Biology subjects

Kunz, B.

Publications and source records attributed to Kunz, B..

9 recordsLinked to original sources

bioRxiv2026

The luminal and cytosolic domains of CLIMP-63 cooperate to set ER sheet width

The endoplasmic reticulum (ER) synthesized and folds secreted and transmembrane protein in regions composed of flattened sheets with a defined luminal width of [~]50 nm. CLIMP-63 has been proposed to be the principal determinant of this spacing, but the molecular mechanism by which it controls luminal width remains unclear. The luminal domain of CLIMP-63 has been proposed to act as a fixed-length spacer through antiparallel coiled-coil dimerization between molecules on opposing membranes. Here we revise this model. Using systematic cysteine trapping across the luminal domain, we demonstrate that CLIMP-63 assembles into parallel trimers. An AlphaFold-guided screen of CLIMP-63 orthologs combined with cryo-electron microscopy reveals that the luminal domain forms an elongated trimeric rod with intrinsic conformational flexibility at hinge regions. This flexibility is functionally required: replacing the human luminal domain with a more rigid ortholog causes complete ER vacuolation. Higher-order assembly of CLIMP-63 trimers beyond the trimer depends on the cytosolic tail rather than on luminal trans interactions, and the tails role in higher order assembly is separable from its other architectural functions. Together, these findings show that luminal trimeric extension and cytosolic tail-mediated clustering cooperate to determine ER sheet width.

cell biology↗

bioRxiv2026

Dynamic S-acylation controls CMG2 maturation, extracellular matrix regulation, and anthrax toxin entry

CMG2/ANTXR2 functions as a Collagen VI receptor and the primary portal for anthrax toxin entry. We find that CMG2 is regulated by ordered cycles of S-acylation and deacylation throughout its life cycle. Following synthesis in the endoplasmic reticulum, acylation by ZDHHC7 on two juxtamembrane cysteines protects folding intermediates from ER-associated degradation, resulting in a 5-fold increase in CMG2 biogenesis. The cytosolic thioesterase APT2 can remove these acyl chains, thereby controlling CMG2 levels. In the Golgi, CMG2 acylation by ZDHHC3 on a third cysteine to permit Arf6-dependent transport to the plasma membrane. At the cell surface, S-acylated CMG2 recruits APT2 in response to ligand binding, enabling release from the actin cytoskeleton and endocytosis. Accordingly, blocking APT2 suppresses the intracellular delivery of anthrax toxin, and inhibits CMG2-dependent Collagen VI degradation. These results define S-acylation-deacylation cycles as key regulators of CMG2 biogenesis and function, and highlight APT2 inhibition as a strategy to modulate CMG2 levels or prevent anthrax intoxication.

cell biology↗

bioRxiv2025

Antxr2-mediated fine-tuning of Collagen VI ensures skeletal muscle function

Tissue function relies on the extracellular matrix (ECM) that surrounds cells, providing structural and biochemical support. The complex ECM composition depends on an adequately tuned balance between the deposition and degradation of each of its components. Disequilibrium may cause disease, as observed for Collagen VI (COL6), for which mutations lead to muscular dystrophy. Here, we investigated the role of Anthrax Toxin Receptor 2 (ANTXR2/CMG2), a receptor that controls the turnover of COL6, in skeletal muscle. We show that ANTXR2 is mostly expressed by fibro-adipogenic precursors and that its deficiency in ANTXR2 null (Antxr2-/-) mice leads to a premature and irregular COL6 accumulation in intramuscular connective tissue. This results in tissue stiffening and gradual, non-functional muscle hypertrophy, marked by impaired locomotion and myopathic signs. Our findings further indicate that COL6 accretion drives these alterations, as revealed by Antxr2-/-::Col6a1-/- double knockout mice, highlighting the essential role of ANTXR2-mediated COL6 remodeling in maintaining ECM homeostasis and muscle functionality. SIGNIFICANCE STATEMENTRemodelling of the extracellular matrix (ECM) was long thought to rely almost exclusively on extracellular proteases. Increasing evidence, however, indicates that some ECM components may undergo intracellular degradation following receptor-mediated endocytosis, as we have found for Collagen VI (COL6). Here, we identify the COL6 receptor ANTXR2 as a critical regulator of ECM turnover in skeletal muscle.. When ANTXR2 is absent, COL6 builds up, followed by the accumulation of fibrillar collagens, without changes in gene expression. These alterations in muscle ECM lead to increased stiffness, myofiber defects and impaired locomotor activity. Our findings establish ANTXR2 as a key regulator in ECM remodelling, offering new insights into potential treatments for conditions associated with defective ECM remodeling, such as aging and congenital muscular dystrophies.

