Biology subjects

Helms, V.

Publications and source records attributed to Helms, V..

15 recordsLinked to original sources

bioRxiv2026

Predicting Differential Protein-Protein Interactions

Motivation Analysis of context-specific protein-protein interactions (PPIs) provides deeper insight into protein function, revealing how cellular pathway components interact in certain perturbation conditions. However, identifying such interactions experimentally often requires combining affinity purification with mass spectrometry, proving costly and time-consuming. On the other hand, it has been shown that an increase in protein abundance does not always correspond to an increase in protein interaction, making it an unsuitable surrogate for interaction studies. Results To address this, a random forest model was created using PPI-specific MINT embeddings and condition-specific mass spectrometry protein abundance information to predict differential interactions across two experimental conditions. The model was trained and tested on validated context-specific PPIs from eight distinct cell-lines, demonstrating proficiency in predicting differential PPIs for both novel proteins and cell lines. To our knowledge, we provide the first computational model that predicts differential PPIs between experimental conditions. Availability and implementation The code is available on GitHub and backed up on Zenodo (DOI: 10.5281/zenodo.23758127). The model is freely available on Zenodo (DOI: 10.5281/zenodo.23757927).

bioinformatics↗

bioRxiv2026

Stress induces DCP2 translation via a stalling-dependent mechanism

The integrated stress response (ISR) globally suppresses protein synthesis while selectively permitting translation of a small subset of stress-responsive mRNAs, many of which contain upstream or overlapping open reading frames (uORFs/oORFs). Although translational induction of transcripts such as ATF4 has classically been attributed to delayed re-initiation caused by reduced ternary complex availability, the mechanisms by which uORFs and oORFs allow ISR-selective translation remain incompletely understood. Here, using ribosome profiling during early ISR activation combined with reporter assays, we identify DCP2, encoding a major mRNA decapping enzyme, as a previously unrecognized ISR-induced transcript. We show that translational induction of DCP2 depends on an overlapping ORF whose conserved 3' region, corresponding to a ribosome pausing site, acts as a potent inhibitory element. Both the DCP2 oORF and main ORF increase in translation during stress, indicating that stress relieves repression by this inhibitory element. This reveals a mode of ISR-dependent gene regulation in which inhibition by a nascent peptide or stalling element embedded either in a uORF or an oORF is relieved upon stress to induce translation.

molecular biology↗

bioRxiv2026

Mammalian MemPrep establishes the lipid composition of ER membranes in HEK293T cells

The endoplasmic reticulum (ER) forms a dynamic network of sheets and tubules, whose molecular lipid composition remains incompletely defined. Using an optimized MemPrep workflow, we establish a high-confidence lipidome of the mammalian ER and selectively enrich membrane vesicles originating from ER tubules as a major ER subdomain. Quantitative lipidomics show that ER membranes are dominated by phosphatidylcholine and mono-unsaturated glycerophospholipids, consistent with a highly compressible bilayer. Although proteomics suggests a functional specialization of ER tubules and an enrichment of tubule-associated proteins therein, the lipidome of an ER tubule-enriched isolate is indistinguishable from the general ER, indicating that principal ER architectures share a common lipid composition. Integration of lipidomic data with bioinformatic analyses of transmembrane helices further demonstrates that the physicochemical features of ER lipids mirror those of ER-resident membrane proteins, including reduced hydrophobicity and increased polarity compared to plasma membrane proteins. These findings support a coordinated evolution of ER proteins and lipids based on shared biophysical constraints. Together, this work provides a definitive characterization of the mammalian ER lipidome and suggest that membrane properties are maintained across the entire ER.

cell biology↗

bioRxiv2025

Sec61 translocon inhibitor Flavitransin blocks selectively dengue virus polyprotein insertion into the ER membrane with pan-flavivirus antiviral potency

