Biology subjects

Graham, T. R.

Publications and source records attributed to Graham, T. R..

11 recordsLinked to original sources

bioRxiv2025

P4-ATPase control over phosphoinositide membrane asymmetry and neomycin resistance

Neomycin, an aminoglycoside antibiotic, has robust antibacterial properties, yet its clinical utility is curtailed by its nephrotoxicity and ototoxicity. The mechanism by which the polycationic neomycin enters specific eukaryotic cell types remains poorly understood. In budding yeast, NEO1 is required for neomycin resistance and encodes a phospholipid flippase that establishes membrane asymmetry. Here, we show that mutations altering Neo1 substrate recognition cause neomycin hypersensitivity by exposing phosphatidylinositol-4-phosphate (PI4P) in the plasma membrane extracellular leaflet. Human cells also expose extracellular PI4P upon knockdown of ATP9A, a Neo1 ortholog and ATP9A expression level correlates to neomycin sensitivity. In yeast, the extracellular PI4P is initially produced in the cytosolic leaflet of the plasma membrane and then delivered by Osh6-dependent nonvesicular transport to the endoplasmic reticulum (ER). Here, a portion of PI4P escapes degradation by the Sac1 phosphatase by entering the ER lumenal leaflet. COPII vesicles transport lumenal PI4P to the Golgi where Neo1 flips this substrate back to the cytosolic leaflet. Cryo-EM reveals that PI4P binds Neo1 within the substrate translocation pathway. Loss of Neo1 activity in the Golgi allows secretion of extracellular PI4P, which serves as a neomycin receptor and facilitates its endocytic uptake. These findings unveil novel mechanisms of aminoglycoside sensitivity and phosphoinositide homeostasis, with important implications for signaling by extracellular phosphoinositides.

cell biology↗

bioRxiv2023

ATP10A promotes endothelial cell insulin sensitivity and protects against diet-induced dyslipidemia in female mice

Genetic association studies have linked ATP10A and closely related type IV P-type ATPases (P4-ATPases) to insulin resistance and vascular complications, such as atherosclerosis. ATP10A translocates phosphatidylcholine and glucosylceramide across cell membranes, and these lipids or their metabolites play important roles in signal transduction pathways regulating metabolism. However, the influence of ATP10A on lipid metabolism in mice has not been explored. Here, we generated gene-specific Atp10A knockout mice and show that Atp10A-/- mice fed a high-fat diet did not gain excess weight relative to wild-type littermates. However, Atp10A-/-mice displayed female-specific dyslipidemia characterized by elevated plasma triglycerides, free fatty acids and cholesterol, as well as altered VLDL and HDL properties. We also observed increased circulating levels of several sphingolipid species along with reduced levels of eicosanoids and bile acids. The Atp10A-/- mice also displayed hepatic insulin resistance without perturbations to whole-body glucose homeostasis. Thus, ATP10A has a sex-specific role in regulating plasma lipid composition and maintaining hepatic liver insulin sensitivity in mice.

molecular biology↗

bioRxiv2022

Lipid transport by Candida albicans Dnf2 is required for hyphal growth and virulence

