C-mannosylation supports folding and enhances stability of thrombospondin repeats

Conserved cysteines prevent C-mannosylation of mucin Cys domains

Mucins are major components of the mucus. Besides the highly O-glycosylated tandem repeat domains, mucins contain Cys domains (CysDs). CysDs contain conserved disulfide-forming cysteine residues as well as a WxxW motif. Since this is the consensus sequence for tryptophan C-mannosylation, mucin CysDs have been suggested to be targets for C-mannosyltransferases, but this has never been directly shown. Here, we recombinantly expressed human mucin CysDs in Chinese hamster ovary (CHO) cells and analyzed the C-mannosylation status. Mass spectrometric analysis revealed that the putative C-mannose site is not or only barely C-mannosylated. However, mutation of the adjacent cysteine residues enabled C-mannosylation to occur. In contrast to mucin CysDs, the homologous CysD of human cartilage intermediate layer protein 1 (CILP1) lacks these cysteine residues preceding the WxxW motif. We show that CILP1 CysD is C-mannosylated, but introducing a cysteine at the -2 position causes this modification to be lost. We thus conclude that the presence of cysteine residues prevents the modification of the WxxW motif in CysDs.

Global View of Domain-Specific O-Linked Mannose Glycosylation in Glycoengineered Cells

Protein O-linked mannose (O-Man) glycosylation is an evolutionary conserved posttranslational modification that fulfills important biological roles during embryonic development. Three nonredundant enzyme families, POMT1/POMT2, TMTC1-4, and TMEM260, selectively coordinate the initiation of protein O-Man glycosylation on distinct classes of transmembrane proteins, including α-dystroglycan, cadherins, and plexin receptors. However, a systematic investigation of their substrate specificities is lacking, in part due to the ubiquitous expression of O-Man glycosyltransferases in cells, which precludes analysis of pathway-specific O-Man glycosylation on a proteome-wide scale. Here, we apply a targeted workflow for membrane glycoproteomics across five human cell lines to extensively map O-Man substrates and genetically deconstruct O-Man initiation by individual and combinatorial knockout of O-Man glycosyltransferase genes. We established a human cell library for the analysis of substrate specificities of individual O-Man initiation pathways by quantitative glycoproteomics. Our results identify 180 O-Man glycoproteins, demonstrate new protein targets for the POMT1/POMT2 pathway, and show that TMTC1-4 and TMEM260 pathways widely target distinct Ig-like protein domains of plasma membrane proteins involved in cell-cell and cell-extracellular matrix interactions. The identification of O-Man on Ig-like folds adds further knowledge on the emerging concept of domain-specific O-Man glycosylation which opens for functional studies of O-Man-glycosylated adhesion molecules and receptors.

Protein O-Fucosyltransferase Is Required for the Efficient Invasion of Hepatocytes by Plasmodium berghei Sporozoites

The O-fucosylation of the thrombospondin type I repeat (TSR) domain is important for TSR-containing proteins' optimal folding and stability. However, the importance of Plasmodium O-fucosyltransferase 2 (POFut2) remains unclear due to two different reports. Here, we disrupted the POFut2 gene in Plasmodium berghei and demonstrated that POFut2 KO parasites develop normally in blood and mosquito stages but show reduced infectivity in mice. We found that the reduced infectivity of POFut2 KO sporozoites was due to a diminished level of TRAP that affected the parasite gliding motility and hepatocyte infectivity. Using all-atom MD simulation, we also hypothesize that O-fucosylation impacts the TSR domain's stability more than its heparin binding capacity.