Flavivirus infections by Dengue and Zika virus impose a significant healthcare threat worldwide. At present no FDA-approved specific antiviral treatment is available, and the safety of a vaccine against Dengue virus is still under debate. Here, we report the identification of the CADA derivative flavitransin (FT), with potent activity against DENV serotype 2 in various cell types. Moreover, FT showed consistent anti-flaviviral activity against all four DENV serotypes, and also against Zika and Yellow fever virus. Viral polyprotein biogenesis was completely abolished by FT treatment of DENV-infected cells. Drug profiling by a-time-of-drug-addition assay revealed a post-entry antiviral effect of FT, in line with its anticipated Sec61 inhibitory effect. Subsequent analysis of the individual viral proteins in transfected HEK293T cells indicated that FT suppresses the expression of the structural proteins (pre-membrane and envelope) only. Furthermore, cell free in vitro protein translation analysis demonstrated a direct inhibitory effect of FT on the co-translational translocation of the DENV polyprotein across the ER membrane. More specifically, FT inhibited the initiation of protein translocation into the ER that relies on the N-terminal transmembrane region of the capsid subunit of the DENV polyprotein, resulting in rerouting of the viral pre-protein to the cytosol for proteasomal degradation. Finally, selection and genotyping of FT-resistant HCT116 cells revealed a unique A70V mutation in the Sec61 subunit that conferred resistance to FT in infected cells. Long-term exposure of DENV to FT demonstrated a high barrier to resistance development. In conclusion, our data demonstrate that FT selectively interferes with the initiation of ER co-translational translocation of the DENV polyprotein and confirm the critical role of this translocation process in the flavivirus replication cycle.

microbiology↗

bioRxiv2024

The conserved membrane-proximal domain of Sbh1/ Sec61β guides signal peptides into the Sec61 channel

In eukaryotes, protein secretion begins with protein translocation through the universally conserved Sec61 channel into the endoplasmic reticulum (ER). Its {beta}-subunit, Sbh1 in yeast, enhances ER import of proteins with specific suboptimal signal peptides by an unknown mechanism. The Sbh1 cytosolic N-terminus consists of an intrinsically disordered, non-conserved region (IDR) that has never been visualized in active channel structures, but is close to the translocating polypeptide in the cytosolic channel vestibule. The Sbh1/Sec61{beta} N-terminal IDR is followed by structured 15 amino acids and its C-terminal transmembrane helix, both of which are conserved. We show here that the proline and adjacent conserved residues at the Sbh1 cytosolic/transmembrane domain interface form a hinge that positions the Sbh1 cytosolic domain across the channel vestibule and orients it with respect to the lateral gate. This orientation is critical for Sbh1-dependent protein insertion into the channel. Sbh1-dependence of Sec61 channel insertion is a function of the signal peptide of the respective secretory protein. By chemical crosslinking of purified cytosolic domains of Sbh1 and its paralog Sbh2 to synthetic signal peptides derived from their respective client proteins, we show that the cytosolic domains of Sbh1 and Sbh2 contain specific signal peptide binding sites. The position of the crosslinked residues suggests that signal peptide binding is mediated by the IDRs. We conclude that Sec61{beta} homologs directly recognize signal peptides of their substrates and guide them into the Sec61 channel; they thus control entry of specific proteins into the secretory pathway.

biochemistry↗

bioRxiv2023

A better brain? Alternative spliced STIM2 in hominoids arises with synapse formation and creates a gain-of-function variant

Balanced Ca2+ homeostasis is essential for cellular functions. STIM2 mediated Store-Operated Ca2+ Entry (SOCE) regulates cytosolic and ER Ca2+ concentrations, stabilizes dendritic spine formation and drives presynaptic spontaneous transmission and ER stress in neurons. Recently identified alternative spliced variants expand the STIM protein repertoire, uncover unique functions and facilitate our understanding of tissue specific regulation of SOCE. Here, we describe an addition to this repertoire, a unique short STIM2 variant (STIM2.3/STIM2G) present only in old world monkeys and humans with expression in humans starting with the beginning of brainwave activity and upon synapse formation within the cerebral cortex. In contrast to the short STIM1B variant, STIM2.3/STIM2G increases SOCE upon stimulation independently of specific spliced in residues. Basal cluster formation is reduced and analyses of several additional deletion and point mutations delineate the role of functional motifs for Ca2+ entry, NFAT activation and changes in neuronal gene expression. In addition, STIM2.3/STIM2G shows reduced binding and activation of the energy sensor AMPK. In the context of reduced STIM2.3 splicing seen in postmortem brains of patients with Huntingtons disease, our data suggests that STIM2.3/STIM2G is an important regulator of neuronal Ca2+ homeostasis, potentially involved in synapse formation/maintenance and evolutionary expansion of brain complexity.

cell biology↗

bioRxiv2022

The ER protein translocation channel subunit Sbh1 controls virulence of Cryptococcus neoformans

