ATP binding cassette (ABC) transporters employ ATP hydrolysis to harness substrate translocation across membranes. The Escherichia coli MalFGK2E maltose importer is an example of a type I ABC importer and a model system for this class of ABC transporters. The MalFGK2E importer is responsible for the intake of malto-oligosaccharides in E.coli. Despite being extensively studied, little is known about the effect of ATP hydrolysis and nucleotide exit on substrate transport. In this work, we studied this phenomenon using extensive molecular dynamics simulations (MD) along with the potential of mean force calculations of maltose transport across the pore, in the pre-hydrolysis, post-hydrolysis, and nucleotide-free states. We concluded that ATP hydrolysis and nucleotide exit trigger conformational changes that result in the decrease of energetic barriers to maltose translocation towards the cytoplasm, with a concomitant increase of the energy barrier in the periplasmic side of the pore, contributing to the irreversibility of the process.
Fig: Structures used in this work. (a) Initial structure for simulating the ATP state. This structure contains an ATP analog bound in the NBDs. The post-hydrolysis state was generated from this structure. PBD code: 3RLF. (b) The initial structure used to mimic the protein after nucleotide exit. This structure reflects the pre-translocation state and can be used to extrapolate the state after nucleotide exit. PBD code: 3PV0. The MalE is represented in bright pink, MalF in cyan, MalG in green, and MalK in yellow and salmon. ATP is represented in teal spheres and maltose in purple spheres. (c) Transport cycle in the maltose importer. MalE is represented in purple, the TMDs in orange, the NBDs in cyan, ATP in red spheres, maltose as a blue sphere, and ADP and Pi as purple and green semi-circles. 1- ATP binding and MalE loading with maltose. 2- MalE binding to the complex. 3- Maltose diffusion in the complex. 4- ATP hydrolysis, the release of the substrate and hydrolysis products, and transporter reset.
We also identified key residues that aid in the positioning and orientation of maltose, as well as a novel binding pocket for maltose in MalG. Additionally, ATP hydrolysis leads to conformations similar to the nucleotide-free state. This study shows the contribution of ATP hydrolysis and nucleotide exit in the transport cycle, shedding light on ABC type I importer mechanisms.
Abreu, B., Cruz, C., Oliveira, A.S.F. et al. ATP hydrolysis and nucleotide exit enhance maltose translocation in the MalFGK2E importer. Sci Rep 11, 10591 (2021). https://doi.org/10.1038/s41598-021-89556-y