Molecular characterization of the craniosynostosis-associated interleukin-11 receptor variants p.T306_S308dup and p.E364_V368del

Interleukin-11 (IL-11) is a member of the IL-6 family of cytokines and is an important factor for bone homeostasis. IL-11 binds to and signals via the membrane-bound IL-11 receptor (IL-11R, classic signaling) or soluble forms of the IL-11R (sIL-11R, trans-signaling). Mutations in the IL11RA gene, which encodes the IL-11R, are associated with craniosynostosis, a human condition in which one or several of the sutures close prematurely, resulting in malformation of the skull. The biological mechanisms of how mutations within the IL-11R are linked to craniosynostosis are mostly unexplored. In this study, we analyze two variants of the IL-11R described in craniosynostosis patients: p.T306_S308dup, which results in a duplication of three amino-acid residues within the membrane-proximal fibronectin type III domain, and p.E364_V368del, which results in a deletion of five amino-acid residues in the so-called stalk region adjacent to the plasma membrane. The stalk region connects the three extracellular domains to the transmembrane and intracellular region of the IL-11R and contains cleavage sites for different proteases that generate sIL-11R variants. Using a combination of bioinformatics and different biochemical, molecular, and cell biology methods, we show that the IL-11R-T306_S308dup variant does not mature correctly, is intracellularly retained, and does not reach the cell surface. In contrast, the IL-11R-E364_V368del variant is fully biologically active and processed normally by proteases, thus allowing classic and trans-signaling of IL-11. Our results provide evidence that mutations within the IL11RA gene may not be causative for craniosynostosis and suggest that other regulatory mechanism(s) are involved but remain to be identified.

Molecular evolution of the Thrombospondin superfamily

Thrombospondins (TSPs) are multidomain, calcium-binding glycoproteins that have wide-ranging roles in vertebrates in cell interactions, extracellular matrix (ECM) organisation, angiogenesis, tissue remodelling, synaptogenesis, and also in musculoskeletal and cardiovascular functions. Land animals encode five TSPs, which assembly co-translationally either as trimers (subgroup A) or pentamers (subgroup B). The vast majority of research has focused on this canonical TSP family, which evolved through the whole-genome duplications that took place early in the vertebrate lineage. With benefit of the growth in genome- and transcriptome-predicted proteomes of a much wider range of animal species, examination of TSPs throughout metazoan phyla has revealed extensive conservation of subgroup B-type TSPs in invertebrates. In addition, these searches established that canonical TSPs are, in fact, one branch within a TSP superfamily that includes other clades designated mega-TSPs, sushi-TSPs and poriferan-TSPs. Despite the apparent simplicity of poriferans and cnidarians as organisms, these phyla encode a greater diversity of TSP superfamily members than vertebrates. We discuss here the molecular characteristics of the TSP superfamily members, current knowledge of their expression profiles and functions in invertebrates, and models for the evolution of this complex ECM superfamily.

Recent Advances in Photoinduced Modification of Amino Acids, Peptides, and Proteins

The chemical modification of biopolymers like peptides and proteins is a key technology to access vaccines and pharmaceuticals. Similarly, the tunable derivatization of individual amino acids is important as they are key building blocks of biomolecules, bioactive natural products, synthetic polymers, and innovative materials. The high diversity of functional groups present in amino acid-based molecules represents a significant challenge for their selective derivatization Recently, visible light-mediated transformations have emerged as a powerful strategy for achieving chemoselective biomolecule modification. This technique offers numerous advantages over other methods, including a higher selectivity, mild reaction conditions and high functional-group tolerance. This review provides an overview of the most recent methods covering the photoinduced modification for single amino acids and site-selective functionalization in peptides and proteins under mild and even biocompatible conditions. Future challenges and perspectives are discussed beyond the diverse types of photocatalytic transformations that are currently available.

Oxidative cyclization reagents reveal tryptophan cation-π interactions

Methods for selective covalent modification of amino acids on proteins can enable a diverse array of applications, spanning probes and modulators of protein function to proteomics1-3. Owing to their high nucleophilicity, cysteine and lysine residues are the most common points of attachment for protein bioconjugation chemistry through acid-base reactivity3,4. Here we report a redox-based strategy for bioconjugation of tryptophan, the rarest amino acid, using oxaziridine reagents that mimic oxidative cyclization reactions in indole-based alkaloid biosynthetic pathways to achieve highly efficient and specific tryptophan labelling. We establish the broad use of this method, termed tryptophan chemical ligation by cyclization (Trp-CLiC), for selectively appending payloads to tryptophan residues on peptides and proteins with reaction rates that rival traditional click reactions and enabling global profiling of hyper-reactive tryptophan sites across whole proteomes. Notably, these reagents reveal a systematic map of tryptophan residues that participate in cation-π interactions, including functional sites that can regulate protein-mediated phase-separation processes.

