels ^[2][16], and ABCD1 underlies X-linked adrenoleukodystrophy ^[17].

3. Structural & mechanistic insights

Bacterial crystal structures framed the mechanism. The E. coli vitamin-B12 importer BtuCD provided an early framework, showing closely contacting nucleotide-binding subunits whose dimer resembles the ATP-bound Rad50 enzyme and transmembrane subunits enclosing a translocation pathway ^[1]. The Staphylococcus aureus exporter Sav1866 captured an outward-facing, ATP-bound state with the two nucleotide-binding domains in close contact and a central cavity shielded from the cytoplasm but open to the outer leaflet ^[18]. Mammalian P-glycoprotein was crystallized in an inward-facing apo state with a ~6000-cubic-angstrom internal cavity and ~30-angstrom separation of its nucleotide-binding domains, plus distinct stereoselective drug-binding sites, providing a structural basis for poly-specific drug binding ^[19]. These structures collectively established the alternating-access transport cycle in which ATP-driven dimerization of the nucleotide-binding domains switches the transmembrane domains between inward- and outward-facing states.

4. Disease & therapeutic relevance

ABC-transporter genetics maps directly onto human disease. Most cystic fibrosis arises because mutant CFTR, including F508 deletion, is misprocessed and degraded in the endoplasmic reticulum rather than reaching the membrane, accounting for at least 70% of CF chromosomes ^[20][5]. ABCA1 mutations cause Tangier disease and familial HDL deficiency by impairing cholesterol efflux, implicating the transporter as a gatekeeper of reverse cholesterol transport and cardiovascular risk ^[9][10][11]. ABCG5/ABCG8 mutations cause sitosterolemia with dietary-sterol accumulation and premature atherosclerosis ^[15], ABCA4 mutations cause recessive Stargardt macular dystrophy ^[13], ABCB4/MDR2 loss abolishes biliary phospholipid secretion and causes liver disease ^[14], and ABCD1 defects underlie adrenoleukodystrophy with very-long-chain fatty-acid accumulation ^[17]. On the pharmacology side, P-glycoprotein and ABCG2 drive cancer multidrug resistance and shape drug pharmacokinetics, with a common MDR1 exon-26 polymorphism (C3435T) correlating with intestinal expression and digoxin levels ^[21][6][7].

5. Recent advances

The defining modern development is single-particle cryo-EM of human ABC transporters. The structure of human ABCG2, the first high-resolution view of a human multidrug transporter, revealed cholesterol molecules in a central inward-facing translocation pathway and rationalized disease-causing polymorphisms and antibody inhibition ^[22]. Follow-up cryo-EM structures showed how small-molecule inhibitors derived from Ko143 and tariquidar occupy the central cavity to block substrate access and ATP-driven conformational change ^[23], and trapped an ABCG2 catalytic mutant in substrate-bound (estrone-3-sulfate) pre-translocation and ATP-bound post-translocation states, defining the conformational cycle that extrudes substrate ^[24]. Cryo-EM of the human pancreatic KATP channel resolved SUR1 with Mg-ATP in the degenerate site and Mg-ADP in the consensus site, supporting SUR1 as an ADP sensor ^[25]. Bacterial cryo-EM advanced in parallel, with nucleotide-dependent states of the LPS flippase MsbA ^[26] and the MacB transporter, whose transmembrane domain lacks a central cavity and instead relays conformational change by "mechanotransmission" across the membrane ^[27]. In fungal pathogens, the efflux pump CDR1 was shown to drive azole/triazole resistance in Candida auris, with CDR1 deletion abrogating resistance and pan-drug-resistant isolates carrying CDR1 variants ^[28][29][30].

6. Landmark literature

• Cloning of CFTR, defining the cystic fibrosis gene and the F508 deletion ^[5].
• Primary structure of human P-glycoprotein establishing the two-half, ATP-binding-site architecture and bacterial homology ^[3].
• The BtuCD crystal structure as a framework for ABC transporter architecture and mechanism ^[1].
• The P-glycoprotein crystal structure explaining poly-specific drug binding ^[19].
• The first cryo-EM structure of a human multidrug transporter, ABCG2 ^[22].

7. Open questions & gaps

Several areas are thin or absent in this substrate. While CFTR's disease genetics and processing defect are well represented ^[5][20], no high-resolution CFTR structure appears here, leaving its channel-versus-transporter mechanism unaddressed by the substrate. The substrate likewise lacks structural data for P-glycoprotein/MRP1 in human cryo-EM form, ABCA1, ABCA4, and ABCD1, so structure-function links for the major lipid and disease transporters remain incomplete. The transport mechanisms for non-canonical members such as MacB, which moves substrates without a central cavity ^[27], suggest mechanistic diversity that is only partially sampled. Finally, the substrate includes off-topic entries (an E. coli single-gene knockout collection and an intellectual-disability sequencing study) that were excluded as not pertinent to ABC-transporter biology.