Candida albicans is a common cause of human mucosal yeast infections, and invasive candidiasis can be fatal. Antifungal medications are limited, but those targeting the pathogen cell wall or plasma membrane have been effective. Therefore, virulence factors controlling membrane biogenesis are potential targets for drug development. P4-ATPases contribute to membrane biogenesis by selecting and transporting specific lipids from the extracellular leaflet to the cytoplasmic leaflet of the bilayer to generate lipid asymmetry. A subset of heterodimeric P4-ATPases, including Dnf1-Lem3 and Dnf2-Lem3 from Saccharomyces cerevisiae, transport phosphatidylcholine (PC), phosphatidylethanolamine (PE), and the sphingolipid glucosylceramide (GlcCer). GlcCer is a critical lipid for Candida albicans polarized growth and virulence, but the role of GlcCer transporters in virulence has not been explored. Here we show that the Candida albicans Dnf2 (CaDnf2) requires association with CaLem3 to form a functional transporter and flip fluorescent derivatives of GlcCer, PC and PE across the plasma membrane. Mutation of conserved substrate-selective residues in the membrane domain strongly abrogates GlcCer transport and partially disrupts PC transport by CaDnf2. Candida strains harboring dnf2 null alleles (dnf2{Delta}{Delta}) or point mutations that disrupt substrate recognition exhibit defects in the yeast to hyphal growth transition, filamentous growth and virulence in systemically infected mice. The influence of CaDNF1 deletion on the morphological phenotypes is negligible although the dnf1{Delta}{Delta} dnf2{Delta}{Delta} strain was less virulent than the dnf2{Delta}{Delta} strain. These results indicate that the transport of GlcCer and/or PC by plasma membrane P4-ATPases is important for pathogenicity of Candida albicans.

microbiology↗

bioRxiv2022

An interaction between betaprime-COP and the ArfGAP, Glo3, maintains post-Golgi cargo recycling

The essential COPI vesicular coat mediates retrieval of key transmembrane proteins at the Golgi and endosomes following recruitment by the small GTPase, Arf1. ArfGAP proteins regulate COPI coats, but molecular details for COPI recognition by ArfGAP proteins remain elusive. Biochemical and biophysical data reveal how {beta}-COP propeller domains directly engage the yeast ArfGAP, Glo3, with a low micromolar binding affinity (KD ~1 {micro}M). Calorimetry data demonstrate both {beta}-COP propeller domains are required to bind Glo3 using electrostatic interactions. An acidic patch on {beta}-COP (D437/D450) interacts with Glo3 lysine residues located within the BoCCS (Binding of Coatomer, Cargo, and SNAREs) region. Targeted point mutations in either Glo3 BoCCS or {beta}-COP abrogate the interaction in vitro, and loss of the {beta}-COP/Glo3 interaction drives Ste2 mis-sorting to the vacuole and aberrant Golgi morphology in budding yeast. Together, these data suggest cells require the {beta}-COP/Glo3 interaction for cargo recycling via endosomes and the TGN, where {beta}-COP may serve as a molecular platform to coordinate binding to multiple protein partners, including Glo3, Arf1, and the COPI F-subcomplex.

cell biology↗

bioRxiv2021

Ubiquitination drives COPI priming and Golgi SNARE localization

Deciphering mechanisms controlling SNARE localization within the Golgi complex is crucial to understanding protein trafficking patterns within the secretory pathway. SNAREs are also thought to prime COPI assembly to ensure incorporation of these essential cargoes into vesicles but the regulation of these events is poorly understood. Here we report a roles for ubiquitin recognition by COPI in SNARE trafficking and in stabilizing interactions between Arf, COPI, and Golgi SNAREs. The ability of COPI to bind ubiquitin through its N-terminal WD repeat domain of {beta}COP or through an unrelated ubiquitin-binding domain (UBD) is essential for the proper localization of Golgi SNAREs Bet1 and Gos1. We find that COPI, the ArfGAP Glo3 and multiple Golgi SNAREs are ubiquitinated. Notably, the binding of Arf and COPI to Gos1 is markedly enhanced by ubiquitination of these components. Glo3 is thought to prime COPI-SNARE interactions; however, Glo3 is not enriched in the ubiquitin-stabilized SNARE-Arf-COPI complex but is instead enriched with COPI complexes that lack SNAREs. These results support a new model for how posttranslational modifications drive COPI priming events crucial for Golgi SNARE localization.