Glycosylation: A new signaling paradigm for the neurovascular diseases

A wide range of life-threatening conditions with complicated pathogenesis involves neurovascular disorders encompassing Neurovascular unit (NVU) damage. The pathophysiology of NVU is characterized by several features including tissue hypoxia, stimulation of inflammatory and angiogenic processes, and the initiation of intricate molecular interactions, collectively leading to an elevation in blood-brain barrier permeability, atherosclerosis and ultimately, neurovascular diseases. The presence of compelling data about the significant involvement of the glycosylation in the development of diseases has sparked a discussion on whether the abnormal glycosylation may serve as a causal factor for neurovascular disorders, rather than being just recruited as a secondary player in regulating the critical events during the development processes like embryo growth and angiogenesis. An essential tool for both developing new anti-ischemic therapies and understanding the processes of ischemic brain damage is undertaking pre-clinical studies of neurovascular disorders. Together with the post-translational modification of proteins, the modulation of glycosylation and its enzymes implicates itself in several abnormal activities which are known to accelerate neuronal vasculopathy. Despite the failure of the majority of glycosylation-based preclinical and clinical studies over the past years, there is a significant probability to provide neuroprotection utilizing modern and advanced approaches to target abnormal glycosylation activity at embryonic stages as well. This article focuses on a variety of experimental evidence to postulate the interconnection between glycosylation and vascular disorders along with possible treatment options.

Diversification of Glycosyl Compounds via Glycosyl Radicals

Glycosyl radical functionalization is one of the central topics in synthetic carbohydrate chemistry. Recent advances in metal-catalyzed cross-coupling chemistry and metallaphotoredox catalysis provided powerful platforms for glycosyl radical diversification. In particular, the discovery of new glycosyl radical precursors in conjunction with these advanced reaction technologies have significantly expanded the space for glycosyl compound synthesis. In this Review, we highlight the most recent progress in this area starting from 2021, and the reports included will be categorized based on different reaction types for better clarity.

C-Glycopyranosyl aldehydes: emerging chiral synthons in organic synthesis

Herein, we have summarized the vast array of synthetic processes that have been developed for the synthesis of C-glycopyranosyl aldehydes and diverse C-glycoconjugates derived from them by covering the literature reported from 1979 to 2023. Notwithstanding its challenging chemistry, C-glycosides are considered stable pharmacophores and are used as important bioactive molecules. The discussed synthetic methodologies to access C-glycopyranosyl aldehydes take advantage of seven key intermediates, viz. allene, thiazole, dithiane, cyanide, alkene, and nitromethane. Furthermore, the integration of complex C-glycoconjugates derived from varied C-glycopyranosyl aldehydes involves nucleophilic addition/substitution, reduction, condensation, oxidation, cyclo condensation, coupling, and Wittig reactions. In this review, we have categorized the synthesis of C-glycopyranosyl aldehydes and C-glycoconjugates on the basis of the methodology used for their synthesis and on types of C-glycoconjugates, respectively.

Protein posttranslational modifications in health and diseases: Functions, regulatory mechanisms, and therapeutic implications

Protein posttranslational modifications (PTMs) refer to the breaking or generation of covalent bonds on the backbones or amino acid side chains of proteins and expand the diversity of proteins, which provides the basis for the emergence of organismal complexity. To date, more than 650 types of protein modifications, such as the most well-known phosphorylation, ubiquitination, glycosylation, methylation, SUMOylation, short-chain and long-chain acylation modifications, redox modifications, and irreversible modifications, have been described, and the inventory is still increasing. By changing the protein conformation, localization, activity, stability, charges, and interactions with other biomolecules, PTMs ultimately alter the phenotypes and biological processes of cells. The homeostasis of protein modifications is important to human health. Abnormal PTMs may cause changes in protein properties and loss of protein functions, which are closely related to the occurrence and development of various diseases. In this review, we systematically introduce the characteristics, regulatory mechanisms, and functions of various PTMs in health and diseases. In addition, the therapeutic prospects in various diseases by targeting PTMs and associated regulatory enzymes are also summarized. This work will deepen the understanding of protein modifications in health and diseases and promote the discovery of diagnostic and prognostic markers and drug targets for diseases.

