Biology subjects

Johnston, H. E.

Publications and source records attributed to Johnston, H. E..

8 recordsLinked to original sources

bioRxiv2026

CLN6 is a glycerophosphoglycerol epimerase in bis(monoacylglycero)phosphate biosynthesis

Lysosomal catabolism depends on bis(monoacylglycero)phosphate (BMP), a phospholipid with atypical S,S stereochemistry of unknown biosynthetic origin. CLN8 and CLN5 catalyze sequential acylation steps in BMP synthesis, and their loss of function causes Batten disease, a group of inherited neurodegenerative lysosomal storage disorders. Here we identify CLN6, a biochemically uncharacterized endoplasmic reticulum protein whose loss also causes Batten disease, as the calcium-dependent glycerophosphoglycerol epimerase that establishes BMP stereochemistry upstream of CLN8. CLN6 deficiency depletes BMP in cells, mice, and patient lysosomes, and reduces lysosomal hydrolase levels. BMP-precursor supplementation bypasses CLN6, restores BMP and increases hydrolase abundance in CLN6-deficient cells and mice. These findings place three Batten disease proteins in a single biosynthetic pathway and reveal BMP deficiency as a reversible cause of disrupted lysosomal hydrolase homeostasis.

biochemistry↗

bioRxiv2026

PqiABC forms a membrane-bridging conduit and mediates bidirectional phospholipid transport across the Gram-negative bacterial envelope

The mechanism by which glycerophospholipids are transported between the inner and outer membranes in Gram-negative bacteria remains poorly understood. In Escherichia coli, the paraquat-inducible (Pqi) pathway, comprising the inner membrane protein PqiA, the periplasm-spanning MCE-family protein PqiB, and the outer membrane lipoprotein PqiC, has been implicated in this process. These components are proposed to assemble into a quaternary complex that forms a continuous channel bridging the inner and outer membranes. Here, using neutron reflectometry and quartz crystal microbalance with dissipation monitoring, we perform a dynamic structural analysis of PqiABC within a planar double bilayer membrane-mimetic system. This approach reveals that PqiABC assembles into a stable, envelope-spanning complex anchored to both membranes, consistent with its proposed conduit architecture. Furthermore, using neutron reflectometry in combination with complementary fluorescence-based assays, we demonstrate that PqiABC mediates passive glycerophospholipid transport, supporting bidirectional lipid exchange between membranes. Together, these findings establish PqiABC as a membrane-bridging lipid transport system and provide direct evidence for a mechanism of passive glycerophospholipid equilibration across the bacterial envelope.

biophysics↗

bioRxiv2026

Phosphoproteomics identifies the DYRK1B protein kinase as a regulator of processing bodies

Dual-specificity tyrosine-phosphorylation-regulated kinase 1B (DYRK1B) modulates the cell cycle, cell fate during development, and is deregulated in cancer and metabolic syndrome. However, only a few DYRK1B substrates have been defined, so we undertook a phosphoproteomics screen in cells that exhibit inducible DYRK1B expression. Motif analysis revealed enrichment for proline-directed serine or threonine phosphorylation sites (pSer/pThr-Pro), consistent with the consensus motif of class I DYRKs. Gene ontology analysis revealed enrichment of proteins involved in mRNA binding, mRNA processing and ribonucleoprotein complexes. Several processing body (PB) components, including DCP1A, PATL1(PAT1B), EDC3 and 4E-T, were identified as DYRK1B-inducible phosphoproteins. DYRK1B also co-immunoprecipitated with DCP1A, PAT1B, EDC3, EDC4, DDX6 and XRN1. Super-resolution microscopy demonstrated that DYRK1B co-localised with DCP1A, DCP1B and DDX6 in PBs. Activation of DYRK1B increased PB abundance, whereas inhibition, depletion or knockout of DYRK1B reduced phosphorylation of DCP1A and 4E-T and decreased PB number. Re-expression of wild type, but not kinase-dead, DYRK1B restored PB numbers in knockout cells. These findings reveal novel DYRK1B targets and establish DYRK1B as a regulator of processing body abundance. HighlightsO_LIDYRK1B induces phosphorylation of a cluster of RNA binding and processing body associated proteins. C_LIO_LIDYRK1B localises to PBs and associates with multiple PB components. C_LIO_LIDYRK1B controls P-body abundance in a kinase-dependent manner. C_LI

