Biology subjects

Claxton, D. P.

Publications and source records attributed to Claxton, D. P..

6 recordsLinked to original sources

bioRxiv2026

Sequence determinants of pathogenicity in glucose-6-phosphatase linked to glycogen storage disease type 1a

Glycogen storage disease type 1a (GSD1a) is an autosomal recessive Mendelian disorder that can be caused by missense variants in glucose-6-phosphatase catalytic subunit 1 (G6PC1). Although hundreds of missense variants have been identified, the vast majority are of unknown clinical significance, and the molecular mechanism(s) of bona fide pathogenic variants are ill-defined. We combine bioinformatic data and clinical associations with the protein language model AlphaMissense to guide mechanistic exploration of 78 missense variants at 55 residue positions using robust biochemical and biophysical assays to distill general principles of enzyme dysfunction. Correlation analysis established a strong linear relationship between folded G6PC1 abundance and catalytic capacity for most variants. Pathogenic variants within this paradigm were linked to compromised stability and activation of the unfolded protein response. However, outliers characterized by relatively high abundance, yet low activity clustered to a network of sidechains adjacent to the active site that allosterically modulate catalysis. Contextualized by recent high-resolution structures and AlphaFold modeling, our holistic analysis of G6PC1 in vitro metrics facilitates clinical (re)classification of variants according to explicit molecular phenotypes and identifies therapeutic design directions. Moreover, our approach illustrates a blueprint for variant characterization that integrates computational prediction with experimental validation to discover disease etiology.

biochemistry↗

bioRxiv2023

Molecular mechanisms of functional impairment for active site mutations in glucose-6-phosphatase catalytic subunit 1 linked to glycogen storage disease type 1a.

Mediating the terminal reaction of gluconeogenesis and glycogenolysis, the integral membrane protein G6PC1 regulates hepatic glucose production by catalyzing hydrolysis of glucose-6-phosphate (G6P) within the lumen of the endoplasmic reticulum. Consistent with its vital contribution to glucose homeostasis, inactivating mutations in G6PC1 cause glycogen storage disease (GSD) type 1a characterized by hepatomegaly and severe hypoglycemia. Despite its physiological importance, the structural basis of G6P binding to G6PC1 and the molecular disruptions induced by missense mutations within the active site that give rise to GSD type 1a are unknown. Exploiting a computational model of G6PC1 derived from the groundbreaking structure prediction algorithm AlphaFold2 (AF2), we combine molecular dynamics (MD) simulations and computational predictions of thermodynamic stability with a robust in vitro screening platform to define the atomic interactions governing G6P binding as well as explore the energetic perturbations imposed by disease-linked variants. We identify a collection of side chains, including conserved residues from the signature phosphatidic acid phosphatase motif, that contribute to a hydrogen bonding and van der Waals network stabilizing G6P in the active site. Introduction of GSD type 1a mutations into the G6PC1 sequence elicits changes in G6P binding energy, thermostability and structural properties, suggesting multiple pathways of catalytic impairment. Our results, which corroborate the high quality of the AF2 model as a guide for experimental design and to interpret outcomes, not only confirm active site structural organization but also suggest novel mechanistic contributions of catalytic and non-catalytic side chains.

biophysics↗

bioRxiv2020

Structure and assembly of CAV1 8S complexes revealed by single particle electron microscopy

Highly stable oligomeric complexes of the monotopic membrane protein caveolin serve as fundamental building blocks of caveolae. Current evidence suggests these complexes are disc shaped, but the details of their structural organization and how they assemble are poorly understood. Here, we address these questions using single particle electron microscopy of negatively stained recombinant 8S complexes of human Caveolin-1. We show that 8S complexes are toroidal structures ~15 nm in diameter that consist of an outer ring, an inner ring, and central protruding stalk. Moreover, we map the position of the N- and C-termini and determine their role in complex assembly, and visualize the 8S complexes in heterologous caveolae. Our findings provide critical insights into the structural features of 8S complexes and allow us to propose a new model for how these highly stable membrane-embedded complexes are generated.

cell biology↗

bioRxiv2019

Sequence and Structural Determinants of Ligand-dependent Alternating Access of a MATE Transporter

