Symmetry and structure in P‐glycoprotein and ABC transporters

Abstract

The ABC superfamily of membrane transporters is one of the largest classes of proteins across all species and one of the most intensely researched. ABC proteins are involved in the trafficking of a diverse variety of biological molecules across cell membranes, with some members implicated in medical syndromes such as cystic fibrosis and multidrug resistance to anti-cancer drugs. In the absence of X-ray crystallographic data, structural information has come from spectroscopy, electron microscopy, secondary structure prediction algorithms and residue substitution, epitope labelling and cysteine cross-linking studies. These have generally supported a model for the topology of the transmembrane domains of ABC transporters in which a single aqueous pore is formed by a toroidal ring of 12 alpha helices, deployed in two arcs of six helices each. Although this so-called 6 + 6 helix model can be arranged in either mirror or rotational symmetry configurations, experimental data supports the former. In this review, we put forward arguments against both configurations of this 6 + 6 helix model, based on what is known generally about symmetry relationships in proteins. We relate these arguments to P-glycoprotein, in particular, and discuss alternative models for the structure of ABC transporters in the light of the most recent research.

Active Transport of Fluorescent P-Glycoprotein Substrates: Evaluation as Markers and Interaction with Inhibitors

“…The evidence for two unique binding sites is further indicated by P-gp substrate binding curves that are biphasic (9), illustrating two dissociation constants for some substrates/inhibitors. Other results indicate that a vinblastine binding site is distinct from a dihydropyridine binding site (19), also Jones and George review published structural data to present an argument for a substrate binding site per half of the P-gp protein (20). Consistent with this conclusion is the identification of photoaffinity binding sites of a photoreactive P-gp substrate, prazosin, indicating multiple distinct binding sites at TM4 -TM5, TM7-TM8, and after the second NBD in hamster P-gp (21).…”

Identification and Characterization of the Binding Sites of P-Glycoprotein for Multidrug Resistance-Related Drugs and Modulators

“…The two homologous cassettes are separated by an intracellular linker region of about 60 amino acid residues [40][41][42][43][44]. In addition to this model of P-glycoprotein, another model has been proposed, which consists of two membrane-embedded sixteen-strand β-barrels, connected by short loops to two six-helix bundles beneath each barrel [45,46]. The involvement of TM segments and NBDs in substrate recognition and ATP binding/hydrolysis, respectively, have been established [47][48][49][50][51][52].…”

Distinct N-glycan glycosylation of P-glycoprotein isolated from the human uterine sarcoma cell line MES-SA/Dx5

“…Pgp is an energy-driven efflux pump and consists of 1,280 amino acids with 12 α-helix transmembrane domains and two cytoplasmic nucleotide-binding domains. This configuration represents the 6+6 helix model [4] or the more recent double β-barrel model described by Jones and George [5].…”