O-fucosylation of thrombospondin type I repeats is dispensable for trafficking thrombospondin 1 to platelet secretory granules

Thrombospondin 1 (THBS1) is a secreted extracellular matrix glycoprotein that regulates a variety of cellular and physiological processes. THBS1's diverse functions are attributed to interactions between the modular domains of THBS1 with an array of proteins found in the extracellular matrix. THBS1's three Thrombospondin type 1 repeats (TSRs) are modified with O-linked glucose-fucose disaccharide and C-mannose. It is unknown whether these modifications impact trafficking and/or function of THBS1 in vivo. The O-fucose is added by Protein O-fucosyltransferase 2 (POFUT2) and is sequentially extended to the disaccharide by β3glucosyltransferase (B3GLCT). The C-mannose is added by one or more of four C-mannosyltransferases. O-fucosylation by POFUT2/B3GLCT in the endoplasmic reticulum has been proposed to play a role in quality control by locking TSR domains into their three-dimensional fold, allowing for proper secretion of many O-fucosylated substrates. Prior studies showed the siRNA knockdown of POFUT2 in HEK293T cells blocked secretion of TSRs 1-3 from THBS1. Here we demonstrated that secretion of THBS1 TSRs 1-3 was not reduced by CRISPR-Cas9-mediated knockout of POFUT2 in HEK293T cells and demonstrated that knockout of Pofut2 or B3glct in mice did not reduce the trafficking of endogenous THBS1 to secretory granules of platelets, a major source of THBS1. Additionally, we demonstrated that all three TSRs from platelet THBS1 were highly C-mannosylated, which has been shown to stabilize TSRs in vitro. Combined, these results suggested that POFUT2 substrates with TSRs that are also modified by C-mannose may be less susceptible to trafficking defects resulting from the loss of the glucose-fucose disaccharide.

Structure, sequon recognition and mechanism of tryptophan C-mannosyltransferase

C-linked glycosylation is essential for the trafficking, folding and function of secretory and transmembrane proteins involved in cellular communication processes. The tryptophan C-mannosyltransferase (CMT) enzymes that install the modification attach a mannose to the first tryptophan of WxxW/C sequons in nascent polypeptide chains by an unknown mechanism. Here, we report cryogenic-electron microscopy structures of Caenorhabditis elegans CMT in four key states: apo, acceptor peptide-bound, donor-substrate analog-bound and as a trapped ternary complex with both peptide and a donor-substrate mimic bound. The structures indicate how the C-mannosylation sequon is recognized by this CMT and its paralogs, and how sequon binding triggers conformational activation of the donor substrate: a process relevant to all glycosyltransferase C superfamily enzymes. Our structural data further indicate that the CMTs adopt an unprecedented electrophilic aromatic substitution mechanism to enable the C-glycosylation of proteins. These results afford opportunities for understanding human disease and therapeutic targeting of specific CMT paralogs.

A structural model of the iRhom-ADAM17 sheddase complex reveals functional insights into its trafficking and activity

Several membrane-anchored signal mediators such as cytokines (e.g. TNFα) and growth factors are proteolytically shed from the cell surface by the metalloproteinase ADAM17, which, thus, has an essential role in inflammatory and developmental processes. The membrane proteins iRhom1 and iRhom2 are instrumental for the transport of ADAM17 to the cell surface and its regulation. However, the structure-function determinants of the iRhom-ADAM17 complex are poorly understood. We used AI-based modelling to gain insights into the structure-function relationship of this complex. We identified different regions in the iRhom homology domain (IRHD) that are differentially responsible for iRhom functions. We have supported the validity of the predicted structure-function determinants with several in vitro, ex vivo and in vivo approaches and demonstrated the regulatory role of the IRHD for iRhom-ADAM17 complex cohesion and forward trafficking. Overall, we provide mechanistic insights into the iRhom-ADAM17-mediated shedding event, which is at the centre of several important cytokine and growth factor pathways.

