Functional implications of MIR domains in protein O-mannosylation
Antonella Chiapparino, Antonija Grbavac, Hendrik RA Jonker, Yvonne Hackmann, Sofia Mortensen, Ewa Zatorska, Andrea Schott, Gunter Stier, Krishna Saxena, Klemens Wild, Harald Schwalbe, Sabine Strahl, Irmgard Sinning
Published: December 24, 2020 doi: 10.7554/eLife.61189
Protein O-mannosyltransferases (PMTs) represent a conserved family of multispanning endoplasmic reticulum membrane proteins involved in glycosylation of S/T-rich protein substrates and unfolded proteins. PMTs work as dimers and contain a luminal MIR domain with a β-trefoil fold, which is susceptive for missense mutations causing α-dystroglycanopathies in humans. Here, we analyze PMT-MIR domains by an integrated structural biology approach using X-ray crystallography and NMR spectroscopy and evaluate their role in PMT function in vivo. We determine Pmt2- and Pmt3-MIR domain structures and identify two conserved mannose-binding sites, which are consistent with general β-trefoil carbohydrate-binding sites (α, β), and also a unique PMT2-subfamily exposed FKR motif. We show that conserved residues in site α influence enzyme processivity of the Pmt1-Pmt2 heterodimer in vivo. Integration of the data into the context of a Pmt1-Pmt2 structure and comparison with homologous β-trefoil – carbohydrate complexes allows for a functional description of MIR domains in protein O-mannosylation.
Fatal outcome after heart surgery in PMM2-CDG due to a rare homozygous gene variant with double effects
Marlen Görlacher, Eleftheria Panagiotou, Nastassja Himmelreich, Andreas Hüllen, Lars Beedgen, Bianca Dimitrov, Virginia Geiger, Matthias Zielonka, Verena Peters, Sabine Strahl, Jaime Vázquez-Jiménez, Gunter Kerst, ChristianThiel
Published: November 7, 2020 DOI: 10.1016/j.ymgmr.2020.100673
Variants in Phosphomannomutase 2 (PMM2) lead to PMM2-CDG, the most frequent congenital disorder of glycosylation (CDG). We here describe the disease course of a ten-month old patient who presented with the classical PMM2-CDG symptoms as cerebellar hypoplasia, retinitis pigmentosa, seizures, short stature, hepato- and splenomegaly, anaemia, recurrent vomiting and inverted mamillae. A severe form of tetralogy of Fallot was diagnosed and corrective surgery was performed at the age of 10 months. At the end of the cardiopulmonary bypass, a sudden oedematous reaction of the myocardium accompanied by biventricular pump failure was observed immediately after heparin antagonization with protamine sulfate. The patient died seven days after surgery, since myocardial function did not recover on ECMO support. We here describe the first patient carrying the homozygous variant g.18313A > T in the PMM2 gene (NG_009209.1) that either can lead to c.394A > T (p.I132F) or even loss of 100 bp due to exon 5 skipping (c.348_447del; p.G117Rfs*4) which is comparable to a null allele. Proliferation and doubling time of the patient’s fibroblasts were affected. In addition, we show that the induction of cellular stress by elevating the cell culture temperature to 40 °C led to a decrease of the patients‘ PMM2 transcript as well as PMM2 protein levels and subsequently to a significant loss of residual activity. We assume that metabolic stressful processes occurring after cardiac surgery led to the drop of the patient’s PMM activity below a life-sustaining niveau which paved the way for the fatal outcome.
NMR characterization of the C-mannose conformation in a thrombospondin repeat using a selective labeling approach
Published: August 3, 2020 DOI: 10.1002/anie.202009489
Despite the great interest in glycoproteins, structural information reporting on conformation and dynamics of the sugar moieties are limited. We present a new biochemical method to express proteins with glycans that are selectively labeled with NMR active nuclei. We report on the incorporation of 13C-labeled mannose in the C-mannosylated UNC-5 thrombospondin repeat. The conformational landscape of the C-mannose sugar puckers attached to tryptophan residues of UNC-5 is characterized by interconversion between the canonical 1C4 state and the B03 / 1S3 state. This flexibility may be essential for protein folding and stabilization. We foresee that this versatile tool to produce proteins with selective labeled C-mannose can be applied and adjusted to other systems and modifications and potentially paves a way to advance glycoprotein research by unravelling the dynamical and conformational properties of glycan structures and their interactions.