The fungal pathogen Cryptococcus neoformans is distinguished by a cell wall-anchored polysaccharide capsule that is critical for virulence. Biogenesis of both cell wall and capsule relies on the secretory pathway. Protein secretion begins with polypeptide translocation across the endoplasmic reticulum (ER) membrane through a highly conserved channel formed by three proteins: Sec61, Sbh1, and Sss1. Sbh1, the most divergent, contains multiple phosphorylation sites, which may allow it to regulate entry into the secretory pathway in a species- and protein-specific manner. Absence of SBH1 causes a cell-wall defect in both Saccharomyces cerevisiae and C. neoformans, although other phenotypes differ. Notably, proteomic analysis showed that when cryptococci are grown in conditions that mimic aspects of the mammalian host environment (tissue culture medium, 37 {degrees}C, 5% CO2), a set of secretory and transmembrane proteins is upregulated in wild-type, but not in{Delta} sbh1 mutant cells. The Sbh1-dependent proteins show specific features of their ER targeting sequences that likely cause them to transit less efficiently into the secretory pathway. Many also act in cell-wall biogenesis, while several are known virulence factors; consistent with these observations, the C. neoformans {Delta}sbh1 mutant is avirulent in a mouse infection model. We conclude that, in the context of conditions encountered during infection, Sbh1 controls the entry of virulence factors into the secretory pathway of C. neoformans, and thereby regulates fungal pathogenicity. ImportanceCryptococcus neoformans is a yeast that causes almost 200,000 deaths worldwide each year, mainly of immunocompromised individuals. The surface structures of this pathogen, a protective cell wall surrounded by a polysaccharide capsule, are made and maintained by proteins that are synthesized inside the cell and travel outwards through the secretory pathway. A protein called Sbh1 is part of the machinery that determines which polypeptides enter this export pathway. We found that when Sbh1 is absent, both C. neoformans and the model yeast S. cerevisiae show cell wall defects. Lack of Sbh1 also changes the pattern of secretion of both transmembrane and soluble proteins, in a manner that depends on characteristics of their sequences. Notably, multiple proteins that are normally upregulated in conditions similar to those encountered during infection, including several needed for cryptococcal virulence, are no longer increased. Sbh1 thereby regulates the ability of this important pathogen to cause disease.

microbiology↗

bioRxiv2022

ER translocation of suboptimal targeting sequences depends on Sec61β/Sbh1 and its phosphorylation

The endoplasmic reticulum (ER) protein translocation channel subunit Sec61{beta}/Sbh1 is non-essential, but contains multiple phosphorylation sites suggesting a regulatory role in ER protein import. We show here that mutating two N-terminal, proline-flanked, phosphorylation sites in the Sbh1 cytosolic domain phenocopies the temperature-sensitivity of a yeast strain lacking SBH1/SBH2, and results in reduced translocation into the ER of an Sbh1-dependent substrate, Gls1. In a microscopic screen we show that about 12% of GFP-tagged secretory proteins depend on Sbh1 for translocation. Sbh1-dependent proteins have targeting sequences with less pronounced hydrophobicity and often no or an inverse charge bias. A subset of these proteins was dependent on N-terminal phosphorylation of Sbh1 and on the phospho-S/T-specific proline isomerase Ess1 (PIN1 in mammals) for ER import. We conclude that Sbh1 promotes ER translocation of substrates with suboptimal targeting sequences and that its activity is regulated by a conformational change induced by N-terminal phosphorylation.

biochemistry↗

bioRxiv2022

Proteomics identifies substrates and a novel component in hSnd2-dependent ER protein targeting

Protein import into the endoplasmic reticulum (ER) is essential for about 30% of the human proteome. It involves targeting of precursor proteins to the ER and insertion into or translocation across the ER membrane. Furthermore, it relies on signals in the precursor polypeptides and components, which read the signals and facilitate their targeting to a protein-conducting channel in the ER membrane, the Sec61 complex. Compared to the SRP- and TRC-dependent pathways, little is known about the SRP-independent/SND pathway. Our aim was to identify additional components and characterize the client spectrum of the human SND pathway. The established strategy of combining depletion of the central hSnd2-component from HeLa cells with proteomic- and differential protein abundance-analysis was used. The SRP and TRC targeting pathways were analyzed in comparison. TMEM109 was characterized as hSnd3. Unlike SRP but similar to TRC, the SND clients are predominantly membrane proteins with N-terminal, central, or C-terminal targeting signals.

