Nucleotide-free MalK drives the transition of the maltose transporter to the inward-facing conformation

A Mathematical Model for the Kinetics of the MalFGK[Formula: see text] Maltose Transporter

The MalFGK[Formula: see text] transporter regulates the movement of maltose across the inner membrane of E. coli and serves as a model system for bacterial ATP binding cassette (ABC) importers. Despite the wealth of biochemical and structural data available, a general model describing the various translocation pathways is still lacking. In this study, we formulate a mathematical model with the goal of determining the transporter reaction pathway, specifically looking at the order of binding events and conformation changes by which transport proceeds. Fitting our mathematical model to equilibrium binding data, we estimate the unknown equilibrium parameters of the system, several of which are key determinants of the transport process. Using these estimates along with steady-state ATPase rate data, we determine which of several possible reaction pathways is dominant, as a function of five underdetermined kinetic parameter values. Because neither experimental measurements nor estimates of certain kinetic rate constants are available, the problem of deciding which of the reaction pathways is responsible for transport remains unsolved. However, using the mathematical framework developed here, a firmer conclusion regarding the dominant reaction pathway as a function of MalE and maltose concentration could be drawn once these unknown kinetic parameters are determined.

An integrated transport mechanism of the maltose ABC importer

ATP-binding cassette (ABC) transporters use the energy of ATP hydrolysis to transport a large diversity of molecules actively across biological membranes. A combination of biochemical, biophysical, and structural studies has established the maltose transporter MalFGK₂ as one of the best characterized proteins of the ABC family. MalF and MalG are the transmembrane domains, and two MalKs form a homodimer of nucleotide-binding domains. A periplasmic maltose-binding protein (MalE) delivers maltose and other maltodextrins to the transporter, and triggers its ATPase activity. Substrate import occurs in a unidirectional manner by ATP-driven conformational changes in MalK₂ that allow alternating access of the substrate-binding site in MalF to each side of the membrane. In this review, we present an integrated molecular mechanism of the transport process considering all currently available information. Furthermore, we summarize remaining inconsistencies and outline possible future routes to decipher the full mechanistic details of transport by MalEFGK₂ complex and that of related importer systems.

Profiling the Escherichia coli membrane protein interactome captured in Peptidisc libraries

Protein-correlation-profiling (PCP), in combination with quantitative proteomics, has emerged as a high-throughput method for the rapid identification of dynamic protein complexes in native conditions. While PCP has been successfully applied to soluble proteomes, characterization of the membrane interactome has lagged, partly due to the necessary use of detergents to maintain protein solubility. Here, we apply the peptidisc, a ‘one-size fits all’ membrane mimetic, for the capture of the Escherichia coli cell envelope proteome and its high-resolution fractionation in the absence of detergent. Analysis of the SILAC-labeled peptidisc library via PCP allows generation of over 4900 possible binary interactions out of >700,000 random associations. Using well-characterized membrane protein systems such as the SecY translocon, the Bam complex and the MetNI transporter, we demonstrate that our dataset is a useful resource for identifying transient and surprisingly novel protein interactions. For example, we discover a trans-periplasmic supercomplex comprising subunits of the Bam and Sec machineries, including membrane-bound chaperones YfgM and PpiD. We identify RcsF and OmpA as bone fide interactors of BamA, and we show that MetQ association with the ABC transporter MetNI depends on its N-terminal lipid anchor. We also discover NlpA as a novel interactor of MetNI complex. Most of these interactions are largely undetected by standard detergent-based purification. Together, the peptidisc workflow applied to the proteomic field is emerging as a promising novel approach to characterize membrane protein interactions under native expression conditions and without genetic manipulation.

The mechanism of nucleotide-binding domain dimerization in the intact maltose transporter as studied by all-atom molecular dynamics simulations

The Escherichia coli maltose transporter MalFGK₂ -E belongs to the protein superfamily of ATP-binding cassette (ABC) transporters. This protein is composed of heterodimeric transmembrane domains (TMDs) MalF and MalG, and the homodimeric nucleotide-binding domains (NBDs) MalK₂ . In addition to the TMDs and NBDs, the periplasmic maltose binding protein MalE captures maltose and shuttle it to the transporter. In this study, we performed all-atom molecular dynamics (MD) simulations on the maltose transporter and found that both the binding of MalE to the periplasmic side of the TMDs and binding of ATP to the MalK₂ are necessary to facilitate the conformational change from the inward-facing state to the occluded state, in which MalK₂ is completely dimerized. MalE binding suppressed the fluctuation of the TMDs and MalF periplasmic region (MalF-P2), and thus prevented the incorrect arrangement of the MalF C-terminal (TM8) helix. Without MalE binding, the MalF TM8 helix showed a tendency to intrude into the substrate translocation pathway, hindering the closure of the MalK₂ . This observation is consistent with previous mutagenesis experimental results on MalF and provides a new point of view regarding the understanding of the substrate translocation mechanism of the maltose transporter.