C-mannosylation supports folding and enhances stability of thrombospondin repeats
Matthias Preller Manuel H Taft Jordi Pujols Salvador Ventura Birgit Tiemann Falk FR Buettner Hans Bakker
Published: December 23, 2019 https://doi: 10.7554/eLife.52978
Previous studies demonstrated importance of C-mannosylation for efficient protein secretion. To study its impact on protein folding and stability, we analyzed both C-mannosylated and non-C-mannosylated thrombospondin type 1 repeats (TSRs) of netrin receptor UNC-5. In absence of C-mannosylation, UNC-5 TSRs could only be obtained at low temperature and a significant proportion displayed incorrect intermolecular disulfide bridging, which was hardly observed when C-mannosylated. Glycosylated TSRs exhibited higher resistance to thermal and reductive denaturation processes, and the presence of C-mannoses promoted the oxidative folding of a reduced and denatured TSR in vitro. Molecular dynamics simulations supported the experimental studies and showed that C-mannoses can be involved in intramolecular hydrogen bonding and limit the flexibility of the TSR tryptophan-arginine ladder. We propose that in the endoplasmic reticulum folding process, C-mannoses orient the underlying tryptophan residues and facilitate the formation of the tryptophan-arginine ladder, thereby influencing the positioning of cysteines and disulfide bridging.
Membrane Topological Model of Glycosyltransferases of the GT-C Superfamily
Published: September 29, 2019 https://doi: 10.3390/ijms20194842
Glycosyltransferases that use polyisoprenol-linked donor substrates are categorized in the GT-C superfamily. In eukaryotes, they act in the endoplasmic reticulum (ER) lumen and are involved in N-glycosylation, glypiation, O-mannosylation, and C-mannosylation of proteins. We generated a membrane topology model of C-mannosyltransferases (DPY19 family) that concurred perfectly with the 13 transmembrane domains (TMDs) observed in oligosaccharyltransferases (STT3 family) structures. A multiple alignment of family members from diverse organisms highlighted the presence of only a few conserved amino acids between DPY19s and STT3s. Most of these residues were shown to be essential for DPY19 function and are positioned in luminal loops that showed high conservation within the DPY19 family. Multiple alignments of other eukaryotic GT-C families underlined the presence of similar conserved motifs in luminal loops, in all enzymes of the superfamily. Most GT-C enzymes are proposed to have an uneven number of TDMs with 11 (POMT, TMTC, ALG9, ALG12, PIGB, PIGV, and PIGZ) or 13 (DPY19, STT3, and ALG10) membrane-spanning helices. In contrast, PIGM, ALG3, ALG6, and ALG8 have 12 or 14 TMDs and display a C-terminal dilysine ER-retrieval motif oriented towards the cytoplasm. We propose that all members of the GT-C superfamily are evolutionary related enzymes with preserved membrane topology.
Swift Large-scale Examination of Directed Genome Editing
Omar T. Hammouda, Frank Böttger, Joachim Wittbrodt, Thomas Thumberger
Published: March 5, 2019
In the era of CRISPR gene editing and genetic screening, there is an increasing demand for quick and reliable nucleic acid extraction pipelines for rapid genotyping of large and diverse sample sets. Despite continuous improvements of current workflows, the handling-time and material costs per sample remain major limiting factors. Here we present a robust method for low-cost DIY-pipet tips addressing these needs; i.e. using a cellulose filter disc inserted into a regular pipet tip. These filter-in-tips allow for a rapid, stand-alone four-step genotyping workflow by simply binding the DNA contained in the primary lysate to the cellulose filter, washing it in water and eluting it directly into the buffer for the downstream application (e.g. PCR). This drastically cuts down processing time to maximum 30 seconds per sample, with the potential for parallelizing and automation. We show the ease and sensitivity of our procedure by genotyping genetically modified medaka (Oryzias latipes) and zebrafish (Danio rerio) embryos (targeted by CRISPR/Cas9 knock-out and knock-in) in a 96-well plate format. The robust isolation and detection of multiple alleles of various abundancies in a mosaic genetic background allows phenotype-genotype correlation already in the injected generation, demonstrating the reliability and sensitivity of the protocol. Our method is applicable across kingdoms to samples ranging from cells to tissues i. e. plant seedlings, adult flies, mouse cell culture and tissue as well as adult fish fin-clips.
