Mutations that alter the transmembrane signalling pathway in an ATP binding cassette (ABC) transporter.

Abstract

The maltose transport system of Escherichia coli is a well‐characterized member of the ATP binding cassette transporter superfamily. Members of this family share sequence similarity surrounding two short sequences (the Walker A and B sequences) which constitute a nucleotide binding pocket. It is likely that the energy from binding and hydrolysis of ATP is used to accomplish the translocation of substrate from one location to another. Periplasmic binding protein‐dependent transport systems, like the maltose transport system of E.coli, possess a water‐soluble ligand binding protein that is essential for transport activity. In addition to delivering ligand to the membrane‐bound components of the system on the external face of the membrane, the interaction of the binding protein with the membrane complex initiates a signal that is transmitted to the ATP binding subunit on the cytosolic side and stimulates its hydrolytic activity. Mutations that alter the membrane complex so that it transports independently of the periplasmic binding protein also result in constitutive activation of the ATPase. Genetic analysis indicates that, in general, two mutations are required for binding protein‐independent transport and constitutive ATPase. The mutations alter residues that cluster to specific regions within the membrane spanning segments of the integral membrane components MalF and MalG. Individually, the mutations perturb the ability of MBP to interact productively with the membrane complex. Genetic alteration of this signalling pathway suggests that other agents might have similar effects. These could be potentially useful for modulating the activities of ABC transporters such as P‐glycoprotein or CFTR, that are implicated in disease.

Structure, Function, and Evolution of Bacterial ATP-Binding Cassette Systems

“…The presumed translocation pathway through the center of the molecule is closed at the external surface and open to the cytoplasm and is shielded from the membrane bilayer. Intriguingly, the BP is bound to the transporter in an orientation that places the substrate-binding cleft over the presumed gate to the translocation channel, and both lobes of the BP contact the transporter, consistent with earlier predictions for maltose-BP (80,282,303). Residues involved in gating access of the translocation channel to the outside of the cell reside in TM helix 3 and TM helix 5 and consist of two highly conserved motifs in the MOI family of molybdate, sulfate, and phosphate importers (205).…”

“…Since the rates of transport by these mutants were too low to be measured readily, they may represent a population of transporters in which the energy barrier associated with conformational change is just marginally overcome by mutation. Interestingly, the point mutations leading to BP-independent transport in this system are located exclusively in the ATPase subunit, HisP, while those in the maltose system are located exclusively in the TM subunits, MalF and MalG (80,459). Another difference is that only single point mutations were observed in HisP, while almost all BP-independent mutants in MalF/MalG contained two point mutations.…”

Two Closely Related ABC Transporters in Streptococcus mutans Are Involved in Disaccharide and/or Oligosaccharide Uptake

“…However, although there are a number of examples of multiple SBPs interacting with a single ABC transporter and refining the specificity of the transporter complex (12,19), we know of no precedent for a single SBP interacting with multiple ABC transporters. Moreover, while the solute specificity of ABC transporters is principally determined by the SBP, it is known that the membrane domains also contribute to specificity (7,24,36,38). Also, MalX and MsmE are only 28% identical, and the membrane domains have only 48% and 29% identity.…”

Characterization of transmembrane domains 6, 7, and 8 of MalF by mutational analysis

“…To date, the only structure-function data available indicate that the amino-terminal 39 amino acids of MalF, including the first transmembrane domain, are dispensable for function (12). The isolation and analysis of MBP-independent mutations in MalF and MalG (6) and suppressor mutations in MBP (33) led to the postulation that possible interaction sites exist in MBP. MBP-independent mutations lead to constitutive hydrolysis of ATP by the MalK subunit (9).…”