Destabilization of vitelline membrane outer layer protein 1 homolog (VMO1) by C-mannosylation

C-mannosylation is a rare type of protein glycosylation whereby a single mannose is added to the first tryptophan in the consensus sequence Trp-Xaa-Xaa-Trp/Cys (in which Xaa represents any amino acid). Its consensus sequence is mainly found in proteins containing a thrombospondin type-1 repeat (TSR1) domain and in type I cytokine receptors. In these proteins, C-mannosylation affects protein secretion, intracellular localization, and protein stability; however, the role of C-mannosylation in proteins that are not type I cytokine receptors and/or do not contain a TSR1 domain is less well explored. In this study, we focused on human vitelline membrane outer layer protein 1 homolog (VMO1). VMO1, which possesses two putative C-mannosylation sites, is a 21-kDa secreted protein that does not contain a TSR1 domain and is not a type I cytokine receptor. Mass spectrometry analyses revealed that VMO1 is C-mannosylated at Trp¹⁰⁵ but not at Trp⁴⁴ . Although C-mannosylation does not affect the extracellular secretion of VMO1, it destabilizes the intracellular VMO1. In addition, a structural comparison between VMO1 and C-mannosylated VMO1 showed that the modification of the mannose changes the conformation of three loops in VMO1. Taken together, our results demonstrate the first example of C-mannosylation for protein destabilization of VMO1.

Conservation, abundance, glycosylation profile, and localization of the TSP protein family in Cryptosporidium parvum

Cryptosporidium parvum is a zoonotic apicomplexan parasite and a common cause of diarrheal disease worldwide. The development of vaccines to prevent or limit infection remains an important goal for tackling cryptosporidiosis. At present, the only approved vaccine against any apicomplexan parasite targets a conserved adhesin possessing a thrombospondin repeat domain. C. parvum possesses 12 orthologous thrombospondin repeat domain-containing proteins known as CpTSP1-12, though little is known about these potentially important antigens. Here, we explore the architecture and conservation of the CpTSP protein family, as well as their abundance at the protein level within the sporozoite stage of the life cycle. We examine the glycosylation states of these proteins using a combination of glycopeptide enrichment techniques to demonstrate that these proteins are modified with C-, O-, and N-linked glycans. Using expansion microscopy, and an antibody against the C-linked mannose that is unique to the CpTSP protein family within C. parvum, we show that these proteins are found both on the cell surface and in structures that resemble the secretory pathway of C. parvum sporozoites. Finally, we generated a polyclonal antibody against CpTSP1 to show that it is found at the cell surface and within micronemes, in a pattern reminiscent of other apicomplexan motility-associated adhesins, and is present both in sporozoites and meronts. This work sheds new light on an understudied family of C. parvum proteins that are likely to be important to both parasite biology and the development of vaccines against cryptosporidiosis.

Structural studies offer a framework for understanding the role of properdin in the alternative pathway and beyond

Structures of alternative pathway proteins have offered a comprehensive structural basis for understanding the molecular mechanisms governing activation and regulation of the amplification pathway of the complement cascade. Although properdin (FP) is required in vivo to sustain a functional alternative pathway, structural studies have been lagging behind due to the extended structure and polydisperse nature of FP. We review recent progress with respect to structure determination of FP and its proconvertase/convertase complexes. These structures identify in detail regions in C3b, factor B and FP involved in their mutual interactions. Structures of FP oligomers obtained by integrative studies have shed light on how FP activity depends on its oligomerization state. The accumulated structural knowledge allows us to rationalize the effect of point mutations causing FP deficiency. The structural basis for FP inhibition by the tick CirpA proteins is reviewed and the potential of alphafold2 predictions for understanding the interaction of FP with other tick proteins and the NKp46 receptor on host immune cells is discussed. The accumulated structural knowledge forms a comprehensive basis for understanding molecular interactions involving FP, pathological conditions arising from low levels of FP, and the molecular strategies used by ticks to suppress the alternative pathway.