cell biology↗

bioRxiv2021

Metabolic reprogramming of cytotoxic T lymphocytes by high glucose enhances pancreatic beta cell apoptosis via TRAIL

Cytotoxic T lymphocytes (CTLs) are involved in development of diabetes. However, the impact of excessive glucose on CTL-mediated antigen-independent killing remains elusive. Here, we report that TNF-related apoptosis inducing ligand (TRAIL) is substantially up- regulated in CTLs in environments with high glucose (HG) both in vitro and in vivo. The PI3K- Akt-NF{kappa}B axis and non-mitochondrial reactive oxygen species are essential in HG-induced TRAIL upregulation in CTLs. TRAILhigh CTLs induce apoptosis of pancreatic beta cell line 1.4E7. Metformin and Vitamin D synergistically reduce HG-enhanced expression of TRAIL in CTLs and coherently protect 1.4E7 cells from TRAIL-mediated apoptosis. Notably, in patients with diabetes, correlation between Vitamin D concentrations in plasma and glucose levels is linked to HG-enhanced TRAIL expression on CTLs. Microarray data reveal that OXCT2, an important enzyme in ketone body catabolism, is a promising target in response to vitamin D. Our work not only reveals a novel mechanism of CTL involvement in progression of diabetes, but also establishes CTLs as a target for combined metformin and vitamin D therapy to protect pancreatic beta cells of diabetic patients.

immunology↗

bioRxiv2021

Mycolactone enhances the Ca2+ leakage from endoplasmic reticulum by trapping Sec61 translocons in a Ca2+ permeable state

The Mycobacterium ulcerans exotoxin, mycolactone, is an inhibitor of co-translational translocation via the Sec61 complex. Mycolactone has previously been shown to bind to, and alter the structure of, the major translocon subunit Sec61, and change its interaction with ribosome nascent chain complexes. In addition to its function in protein translocation into the ER, Sec61 also plays a key role in cellular Ca2+ homeostasis, acting as a leak channel between the endoplasmic reticulum (ER) and cytosol. Here, we have analysed the effect of mycolactone on cytosolic and ER Ca2+ levels using compartment-specific sensors. We also used molecular docking analysis to explore potential interaction sites for mycolactone on translocons in various states. These results show that mycolactone enhances the leak of Ca2+ ions via the Sec61 translocon, resulting in a slow but substantial depletion of ER Ca2+. This leak was dependent on mycolactone binding to Sec61 because resistance mutations in this protein completely ablated the increase. Molecular docking supports the existence of a mycolactone-binding transient inhibited state preceding translocation and suggests mycolactone may also bind Sec61 in its idle state. We propose that delayed ribosomal release after translation termination and/or translocon "breathing" during rapid transitions between the idle and intermediate-inhibited states allow for transient Ca2+ leak, and mycolactones stabilisation of the latter underpins the phenotype observed.

biochemistry↗

bioRxiv2020

Identification of molecular candidates which regulate calcium-dependent CD8+ T-cell cytotoxicity

Cytotoxic CD8+ T lymphocytes (CTL) eliminate infected cells or transformed tumour cells by releasing perforin-containing cytotoxic granules at the immunological synapse. The secretion of such granules depends on Ca2+-influx through store operated Ca2+ channels, formed by STIM-activated Orai proteins. Whereas molecular mechanisms of the secretion machinery are well understood, much less is known about the molecular machinery that regulates the efficiency of Ca2+-dependent target cell killing. Here, we isolated total RNA from natural killer (NK) cells, non-stimulated CD8+ T-cells, and from Staphylococcus aureus enterotoxin A (SEA) stimulated CD8+ T-cells (SEA-CTL) and conducted whole genome expression profiling by microarray experiments. Based on differential expression analysis of the transcriptome data and analysis of master regulator genes, we identified 31 candidates which potentially regulate Ca2+-homeostasis in CTL. To investigate a putative function of these candidates in CTL cytotoxicity, we transfected either SEA-stimulated CTL (SEA-CTL) or antigen specific CD8+ T-cell clones (CTL-MART-1) with siRNAs specific against the identified candidates and analyzed the killing capacity using a real-time killing assay. In addition, we complemented the analysis by studying the effect of inhibitory substances acting on the candidate proteins if available. Finally, to unmask their involvement in Ca2+ dependent cytotoxicity, candidates were also analyzed under Ca2+-limiting conditions. Overall, this strategy led to the identification of KCNN4, RCAN3, CCR5 and BCL2 as potential candidates to regulate the efficiency of Ca2+-dependent target cell killing.