cell biology↗

bioRxiv2025

Identification of the Lipid Oxygen Radical Defense pathway and its epigenetic control

Membrane phospholipids are vulnerable to oxidative radicals, and uncontrolled lipid peroxidation affects cell viability. Cells have evolved quality control and defense mechanisms, of which the genetic regulation is not fully understood. Here, we identify what we have coined the Lipid Oxygen Radical Defense (LORD) pathway. It is epigenetically repressed by a complex comprising the KRAB-zinc finger protein ZNF354A, the scaffold protein KAP1/TRIM28, the histone methyltransferase SETDB1, and the transcriptional activator ATF2. Upon lipid peroxide accumulation, p38- and JNK-dependent phosphorylation of ATF2, KAP1/TRIM28, and ZNF354A leads to disassembly of the repressive complex, releasing ZNF354A from specific DNA loci and activating a network of protective genes, including NRF2 targets. The pathway affects the cellular sensitivity to oxidative stress and ferroptosis, revealing a novel layer of epigenetic control in lipid quality control and damage repair. This positions the LORD pathway as a promising therapeutic target for diseases linked to chronic inflammation, neurodegeneration and cancer.

cell biology↗

bioRxiv2025

Intestinal stem cell renewal controlled by capillary morphogenesis gene 2 following injury

Patients with the rare genetic disorder Hyaline Fibromatosis Syndrome (HFS) often succumb before 18 months of age due to severe diarrhea and protein-losing enteropathy. As HFS is caused by loss-of-function mutations in the gene encoding capillary morphogenesis gene 2 (CMG2), also known as Anthrax Toxin Receptor 2, these symptoms highlight a critical yet unclear role for CMG2 in the gut. Here, we demonstrate that CMG2 knockout mice exhibit normal colon morphology and no signs of inflammation until the chemical induction of colitis. In these conditions, the colons of knockout mice do not regenerate despite previously experiencing similarly severe colitis, due to an inability to replenish their intestinal stem cell pool. Specifically, CMG2 knockout impairs the transition from fetal-like to Lgr5+ adult stem cells, which is associated with a defect in {beta}-catenin nuclear translocation. Our findings suggest that CMG2 functions as a context-specific modulator of Wnt signaling, essential for replenishing the pool of intestinal stem cells following injury. This study provides new insights into the molecular mechanisms underlying protein-losing enteropathy in HFS and offers a broader understanding of fetal-like regenerative responses. GRAPHICAL ABSTRACT O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=87 SRC="FIGDIR/small/631493v1_ufig1.gif" ALT="Figure 1"> View larger version (29K): org.highwire.dtl.DTLVardef@1a38f2corg.highwire.dtl.DTLVardef@10ca815org.highwire.dtl.DTLVardef@1ef3bddorg.highwire.dtl.DTLVardef@1433671_HPS_FORMAT_FIGEXP M_FIG C_FIG

cell biology↗

bioRxiv2023

SARS-CoV-2 shifts transcription of host gene to increase Spike acylation and boost infectivity

SARS-CoV-2 infection requires Spike protein mediating fusion between the viral and cellular membranes. The fusogenic activity of Spike requires its post-translational lipid modification by host S-acyltransferases, predominantly ZDHHC20. Previous observations indicate that SARS-CoV-2 infection augments the S-acylation of Spike when compared to transfection. Here, we find that SARS-CoV-2 infection triggers a change in the transcriptional start site of the zddhc20 gene, both in cells and in an in vivo infection model, resulting in a 67-amino-acid-long N-terminally extended protein with 37-times higher Spike acylating activity, leading to enhanced viral infectivity. Furthermore, we observed the same induced transcriptional change in response to other challenges, such as chemically induced colitis, indicating that SARS-CoV-2 hijacks an existing cell damage response pathway to generate more infectious viruses.