Negative Stain Single-particle EM of the Maltose Transporter in Nanodiscs Reveals Asymmetric Closure of MalK2 and Catalytic Roles of ATP, MalE, and Maltose

The Escherichia coli MalE-MalFGK₂ complex is one of the best characterized members of the large and ubiquitous family of ATP-binding cassette (ABC) transporters. It is composed of a membrane-spanning heterodimer, MalF-MalG; a homodimeric ATPase, MalK₂; and a periplasmic maltose receptor, MalE. Opening and closure of MalK₂ is coupled to conformational changes in MalF-MalG and the alternate exposition of the substrate-binding site to either side of the membrane. To further define this alternate access mechanism and the impact of ATP, MalE, and maltose on the conformation of the transporter during the transport cycle, we have reconstituted MalFGK₂ in nanodiscs and analyzed its conformations under 10 different biochemical conditions using negative stain single-particle EM. EM map results (at 15-25 Å resolution) indicate that binding of ATP to MalK₂ promotes an asymmetric, semi-closed conformation in accordance with the low ATPase activity of MalFGK₂ In the presence of MalE, the MalK dimer becomes fully closed, gaining the ability to hydrolyze ATP. In the presence of ADP or maltose, MalE·MalFGK₂ remains essentially in a semi-closed symmetric conformation, indicating that release of these ligands is required for the return to the initial state. Taken together, this structural information provides a rationale for the stimulation of MalK ATPase activity by MalE as well as by maltose.

Coupling between ATP hydrolysis and protein conformational change in maltose transporter

As the intracellular part of maltose transporter, MalK dimer utilizes the energy of ATP hydrolysis to drive protein conformational change, which then facilitates substrate transport. Free energy evaluation of the complete conformational change before and after ATP hydrolysis is helpful to elucidate the mechanism of chemical-to-mechanical energy conversion in MalK dimer, but is lacking in previous studies. In this work, we used molecular dynamics simulations to investigate the structural transition of MalK dimer among closed, semi-open and open states. We observed spontaneous structural transition from closed to open state in the ADP-bound system and partial closure of MalK dimer from the semi-open state in the ATP-bound system. Subsequently, we calculated the reaction pathways connecting the closed and open states for the ATP- and ADP-bound systems and evaluated the free energy profiles along the paths. Our results suggested that the closed state is stable in the presence of ATP but is markedly destabilized when ATP is hydrolyzed to ADP, which thus explains the coupling between ATP hydrolysis and protein conformational change of MalK dimer in thermodynamics. Proteins 2017; 85:207-220. © 2016 Wiley Periodicals, Inc.

Formation of a Chloride-conducting State in the Maltose ATP-binding Cassette (ABC) Transporter

ATP-binding cassette transporters use an alternating access mechanism to move substrates across cellular membranes. This mode of transport ensures the selective passage of molecules while preserving membrane impermeability. The crystal structures of MalFGK2, inward- and outward-facing, show that the transporter is sealed against ions and small molecules. It has yet to be determined whether membrane impermeability is maintained when MalFGK2 cycles between these two conformations. Through the use of a mutant that resides in intermediate conformations close to the transition state, we demonstrate that not only is chloride conductance occurring, but also to a degree large enough to compromise cell viability. Introduction of mutations in the periplasmic gate lead to the formation of a channel that is quasi-permanently open. MalFGK2 must therefore stay away from these ion-conducting conformations to preserve the membrane barrier; otherwise, a few mutations that increase access to the ion-conducting states are enough to convert an ATP-binding cassette transporter into a channel.

Sequential Action of MalE and Maltose Allows Coupling ATP Hydrolysis to Translocation in the MalFGK2 Transporter

ATP-binding cassette (ABC) transporters have evolved an ATP-dependent alternating-access mechanism to transport substrates across membranes. Despite important progress, especially in their structural analysis, it is still unknown how the substrate stimulates ATP hydrolysis, the hallmark of ABC transporters. In this study, we measure the ATP turnover cycle of MalFGK2 in steady and pre-steady state conditions. We show that (i) the basal ATPase activity of MalFGK2 is very low because the cleavage of ATP is rate-limiting, (ii) the binding of open-state MalE to the transporter induces ATP cleavage but leaves release of Pi limiting, and (iii) the additional presence of maltose stimulates release of Pi, and therefore increases the overall ATP turnover cycle. We conclude that open-state MalE stabilizes MalFGK2 in the outward-facing conformation until maltose triggers return to the inward-facing state for substrate and Pi release. This concerted action explains why ATPase activity of MalFGK2 depends on maltose, and why MalE is essential for transport.

An annular lipid belt is essential for allosteric coupling and viral inhibition of the antigen translocation complex TAP (transporter associated with antigen processing)

The transporter associated with antigen processing (TAP) constitutes a focal element in the adaptive immune response against infected or malignantly transformed cells. TAP shuttles proteasomal degradation products into the lumen of the endoplasmic reticulum for loading of major histocompatibility complex (MHC) class I molecules. Here, the heterodimeric TAP complex was purified and reconstituted in nanodiscs in defined stoichiometry. We demonstrate that a single heterodimeric core-TAP complex is active in peptide binding, which is tightly coupled to ATP hydrolysis. Notably, with increasing peptide length, the ATP turnover was gradually decreased, revealing that ATP hydrolysis is coupled to the movement of peptide through the ATP-binding cassette transporter. In addition, all-atom molecular dynamics simulations show that the observed 22 lipids are sufficient to form an annular belt surrounding the TAP complex. This lipid belt is essential for high affinity inhibition by the herpesvirus immune evasin ICP47. In conclusion, nanodiscs are a powerful approach to study the important role of lipids as well as the function, interaction, and modulation of the antigen translocation machinery.