MATE transporters are ubiquitous ion-coupled antiporters that extrude structurally- and chemically-dissimilar molecules and have been implicated in conferring multidrug resistance. Here, we integrate Double Electron Electron Resonance (DEER) in conjunction with functional assays and site-directed mutagenesis of conserved residues to illuminate principles of ligand-dependent alternating access of PfMATE, a proton-coupled MATE from the hyperthermophilic archaeon Pyrococcus furiosus. Pairs of spin labels monitoring the two sides of the transporter reconstituted into nanodiscs reveal large amplitude movement of helices that alter the orientation of a putative substrate binding cavity. We found that acidic pH favors formation of an inward-facing (IF) conformation, whereas elevated pH (>7) and the substrate rhodamine 6G stabilizes an outward-facing (OF) conformation. PfMATE isomerization between outward-facing and inward-facing conformations is driven by protonation of a previously unidentified intracellular glutamate residue that is critical for drug resistance. Our results can be framed in a mechanistic model of transport that addresses central aspects of ligand coupling and alternating access.

biophysics↗

bioRxiv2018

Expression and purification of a functional heteromeric GABAA receptor for structural studies

The multi-subunit GABA-gated chloride channels of the Cys-loop receptor family, known as GABAA receptors, function as the primary gatekeepers of fast inhibitory neurotransmission in the central nervous system. In addition to their role in controlling synaptic tone, these receptors are the targets of a vast array of therapeutic compounds that potentiate channel gating. Importantly, functional activity and pharmacological efficacy of GABAA receptors is coupled directly to the subunit composition. However, the absence of high resolution structural information precludes an explicit determination of the molecular mechanism of ligand binding to ion channel gating and modulation. Efforts to obtain this data are hindered largely by the lack of heterologous expression and purification protocols for high expressing receptor constructs. To address this issue, we describe a unique approach to identify bona fide functional GABAA receptor subunit combinations by using the Xenopus oocyte as an expression host in combination with fluorescence detection size exclusion chromatography. The results demonstrate that formation of a defined pentameric species is dependent on subunit composition. Furthermore, receptor subunits can tolerate large truncations in non-conserved M3/M4 cytoplasmic loop, although removal of N-linked glycosylation sites is negatively correlated with expression level. Additionally, we report methods to improve GABAA receptor expression in mammalian cell culture that employ recombinant baculovirus transduction. From these methods we have identified a well-behaving minimal functional construct for the 1/{beta}1 GABAA receptor subtype that can be purified in milligram quantities while retaining high affinity agonist binding activity.

biochemistry↗

bioRxiv2018

Engineering of a Polydisperse Small Heat-Shock Protein Reveals Conserved Motifs of Oligomer Plasticity

Small heat-shock proteins (sHSP) are molecular chaperones that bind and sequester partially and globally unfolded states of their client proteins. Of paramount importance to their physiological roles is the assembly into large oligomers, which for mammalian sHSP are polydisperse and undergo subunit exchange. The flexibility and dynamic nature of these oligomers mediates functional regulation by phosphorylation and underpins the deleterious effects of disease-linked mutations. Previously, we discovered that the archaeal Hsp16.5, which natively forms ordered and symmetric 24-subunit oligomers, can be engineered to transition to an ordered and symmetric 48-subunit oligomer by insertion of a peptide from human HspB1 (Hsp27) at the junction of the N-terminal and -crystallin domains. Here, we carried out a detailed analysis of the determinants of Hsp16.5 oligomeric plasticity by altering the sequence and length of the inserted peptide. Utilizing light scattering, blue native gel electrophoresis, native mass spectrometry and electron microscopy, we uncovered the existence of an array of oligomeric states (30 to 38 subunits) that can be populated as a consequence of different insertions. These oligomers are intermediate states on the assembly pathway of the 48-subunit oligomer as two of the variants can concurrently form 24-subunit or 30-38 subunit polydisperse oligomers. Polydisperse Hsp16.5 oligomers displayed higher affinity to a model client protein consistent with a general mechanism for recognition and binding that involves increased access of the hydrophobic N-terminal region. Our findings, which integrate structural and functional analyses from evolutionarily-distant sHSP, support a model wherein the modular architecture of these proteins encodes motifs of oligomer polydispersity, dissociation and expansion to achieve functional diversity and regulation.

biochemistry↗