immunology↗

bioRxiv2020

Deregulation of epigenetic marks is correlated to differential exon usage of developmental genes

Alternative exon usage is known to affect a large portion of genes in mammalian genomes. Importantly, different splice forms sometimes lead to distinctly different protein functions. We analyzed data from the Human Epigenome Atlas (version 9) whereby we connected the differential usage of exons in various developmental stages of human cells/tissues to differential epigenetic modifications at the exon level. In total, we analyzed 19 human tissues, adult cells, and cultured cells that mimic early developmental stages. We found that the differential occurrence of protein isoforms across developmental stages was often associated with changes in histone marks at exon boundary regions. Many of the genes that are differentially regulated at the exon level were found to be functionally associated with development and metabolism.

bioinformatics↗

bioRxiv2019

Proteomics identifies signal peptide features determining the substrate specificity in human Sec62/Sec63-dependent ER protein import

In mammalian cells one-third of all polypeptides are integrated into the membrane or translocated into the lumen of the endoplasmic reticulum (ER) via the Sec61-channel. While the Sec61-complex facilitates ER-import of most precursor polypeptides, the Sec61-associated Sec62/Sec63-complex supports ER-import in a substrate-specific manner. So far, mainly posttranslationally imported precursors and the two cotranslationally imported precursors of ERj3 and prion protein were found to depend on the Sec62/Sec63-complex in vitro. Therefore, we determined the rules for engagement of Sec62/Sec63 in ER-import in intact human cells using a recently established unbiased proteomics approach. In addition to confirming ERj3, we identified twenty-two novel Sec62/Sec63-substrates under these in vivo-like conditions. As a common feature, those previously unknown substrates share signal peptides with comparatively longer but less hydrophobic H-region and lower C-region polarity. Further analyses with four substrates, and ERj3 in particular, revealed the combination of a slowly-gating signal peptide and a downstream translocation-disruptive positively charged cluster of amino acid residues as decisive for the Sec62-/Sec63-requirement. In the case of ERj3, these features were found to be responsible for an additional BiP-requirement and to correlate with sensitivity towards the Sec61-channel inhibitor CAM741. Thus, the human Sec62/Sec63-complex may support Sec61-channel opening for precursor polypeptides with slowly-gating signal peptides by direct interaction with the cytosolic amino-terminal peptide of Sec61 or via recruitment of BiP and its interaction with the ER-lumenal loop 7 of Sec61. These novel insights into the mechanism of human ER protein import contribute to our understanding of the etiology of SEC63-linked Polycystic Liver Disease. DatabasesThe mass spectrometry proteomics data have been deposited to the ProteomeXchange Consortium via the PRIDE partner repository (http://www.ebi.ac.uk/pride/archive/projects/Identifiers) with the dataset identifiers: PXD008178, PXD011993, and PXD012078. Supplementary information was deposited at Mendeley Data under the DOI:10.17632/6s5hn73jcv.1 (http://dx.doi.or/10.17632/6s5hn73jcv.1).

biochemistry↗

bioRxiv2018

Relative Principal Components Analysis: Application to Analyzing Biomolecular Conformational Changes

A new method termed \"Relative Principal Components analysis\" (RPCA) is introduced that extracts optimal relevant principal components to describe the change between two data samples representing two macroscopic states. The method is widely applicable in data-driven science. Calculating the components is based on a unified physical framework which introduces the objective function, namely the Kullback-Leibler divergence, appropriate for quantifying the change of the macroscopic state as it is effected by the microscopic features. To demonstrate the applicability of RPCA, we analyze the thermodynamically relevant conformational changes of the protein HIV-1 protease upon binding to different drug molecules. In this case, the RPCA method provides a sound thermodynamic foundation for the analysis of the binding process. The relevant collective (global) conformational changes can be reconstructed from the informative latent variables to exhibit both the enhanced and the restricted conformational fluctuations upon ligand association. Moreover, RPCA characterizes the locally relevant conformational changes which can be presented on the structure of the protein.

biophysics↗