C-mannosylation regulates stabilization of RAMP1 protein and RAMP1-mediated cell migration

C-mannosylation is a unique type of protein glycosylation via C-C linkage between an α-mannose and a tryptophan residue. This modification has been identified in about 30 proteins and regulates several functions, such as protein secretion and intracellular localization, as well as protein stability. About half of C-mannosylated proteins are categorized as proteins containing thrombospondin type 1 repeat domain or type I cytokine receptors. To evaluate whether C-mannosylation broadly affects protein functions regardless of protein domain or family, we have sought to identify other types of C-mannosylated protein and analyse their functions. In this study, we focused on receptor activity modifying protein 1, which neither contains thrombospondin type 1 repeat domain nor belongs to the type I cytokine receptors. Our mass spectrometry analysis demonstrated that RAMP1 is C-mannosylated at Trp⁵⁶ . It has been shown that RAMP1 transports to the plasma membrane after dimerization with calcitonin receptor-like receptor and is important for ligand-dependent downstream signalling activation. Our results showed that C-mannosylation has no effect on this transport activity. On the other hand, C-mannosylation did enhance protein stability and cell migration activity. Our data may provide new insight into both C-mannosylation research and novel RAMP1 analysis.

Isthmin-A Multifaceted Protein Family

Isthmin (ISM) is a secreted protein family with two members, namely ISM1 and ISM2, both containing a TSR1 domain followed by an AMOP domain. Its broad expression pattern suggests diverse functions in developmental and physiological processes. Over the past few years, multiple studies have focused on the functional analysis of the ISM protein family in several events, including angiogenesis, metabolism, organ homeostasis, immunity, craniofacial development, and cancer. Even though ISM was identified two decades ago, we are still short of understanding the roles of the ISM protein family in embryonic development and other pathological processes. To address the role of ISM, functional studies have begun but unresolved issues remain. To elucidate the regulatory mechanism of ISM, it is crucial to determine its interactions with other ligands and receptors that lead to the activation of downstream signalling pathways. This review provides a perspective on the gene organization and evolution of the ISM family, their links with developmental and physiological functions, and key questions for the future.

Molecular basis of C-mannosylation - a structural perspective

The structural and functional diversity of proteins can be enhanced by numerous post-translational modifications. C-mannosylation is a rare form of glycosylation consisting of a single alpha or beta D-mannopyranose forming a carbon-carbon bond with the pyrrole ring of a tryptophan residue. Despite first being discovered in 1994, C-mannosylation is still poorly understood and 3D structures are available for only a fraction of the total predicted C-mannosylated proteins. Here, we present the first comprehensive review of C-mannosylated protein structures by analysing the data for all 10 proteins with C-mannosylation/s deposited in the Research Collaboratory for Structural Bioinformatics Protein Data Bank (RCSB PDB). We analysed in detail the WXXW/WXXWXXW consensus motif and the highly conserved pair of arginine residues in thrombospondin type 1 repeat C-mannosylation sites or homologous arginine residues in other domains. Furthermore, we identified a conserved PXP sequence C-terminal of the C-mannosylation site. The PXP motif forms a tight turn region in the polypeptide chain and its universal conservation in C-mannosylated protein is worthy of further experimental study. The stabilization of C-mannopyranosyl groups was demonstrated through hydrogen bonding with arginine and other charged or polar amino acids. Where possible, the structural findings were linked to other functional studies demonstrating the role of C-mannosylation in protein stability, secretion or function. With the current technological advances in structural biology, we hope to see more progress in the study of C-mannosylation that may correspond to discoveries of novel C-mannosylation pathways and functions with implications for human health and biotechnology.

GPC3-Unc5 receptor complex structure and role in cell migration

Neural migration is a critical step during brain development that requires the interactions of cell-surface guidance receptors. Cancer cells often hijack these mechanisms to disseminate. Here, we reveal crystal structures of Uncoordinated-5 receptor D (Unc5D) in complex with morphogen receptor glypican-3 (GPC3), forming an octameric glycoprotein complex. In the complex, four Unc5D molecules pack into an antiparallel bundle, flanked by four GPC3 molecules. Central glycan-glycan interactions are formed by N-linked glycans emanating from GPC3 (N241 in human) and C-mannosylated tryptophans of the Unc5D thrombospondin-like domains. MD simulations, mass spectrometry and structure-based mutants validate the crystallographic data. Anti-GPC3 nanobodies enhance or weaken Unc5-GPC3 binding and, together with mutant proteins, show that Unc5/GPC3 guide migrating pyramidal neurons in the mouse cortex, and cancer cells in an embryonic xenograft neuroblastoma model. The results demonstrate a conserved structural mechanism of cell guidance, where finely balanced Unc5-GPC3 interactions regulate cell migration.