Novel variants and clinical symptoms in four new ALG3‐CDG patients review of the literature, and identification of AAGRP‐ALG3 as a novel ALG3 variant with alanine and glycine‐rich N‐terminus
ALG3‐CDG is one of the very rare types of congenital disorder of glycosylation (CDG) caused by variants in the ER‐mannosyltransferase ALG3. Here, we summarize the clinical, biochemical, and genetic data of four new ALG3‐CDG patients, who were identified by a type I pattern of serum transferrin and the accumulation of Man5GlcNAc2‐PP‐dolichol in LLO analysis. Additional clinical symptoms observed in our patients comprise sensorineural hearing loss, right‐descending aorta, obstructive cardiomyopathy, macroglossia, and muscular hypertonia. We add four new biochemically confirmed variants to the list of ALG3‐CDG inducing variants: c.350G>C (p.R117P), c.1263G>A (p.W421*), c.1037A>G (p.N346S), and the intron variant c.296+4A>G. Furthermore, in Patient 1 an additional open‐reading frame of 141 bp (AAGRP) in the coding region of ALG3 was identified. Additionally, we show that control cells synthesize, to a minor degree, a hybrid protein composed of the polypeptide AAGRP and ALG3 (AAGRP‐ALG3), while in Patient 1 expression of this hybrid protein is significantly increased due to the homozygous variant c.160_196del (g.165C>T). By reviewing the literature and combining our findings with previously published data, we further expand the knowledge of this rare glycosylation defect.
Cutis laxa, exocrine pancreatic insufficiency and altered cellular metabolomics as additional symptoms in a new patient with ATP6AP1-CDG.
Dimitrov B, Himmelreich N, Hipgrave Ederveen AL, Lüchtenborg C, Okun JG, Breuer M, Hutter AM, Carl M, Guglielmi L, Hellwig A, Thiemann KC, Jost M, Peters V, Staufner C, Hoffmann GF, Hackenberg A, Paramasivam N, Wiemann S, Eils R, Schlesner M, Strahl S, Brügger B, Wuhrer M, Christoph Korenke G, Thiel C
Congenital disorders of glycosylation (CDG) are genetic defects in the glycoconjugate biosynthesis. >100 types of CDG are known, most of them cause multi-organ diseases. Here we describe a boy whose leading symptoms comprise cutis laxa, pancreatic insufficiency and hepatosplenomegaly. Whole exome sequencing identified the novel hemizygous mutation c.542T>G (p.L181R) in the X-linked ATP6AP1, an accessory protein of the mammalian vacuolar H+-ATPase, which led to a general N-glycosylation deficiency. Studies of serum N-glycans revealed reduction of complex sialylated and appearance of truncated diantennary structures. Proliferation of the patient’s fibroblasts was significantly reduced and doubling time prolonged. Additionally, there were alterations in the fibroblasts‘ amino acid levels and the acylcarnitine composition. Especially, short-chain species were reduced, whereas several medium- to long-chain acylcarnitines (C14-OH to C18) were elevated. Investigation of the main lipid classes revealed that total cholesterol was significantly enriched in the patient’s fibroblasts at the expense of phophatidylcholine and phosphatidylethanolamine. Within the minor lipid species, hexosylceramide was reduced, while its immediate precursor ceramide was increased. Since catalase activity and ACOX3 expression in peroxisomes were reduced, we assume an ATP6AP1-dependent impact on the β-oxidation of fatty acids. These results help to understand the complex clinical characteristics of this new patient.
A mutation in mannose‐phosphate‐dolichol utilization defect 1 reveals clinical symptoms of congenital disorders of glycosylation type I and dystroglycanopathy
Walinka van Tol, Angel Ashikov, Eckhard Korsch, Nurulamin Abu Bakar, Michèl A. Willemsen, Christian Thiel, Dirk J. Lefeber
Journal of Inherited Metabolic Disease
Published: September 30, 2019
Congenital disorders of glycosylation type I (CDG‐I) are inborn errors of metabolism, generally characterized by multisystem clinical manifestations, including developmental delay, hepatopathy, hypotonia, and skin, skeletal, and neurological abnormalities. Among others, dolichol‐phosphate‐mannose (DPM) is the mannose donor for N‐glycosylation as well as O‐mannosylation. DOLK‐CDG, DPM1‐CDG, DPM2‐CDG, and DPM3‐CDG are defects in the DPM synthesis showing both CDG‐I abnormalities and reduced O‐mannosylation of alpha‐dystroglycan (αDG), which leads to muscular dystrophy‐dystroglycanopathy. Mannose‐phosphate‐dolichol utilization defect 1 (MPDU1) plays a role in the utilization of DPM. Here, we report two MPDU1‐CDG patients without skin involvement, but with massive dilatation of the biliary duct system and dystroglycanopathy characteristics including hypotonia, elevated creatine kinase, dilated cardiomyopathy, buphthalmos, and congenital glaucoma. Biochemical analyses revealed elevated disialotransferrin in serum, and analyses in fibroblasts showed shortened lipid linked oligosaccharides and DPM, and reduced O‐mannosylation of αDG. Thus, MPDU1‐CDG can be added to the list of disorders with overlapping biochemical and clinical abnormalities of CDG‐I and dystroglycanopathy.