cell biology↗

bioRxiv2026

Ubi-SCAPE enables deep exploration of the poly-ubiquitylome

Post-translational modification with chains of the 76-amino-acid protein ubiquitin ( poly-ubiquitylation) confers diverse fates to the targeted protein and non-protein substrates, including degradation, intracellular trafficking, and signal transduction. Despite being one of the most frequent modifications, the complexity of poly-ubiquitin chains adds methodological challenges to their characterization. Trypsin-resistant tandem-ubiquitin-binding entities (trTUBEs), engineered from natural ubiquitin-binding domains, can capture intact poly-ubiquitylated proteins with high cumulative avidity. However, such approaches have suffered from considerable co-eluting contaminants in proteomics applications. Here, we introduce an optimized trTUBE-based method for poly-ubiquitylated proteome purification, drastically depleting non-ubiquitylated protein contaminants and mono-ubiquitylated proteins. The method, termed Ubiquitylomics by Stringent, Cleavable, Affinity-based Proteome Extraction (Ubi-SCAPE), offers a streamlined and reproducible (median R2 > 0.98; CV < 8%) means of characterizing the poly-ubiquitylome. Over 7,800 poly-ubiquitylated proteins and 8,500 ubiquitin-modified peptides (diGly) were quantified with Ubi-SCAPE at a throughput of 40 samples per day, as well as over 6,000 from an equivalent of 33 g unstressed cell lysate. Upon acute stress by heat-shock, we identified 2,700 proteins and 8,000 diGly peptides with increased poly-ubiquitylation--offering similar biological insight as with far more material-, labor-, and cost-intensive peptide-based ubiquitin enrichment methods. Ubi-SCAPE therefore provides a simple and effective means of comprehensively quantifying a selective-enriched poly-ubiquitylome. GRAPHICAL ABSTRACT O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=108 SRC="FIGDIR/small/703496v1_ufig1.gif" ALT="Figure 1"> View larger version (51K): org.highwire.dtl.DTLVardef@a28120org.highwire.dtl.DTLVardef@cb801aorg.highwire.dtl.DTLVardef@47577aorg.highwire.dtl.DTLVardef@1c030af_HPS_FORMAT_FIGEXP M_FIG C_FIG

molecular biology↗

bioRxiv2025

Declining intracellular proteostasis capacity drives misfolded protein secretion in senescent human cells.

Healthy protein homeostasis ( proteostasis) relies on tightly-regulated protein quality-control (PQC) circuits that co-ordinate sequestration and clearance of potentially toxic aggregation-prone proteins, arising from various internal or external stress throughout an organisms lifespan. At the protein level, proteotoxic stress responses typically involve extensive poly-ubiquitylation and sequestration of aggregation-prone proteins and PQC factors into various protective cytoplasmic and nuclear granules. However, much of our current understanding regarding this aspect of stress responses in humans stems from research in proliferating cells--despite growing evidence that stress responses vary considerably at the transcriptional level across cell proliferation states. Here, we show that the senescent cellular state--considered a major contributor to ageing-associated degeneration due to a chronic inflammatory phenotype--re-wires PQC and expels the misfolded protein load to mitigate proteotoxic stresses. Starting with a multi-dimensional transcriptomics and proteomics approach for measuring levels of total, poly-ubiquitylated, and granule-forming proteins, we have discovered a clear point of divergence between senescent and proliferating or quiescent human cell states in their responses to proteotoxic stress. Although the proteins that were poly-ubiquitylated and degraded during stress were largely conserved across states, the stress-induced sedimentation of a large number of disease-associated RNA-binding proteins (including TDP-43) was impaired only in the senescent state. Strikingly, TDP-43, as well as several other misfolded proteins, were actively secreted through the endo-lysosomal system by a diverse range of senescent cells during acute or chronic stress, through a process that requires the vesicle-associated HSP70 co-chaperone DNAJC5--an established risk factor for several neurodegenerative diseases. Misfolded protein secretion could be rescued by increasing intracellular HSP levels in shallow but not deep senescence, suggesting that secretion is a proteostatic adaptation that becomes less reversible over time. Our findings reveal an unappreciated aspect of the senescent-cell secretory phenotype, which may have important consequences for the non-cell-autonomous impact of senescence at the level of tissue resilience and frailty.

cell biology↗

bioRxiv2024

CryoEM structure of the di-domain core of Mycobacterium tuberculosis polyketide synthase 13, essential for mycobacterial mycolic acid synthesis.

Mycobacteria are known for their complex cell wall, which comprises layers of peptidoglycan, polysaccharides and unusual fatty acids known as mycolic acids that form their unique outer membrane. Polyketide synthase 13 (Pks13) of Mycobacterium tuberculosis, the bacterial organism causing tuberculosis (TB), catalyses the last step of mycolic acid synthesis prior to export to and assembly in the cell wall. Due to its essentiality, Pks13 is a target for several novel anti-tubercular inhibitors, but its 3D structure and catalytic reaction mechanism remain to be fully elucidated. Here, we report the molecular structure of the catalytic core domains of M. tuberculosis Pks13 (Mt-Pks13), determined by transmission cryo-electron microscopy (cryoEM) to a resolution of 3.4 [A]. We observe a homodimeric assembly comprising the ketoacyl synthase (KS) domain at the centre, mediating dimerization, and the acyltransferase (AT) domains protruding in opposite directions from the central KS domain dimer. In addition to the KS-AT di-domains, the cryoEM map includes features not covered by the di-domain structural model that we predict to contain a dimeric domain with similarity to dehydratases, yet likely lacking catalytic function. Analytical ultracentrifugation data indicate a pH-dependent equilibrium between monomeric and dimeric assembly states, while comparison with the previously determined structures of M. smegmatis Pks13 indicates architectural flexibility. Combining the experimentally determined structure with modelling in AlphaFold2 suggests a structural scaffold with a relatively stable dimeric core, that combines with considerable conformational flexibility to facilitate the successive steps of the Claisen-type condensation reaction catalysed by Pks13.