Tryptophan C-mannosylation is critical for Plasmodium falciparum transmission

Tryptophan C-mannosylation stabilizes proteins bearing a thrombospondin repeat (TSR) domain in metazoans. Here we show that Plasmodium falciparum expresses a DPY19 tryptophan C-mannosyltransferase in the endoplasmic reticulum and that DPY19-deficiency abolishes C-glycosylation, destabilizes members of the TRAP adhesin family and inhibits transmission to mosquitoes. Imaging P. falciparum gametogenesis in its entirety in four dimensions using lattice light-sheet microscopy reveals defects in ΔDPY19 gametocyte egress and exflagellation. While egress is diminished, ΔDPY19 microgametes still fertilize macrogametes, forming ookinetes, but these are abrogated for mosquito infection. The gametogenesis defects correspond with destabilization of MTRAP, which we show is C-mannosylated in P. falciparum, and the ookinete defect is concordant with defective CTRP secretion on the ΔDPY19 background. Genetic complementation of DPY19 restores ookinete infectivity, sporozoite production and C-mannosylation activity. Therefore, tryptophan C-mannosylation by DPY19 ensures TSR protein quality control at two lifecycle stages for successful transmission of the human malaria parasite.

Here, Lopaticki et al. show that Plasmodium falciparum expresses a Dpy19 C-mannosyltransferase in the endoplasmic reticulum that glycosylates TSR domains. Functional characterization shows that PfDpy19 plays a critical role in transmission through mosquitoes as PfDpy19-deficiency abolishes C-glycosylation and destabilizes proteins relevant for gametogenesis and oocyst formation.

GANAB and N-Glycans Substrates Are Relevant in Human Physiology, Polycystic Pathology and Multiple Sclerosis: A Review

Glycans are one of the four fundamental macromolecular components of living matter, and they are highly regulated in the cell. Their functions are metabolic, structural and modulatory. In particular, ER resident N-glycans participate with the Glc3Man9GlcNAc2 highly conserved sequence, in protein folding process, where the physiological balance between glycosylation/deglycosylation on the innermost glucose residue takes place, according GANAB/UGGT concentration ratio. However, under abnormal conditions, the cell adapts to the glucose availability by adopting an aerobic or anaerobic regimen of glycolysis, or to external stimuli through internal or external recognition patterns, so it responds to pathogenic noxa with unfolded protein response (UPR). UPR can affect Multiple Sclerosis (MS) and several neurological and metabolic diseases via the BiP stress sensor, resulting in ATF6, PERK and IRE1 activation. Furthermore, the abnormal GANAB expression has been observed in MS, systemic lupus erythematous, male germinal epithelium and predisposed highly replicating cells of the kidney tubules and bile ducts. The latter is the case of Polycystic Liver Disease (PCLD) and Polycystic Kidney Disease (PCKD), where genetically induced GANAB loss affects polycystin-1 (PC1) and polycystin-2 (PC2), resulting in altered protein quality control and cyst formation phenomenon. Our topics resume the role of glycans in cell physiology, highlighting the N-glycans one, as a substrate of GANAB, which is an emerging key molecule in MS and other human pathologies.

