Bradshaw, R. T. | Search bioRxiv

bioRxiv2020

Chloride-dependent conformational changes in the GlyT1 glycine transporter

The human GlyT1 glycine transporter requires chloride for its function. However, the mechanism by which Cl- exerts its influence is unknown. To examine the role that Cl- plays in the transport cycle, we measured the effect of Cl- on both glycine binding and conformational changes. The ability of glycine to displace the high-affinity radioligand [3H]CHIBA-3007 required Na+ and was potentiated over 1000-fold by Cl-. We generated GlyT1b mutants containing reactive cysteine residues in either the extracellular or cytoplasmic permeation pathways and measured changes in the reactivity of those cysteine residues as indicators of conformational changes in response to ions and substrate. Na+ increased accessibility in the extracellular pathway and decreased it in the cytoplasmic pathway, consistent with stabilizing an outward-open conformation as observed in other members of this transporter family. In the presence of Na+, both glycine and Cl- independently shifted the conformation of GlyT1b toward an outward-closed conformation. Together, Na+, glycine and Cl- stabilized an inward-open conformation of GlyT1b. We then examined whether Cl- acts by interacting with a conserved glutamine to allow formation of an ion pair that stabilizes the closed state of the extracellular pathway. Molecular dynamics simulations of a GlyT1 homologue indicated that this ion pair is formed more frequently as that pathway closes. Mutation of the glutamine blocked the effect of Cl-, and substituting it with glutamate or lysine resulted in outward- or inward-facing transporter conformations, respectively. These results provide novel and unexpected insight into the role of Cl- in this family of transporters.

biochemistry↗

bioRxiv2019

Structural Interpretation of Hydrogen-Deuterium Exchange with Maximum-Entropy Simulation Reweighting

Hydrogen-deuterium exchange combined with mass spectrometry (HDX-MS) is a widely applied biophysical technique that probes the structure and dynamics of biomolecules in native environments without the need for site-directed modifications or bio-orthogonal labels. The mechanistic interpretation of measured HDX data, however, is often qualitative and subjective, owing to a lack of quantitative methods to rigorously translate observed deuteration levels into atomistic structural information. To help address this problem, we have developed a methodology to generate structural ensembles that faithfully reproduce HDX-MS measurements. In this approach, an ensemble of protein conformations is first generated, typically using molecular dynamics simulations. A maximum-entropy bias is then applied post-hoc to the resulting ensemble, such that averaged peptide-deuteration levels, as predicted by an empirical model of a value called the protection factor, agree with target values within a given level of uncertainty. We evaluate this approach, referred to as HDX ensemble reweighting (HDXer), for artificial target data reflecting the two major conformational states of a binding protein. We demonstrate that the information provided by HDX-MS experiments, and by the model of exchange, are sufficient to recover correctly-weighted structural ensembles from simulations, even when the relevant conformations are observed rarely. Degrading the information content of the target data, e.g., by reducing sequence coverage or by averaging exchange levels over longer peptide segments, reduces the quantitative structural accuracy of the reweighted ensemble but still allows for useful, molecular-level insights into the distinctive structural features reflected by the target data. Finally, we describe a quantitative metric with which candidate structural ensembles can be ranked based on their correspondence with target data, or revealed to be inadequate. Thus, not only does HDXer facilitate a rigorous mechanistic interpretation of HDX-MS measurements, but it may also inform experimental design and further the development of empirical models of the HDX reaction.\n\nStatement of significanceHDX-MS experiments are a powerful approach for probing the conformational dynamics and mechanisms of proteins. However, the mechanistic implications of HDX-MS observations are frequently difficult to interpret, due to the limited spatial resolution of the technique as well as the lack of quantitative tools to translate measured data into structural information. To overcome these problems, we have developed a computational approach to construct structural ensembles that are maximally diverse while reproducing target experimental HDX-MS data within a given level of uncertainty. Using artificial test data, we demonstrate that the approach can correctly discern distinct structural ensembles reflected in the target data, and thereby facilitate statistically robust evaluations of competing mechanistic interpretations of HDX-MS experiments.

biophysics↗