microbiology↗

bioRxiv2022

Native size exclusion chromatography-based mass spectrometry (SEC-MS) identifies novel components of the Heat Shock Protein 90-dependent proteome

The molecular chaperone heat shock protein 90 (HSP90) works in concert with co-chaperones to stabilize its client proteins, which include multiple drivers of oncogenesis and malignant progression. Pharmacologic inhibitors of HSP90 have been observed to exert a wide range of effects on the proteome, including depletion of client proteins, induction of heat shock proteins, dissociation of co-chaperones from HSP90, disruption of client protein signaling networks, and recruitment of the protein ubiquitylation and degradation machinery--suggesting widespread remodeling of cellular protein complexes. However, proteomics studies to date have focused on inhibitor-induced changes in total protein levels, often overlooking protein complex alterations. Here, we use size-exclusion chromatography in combination with mass spectrometry (SEC-MS) to characterize the changes in native protein complexes following treatment with the HSP90 inhibitor tanespimycin (17-AAG) in the HT29 colon adenocarcinoma cell line. After confirming the signature cellular response to HSP90 inhibition (e.g., induction of heat shock proteins, decreased total levels of client proteins), we were surprised to find only modest perturbations to the global distribution of protein elution profiles in inhibitor-treated cells. Similarly, co-chaperones that co-eluted with HSP90 displayed no clear difference between control and treated conditions. However, two distinct analysis strategies identified multiple inhibitor-induced changes, including several known components of the HSP90 proteome, as well as numerous proteins and protein complexes with no previous links to HSP90. We present this dataset as a resource for the HSP90, proteostasis, and cancer communities (https://www.bioinformatics.babraham.ac.uk/shiny/HSP90/SEC-MS/), laying the groundwork for future mechanistic and therapeutic studies related to HSP90 pharmacology. Data are available via ProteomeXchange with identifier PXD033459.

biochemistry↗

bioRxiv2021

Solvent Precipitation SP3 (SP4) enhances recovery for proteomics sample preparation without magnetic beads

Complete, reproducible extraction of protein material is essential for comprehensive and unbiased proteome analyses. A current gold standard is single-pot, solid-phase-enhanced sample preparation (SP3), in which organic solvent and magnetic beads are used to denature and capture proteins, with subsequently washes allowing contaminant removal. However, SP3 is dependent on effective protein immobilisation onto beads, risks losses during wash steps, and experiences a drop-off in protein recovery at higher protein inputs. Magnetic beads may also contaminate samples and instruments, and become costly for larger scale protein preparations. Here, we propose solvent precipitation SP3 (SP4) as an alternative to SP3, omitting magnetic beads and employing brief centrifugation--either with or without low-cost inert glass beads--as the means of aggregated protein capture. SP4 recovered equivalent or greater protein yields for 1-5000 {micro}g preparations and improved reproducibility (median protein R2 0.99 (SP4) vs. 0.97 (SP3)). Deep proteome profiling (n = 9,076) also demonstrated improved recovery by SP4 and a significant enrichment of membrane and low-solubility proteins vs. SP3. The effectiveness of SP4 was verified in three other labs, each confirming equivalent or improved proteome characterisation over SP3. This work suggests that protein precipitation is the primary mechanism of SP3, and reliance on magnetic beads presents protein losses, especially at higher concentrations and amongst hydrophobic proteins. SP4 represents an efficient and effective alternative to SP3, provides the option to omit beads entirely, and offers virtually unlimited scalability of input and volume--all whilst retaining the speed and universality of SP3. BRIEFSolvent precipitation SP3 (SP4) captures aggregated protein for proteomics sample clean-up by omitting magnetic beads, instead employing brief centrifugation--with or without low-cost inert glass beads. SP4 offers improvements to protein yields, higher reproducibility, and greater recovery of membrane proteins, with verifications from three labs. Protein precipitation appears to be the primary mechanism of SP3, with reliance on magnetic beads presenting protein losses, especially at higher concentrations. SP4 presents an effective alternative to SP3 with improved scalability and equal speed and universality. GRAPHICAL ABSTRACT O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=135 SRC="FIGDIR/small/461247v1_ufig1.gif" ALT="Figure 1"> View larger version (49K): org.highwire.dtl.DTLVardef@1ba615corg.highwire.dtl.DTLVardef@14df25dorg.highwire.dtl.DTLVardef@62f0c1org.highwire.dtl.DTLVardef@1ac436a_HPS_FORMAT_FIGEXP M_FIG C_FIG

biochemistry↗