cell biology↗

bioRxiv2021

S-acylation controls SARS-Cov-2 membrane lipid organization and enhances infectivity

SARS-CoV-2 virions are surrounded by a lipid bilayer which contains membrane proteins such as Spike, responsible for target-cell binding and virus fusion, the envelope protein E and the accessory protein Orf3a. Here, we show that during SARS-CoV-2 infection, all three proteins become lipid modified, through action of the S-acyltransferase ZDHHC20. Particularly striking is the rapid acylation of Spike on 10 cytosolic cysteines within the ER and Golgi. Using a combination of computational, lipidomics and biochemical approaches, we show that this massive lipidation controls Spike biogenesis and degradation, and drives the formation of localized ordered cholesterol and sphingolipid rich lipid nanodomains, in the early Golgi where viral budding occurs. ZDHHC20-mediated acylation allows the formation of viruses with enhanced fusion capacity and overall infectivity. Our study points towards S-acylating enzymes and lipid biosynthesis enzymes as novel therapeutic anti-viral targets.

cell biology↗

bioRxiv2018

The architecture of the endoplasmic reticulum is regulated by the reversible lipid modification of the shaping protein CLIMP-63

The endoplasmic reticulum (ER) has a complex morphology generated and maintained by membrane-shaping proteins and membrane energy minimization, though not much is known about how it is regulated. The architecture of this intracellular organelle is balanced between large, thin sheets that are densely packed in the perinuclear region and a connected network of branched, elongated tubules that extend throughout the cytoplasm. Sheet formation is known to involve the cytoskeleton-linking membrane protein 63 (CLIMP-63), though its regulation and the depth of its involvement remain unknown. Here we show that the post-translational modification of CLIMP-63 by the palmitoyltransferase ZDHHC6 controls the relative distribution of CLIMP-63 between the ER and the plasma membrane. By combining data-driven mathematical modeling, predictions, and experimental validation, we found that the attachment of a medium chain fatty acid, so-called S-palmitoylation, to the unique CLIMP-63 cytoplasmic cysteine residue drastically reduces its turnover rate, and thereby controls its abundance. Light microscopy and focused ion beam electron microcopy further revealed that enhanced CLIMP-63 palmitoylation leads to strong ER-sheet proliferation. Altogether, we show that ZDHHC6-mediated S-palmitoylation regulates the cellular localization of CLIMP-63, the morphology of the ER, and the interconversion of ER structural elements in mammalian cells through its action on the CLIMP-63 protein.\n\nSignificance StatementEukaryotic cells subcompartmentalize their various functions into organelles, the shape of each being specific and necessary for its proper role. However, how these shapes are generated and controlled is poorly understood. The endoplasmic reticulum is the largest membrane-bound intracellular compartment, accounting for more than 50% of all cellular membranes. We found that the shape and quantity of its sheet-like structures are controlled by a specific protein, cytoskeleton-linking membrane protein 63, through the acquisition of a lipid chain attached by an enzyme called ZDHHC6. Thus, by modifying the ZDHHC6 amounts, a cell can control the shape of its ER. The modeling and prediction technique used herein also provides a method for studying the interconnected function of other post-translational modifications in organelles.

cell biology↗

bioRxiv2017

Identification and dynamics of the DHHC16-DHHC6 palmitoylation cascade

S-Palmitoylation is the only reversible post-translational lipid modification. Knowledge about the DHHC family of palmitoyltransferases is very limited. Here we show that mammalian DHHC6, which modifies key proteins of the endoplasmic reticulum, is controlled by an upstream palmitoyltransferase, DHHC16, revealing the first palmitoylation cascade. Combination of site specific mutagenesis of the three DHHC6 palmitoylation sites, experimental determination of kinetic parameters and data-driven mathematical modelling allowed us to obtain detailed information on the 8 differentially palmitoylated DHHC6 species. We found that species rapidly interconvert through the action of DHHC16 and the Acyl Protein Thioesterase APT2, that each species varies in terms of turnover rate and activity, altogether allowing the cell to robustly tune its DHHC6 activity.

cell biology↗