cell biology↗

bioRxiv2021

Structural basis of the P4B ATPase lipid flippase activity

P4 ATPases are lipid flippases that are phylogenetically grouped into P4A, P4B and P4C clades. The P4A ATPases are heterodimers composed of a catalytic -subunit and accessory {beta}-subunit, and the structures of several heterodimeric flippases have been reported. The S. cerevisiae Neo1 and its orthologs represent the P4B ATPases, which function as monomeric flippases without a {beta}-subunit. It has been unclear whether monomeric flippases retain the architecture and transport mechanism of the dimeric flippases. Here we report the first structure of a P4B ATPase, Neo1, in its E1-ATP, E2P-transition, and E2P states. The structure reveals a conserved architecture as well as highly similar functional intermediate states relative to dimeric flippases. Consistently, structure-guided mutagenesis of residues in the proposed substrate translocation path disrupted Neo1s ability to establish membrane asymmetry. These observations indicate that evolutionarily distant P4 ATPases use a structurally conserved mechanism for substrate transport.

biochemistry↗

bioRxiv2020

Transport mechanism of class-3 P4 ATPase lipid flippases

The P4 ATPases are a large family of membrane-embedded enzymes that use ATP hydrolysis to transport large lipid substrates across lipid bilayers. P4 ATPases differ in their cellular membrane location and their substrates. The structures of the endosome- and Golgi-localized class-1 phosphatidylserine flippases--such as the yeast Drs2 and human ATP8A1--have recently been reported, revealing a substrate binding site on the lumenal side and several transport states. However, a substrate binding site on the cytosolic side has not been found, and the transport mechanisms of P4 ATPases in other classes are still unknown. Here we report a systematic structural and functional study on two plasma-membrane localized, class-3 P4 ATPases that have broader substrate specificity, the S. cerevisiae Dnf1-Lem3 and Dnf2-Lem3 complexes. We have captured substrate lipids on both the exoplasmic and cytosolic sides, and we found that these two enzymes have very similar structures, consistent with their high sequence identity and redundant function. Unexpectedly, Lem3 contributes to substrate binding near the cytosolic surface. We found that the conformational transitions through the substrate transport cycle of these two class-3 enzymes match those of the class-1 enzymes, suggesting a conserved lipid-flipping mechanism among all classes of the P4 ATPases. Our study also revealed a helix-turn-helix insertion in the cytosolic P domain that is unique to the class-3 enzymes and plays a crucial role in their function. Therefore, the P4 ATPases may have retained an overall transport mechanism while evolving distinct features for cellular membrane localization, regulatory mechanisms, and transporting different lipid substrates.

biochemistry↗

bioRxiv2020

Exofacial membrane composition and lipid metabolism regulates plasma membrane P4-ATPase substrate specificity

The plasma membrane of a cell is characterized by an asymmetric distribution of lipid species across the exofacial and cytofacial aspects of the bilayer. The regulation of membrane asymmetry is a fundamental characteristic of membrane biology, and is crucial for signal transduction, vesicle transport, and cell division. The type-IV family of P-ATPases, or P4-ATPases, establish membrane asymmetry by selection and transfer of a subset of membrane lipids from the lumenal or exofacial leaflet to the cytofacial aspect of the bilayer. It is still unclear how these enzymes sort through the spectrum of lipids within the membrane to identify their desired substrate(s) and how the membrane environment modulates this activity. Therefore, we tested how the yeast plasma membrane P4-ATPase, Dnf2, responds to changes in membrane composition induced by perturbation of endogenous lipid biosynthetic pathways or exogenous application of lipid. The primary substrates of Dnf2 are two chemically divergent lipids, glucosylceramide (GlcCer) and phosphatidylcholine ((PC) or their lyso-lipid derivatives), and we find that these substrates compete with each other for transport. Acutely inhibiting sphingolipid synthesis using myriocin attenuates transport of exogenously applied GlcCer without perturbing PC transport. Deletion of genes controlling later steps of glycosphingolipid production also perturb GlcCer transport to a greater extent than PC transport. Surprisingly, application of lipids that are poor transport substrates differentially affect PC and GlcCer transport by Dnf2, thus altering substrate preference. Our data indicate that Dnf2 exhibits exquisite sensitivity to the membrane composition; thus, providing feedback onto the function of the P4-ATPases.

