Structures of ABC transporters: handle with care

Abstract

In the past two decades, the ATP-binding cassette (ABC) transporters' field has undergone a structural revolution. The importance of structural biology to the development of the field of ABC transporters cannot be overstated, as the ensemble of structures not only revealed the architecture of ABC transporters but also shaped our mechanistic view of these remarkable molecular machines. Nevertheless, we advocate that the mechanistic interpretation of the structures is not trivial and should be carried out with prudence. Herein, we bring several examples of structures of ABC transporters that merit re-interpretation via careful comparison to experimental data. We propose that it is of the upmost importance to place new structures within the context of the available experimental data.

ATP Analogues for Structural Investigations: Case Studies of a DnaB Helicase and an ABC Transporter

“…The most suited for comparison appears to be the set of structures that shows the maltose ABC-transporter of E. coli (MBP-MalFGK2) in the outward-facing conformation, with two interacting NBDs and in the presence of AMPPNP (PDB 3RLF), ADP:BeF 3 (PDB 3PUX), ADP:Vi (PDB 3PUV), and ADP:AlF 4 - (PDB 3PUW) [ 64 ]. The collection of these different structures shapes the view of the transport cycle [ 122 ]. Chen and Oldham noticed that, despite the different ATP-analogues used, all residues within the NBD are essentially superimposable.…”

“…Background corrected 94 GHz EDNMR spectra of BmrA ( A ) and DnaB ( B ) incubated with Mn 2+ and ADP, with and without vanadate. For 31 P, a doublet due to the hyperfine coupling to 31 P is observed (~4 MHz), as well as a singlet not assigned so far [ 122 ]. 51 V spectra of DnaB:ADPMg:DNA:Vi recorded at a MAS frequency of 17 and 19 kHz ( C ).…”

The incredible diversity of structures and functions of ABC transporters

“…While cryo‐EM produced structures at an astonishing pace, there was a cost, since structures could precede biochemical validation of deduced mechanisms. Several excellent papers in this special issue are dedicated to this controversy, arguing against the idea that the cryo‐EM snapshots can be straightforwardly interpreted and used to easily resolve many of the existing controversies within the ABC transporter field [5,6]. Importantly, cryo‐EM structures captured along the entire frame of the catalytic cycle confirm the long suspected conformational flexibility as a fundamental feature of ABC proteins [7].…”

Multidrug Resistance in Mammals and Fungi—From MDR to PDR: A Rocky Road from Atomic Structures to Transport Mechanisms

“…Most ABC proteins from non-pathogenic baker’s yeasts ( Table 1 ) are also found in various numbers and functions in pathogenic fungi ( Table 2 ). A phylogenetic tree analysis suggests two major groups, referred to as exporter type I and type II [ 4 , 248 , 265 , 266 ], contain three MDR, MRP and PDR subfamilies ( Figure 1 A). While type I exporters (MDR/ABCB and MRP/ABCC) hold a TMD1­–NBD1–TMD2–NBD2 configuration, all PDR/ABCG type II exporters adopt a “reverse” architecture, such as NBD1–TMD1–NBD2–TMD2 ( Figure 1 B).…”