cell biology↗

bioRxiv2019

Mammalian retromer is an adaptable scaffold for cargo sorting from endosomes

In metazoans, retromer (VPS26/VPS35/VPS29) associates with sorting nexin (SNX) proteins to form coats on endosomal tubules and sort cargo proteins to the trans-Golgi network (TGN) or plasma membrane. This core complex is highly conserved from yeast to humans, but molecular mechanisms of metazoan retromer assembly remain undefined. Here we combine single particle cryo-electron microscopy with biophysical methods to uncover multiple oligomer structures formed by mammalian retromer. Two-dimensional class averages in ice reveal the retromer heterotrimer; dimers of trimers; tetramers of trimers; and flat chains. These species are further supported by biophysical studies in solution. We provide cryo-EM reconstructions of all species, including pseudo-atomic resolution detail for key sub-structures. Multi-body refinement demonstrates how retromer heterotrimers and dimers adopt a range of conformations. Our structures identify a flexible yet highly conserved electrostatic interface in dimers formed by interactions between VPS35 subunits. We generate a structure-based mutant to disrupt this key interface in vitro and introduce equivalent mutations into S. cerevisiae to demonstrate the mutant exhibits a cargo sorting defect. Together, structures and complementary functional data in budding yeast imply a conserved assembly interface across eukaryotes. These data further suggest mammalian retromer acts as an adaptable and plastic scaffold that accommodates interactions with different SNXs to sort multiple cargoes from endosomes their final destinations.

cell biology↗

bioRxiv2018

Identification and characterization of yeast and human glycosphingolipid flippases

Lipid transport is an essential process with manifest importance to human health and disease. Phospholipid flippases (P4-ATPases) transport lipids across the membrane bilayer, and are involved in signal transduction, cell division, and vesicular transport. Mutations in flippase genes cause or contribute to a host of diseases such as cholestasis, neurological deficits, immunological dysfunction, and metabolic disease. Genome-wide association studies have shown that ATP10A and ATP10D variants are associated with an increased risk of diabetes, obesity, myocardial infarction, and atherosclerosis; and ATP10D SNPs are associated with elevated levels of glucosylceramide (GlcCer) in plasma from diverse European populations. Although sphingolipids are strong contributors to metabolic disease, little is known about how GlcCer is transported across cell membranes. We have identified an evolutionarily conserved clade of P4-ATPases from Saccharomyces cerevisiae (Dnf1, Dnf2), Schizosaccharomyces pombe (Dnf2), and Homo sapiens (ATP10A, ATP10D) that transport GlcCer. Further, we establish the structural determinants necessary for the specific recognition of this sphingolipid substrate. Our molecular observations clarify the relationship between these flippases and human disease, and have fundamental implications for membrane organization and sphingolipid homeostasis.

biochemistry↗

bioRxiv2017

COPI Mediates Recycling Of An Exocytic SNARE From Endosomes By Recognition Of A Ubiquitin Sorting Signal

ABSTRACTThe COPI coat forms transport vesicles from the Golgi complex and plays a poorly defined role in endocytic trafficking. Here we show that COPI mediates delivery of a budding yeast SNARE (Snc1) from early endosomes to the Golgi complex through recognition of a polyubiquitin sorting signal. Snc1 is a v-SNARE that drives fusion of exocytic vesicles with the plasma membrane, and then recycles through early endosomes back to the Golgi for reuse. Removal of ubiquitin from Snc1, or deletion of a {beta}-COP subunit propeller domain that binds K63-linked polyubiquitin, causes aberrant accumulation of Snc1 in early endosomes. Moreover, replacement of the {beta}-COP propeller domain with unrelated ubiquitin-binding domains restores Snc1 recycling. These results indicate that ubiquitination, a modification well known to target membrane proteins to the lysosome or vacuole for degradation, can also function as recycling signal to sort a SNARE into COPI vesicles at early endosomes for Golgi delivery.

cell biology↗