C-Mannosyltransferase Is Essential for Malaria Transmission in Plasmodium berghei

C-Mannosylation of the thrombospondin type I repeat (TSR) domains is one of the most important factors involved in their function. It occurs on the first tryptophan of the WXXWXXC conserved motif where the tryptophan is usually surrounded by arginine or lysine forming the ligand-binding stretch of this sticky domain. It is found in its canonical or modified forms in many Plasmodium proteins. TSR containing proteins such as thrombospondin-like anonymous protein (TRAP), circumsporozoite protein (CSP), CSP and TRAP related protein (CTRP), and secreted protein with altered thrombospondin repeat (SPATR) have all been shown to be important for various parasite processes and life cycle stages. Here, we show that C-mannosylation catalyzing enzyme C-mannosyltransferase (CmanT) plays an essential role in malaria transmission in Plasmodium berghei. Disruption of the CmanT does not affect asexual blood stage propagation or gametocyte development but abolishes the formation of oocysts in mosquitoes. CmanT knockout (CmanT^-) parasites showed normal ookinete formation; however, these ookinetes failed in their ability to glide. CmanT^- was complemented by reintroducing the gene, restoring mosquito transmission to wild-type level. We also investigated the effect of C-mannosylation on the folding and heparin-binding capacity of the Plasmodium falciparum TRAP TSR domain in silico, which suggested that this phenotype should be due to its involvement in the global stabilization of TSR residue side chain interactions.

O-fucosylation stabilizes the TSR3 motif in thrombospondin-1 by interacting with nearby amino acids and protecting a disulfide bond

Thrombospondin type-1 repeats (TSRs) are small protein motifs containing six conserved cysteines forming three disulfide bonds that can be modified with an O-linked fucose. Protein O-fucosyltransferase 2 (POFUT2) catalyzes the addition of O-fucose to TSRs containing the appropriate consensus sequence, and the O-fucose modification can be elongated to a Glucose-Fucose disaccharide with the addition of glucose by β3-glucosyltransferase (B3GLCT). Elimination of Pofut2 in mice results in embryonic lethality in mice, highlighting the biological significance of O-fucose modification on TSRs. Knockout of POFUT2 in HEK293T cells has been shown to cause complete or partial loss of secretion of many proteins containing O-fucosylated TSRs. In addition, POFUT2 is localized to the endoplasmic reticulum (ER) and only modifies folded TSRs, stabilizing their structures. These observations suggest that POFUT2 is involved in an ER quality control mechanism for TSR folding and that B3GLCT also participates in quality control by providing additional stabilization to TSRs. However, the mechanisms by which addition of these sugars result in stabilization are poorly understood. Here, we conducted molecular dynamics (MD) simulations and provide crystallographic and NMR evidence that the Glucose-Fucose disaccharide interacts with specific amino acids in the TSR3 domain in thrombospondin-1 that are within proximity to the O-fucosylation modification site resulting in protection of a nearby disulfide bond. We also show that mutation of these amino acids reduces the stabilizing effect of the sugars in vitro. These data provide mechanistic details regarding the importance of O-fucosylation and how it participates in quality control mechanisms inside the ER.

A Bacterial Mannose Binding Lectin as a Tool for the Enrichment of C- and O-Mannosylated Peptides

Mass spectrometry (MS) easily detects C-mannosylated peptides from purified proteins but not from complex biological samples. Enrichment of specific glycopeptides by lectin affinity prior to MS analysis has been widely applied to support glycopeptide identification but was until now not available for C-mannosylated peptides. Here, we used the α-mannose-specific Burkholderia cenocepacia lectin A (BC2L-A) and show that, in addition to its previously demonstrated high-mannose N-glycan binding capability, this lectin is able to retain C- and O-mannosylated peptides. Besides testing binding abilities to standard peptides, we applied BC2L-A affinity to enrich C-mannosylated peptides from complex samples of tryptic digests of HEK293 and MCF10A whole cell extracts, which led to the identification of novel C-mannosylation sites. In conclusion, BC2L-A enabled specific enrichment of C- and O-mannosylated peptides and might have superior properties over other mannose binding lectins for this purpose.

Recent advances in palladium-catalyzed C(sp3)/C(sp2)-H bond functionalizations: access to C-branched glycosides

Over the recent decades, tremendous interest has developed in the transformation of complex substrates by C-H activation and functionalization. In particular, palladium-catalyzed directing and non-directing group-assisted C-H functionalization has emerged as a powerful avenue to access C-branched glycosides. Due to the extreme complexity, delicate functionalities, and high stability of C-H bonds, site-selective functionalization of carbohydrate under mild conditions is highly desirable. The purpose of this review is to cover most of the recent advances in palladium-catalyzed C(sp³) and C(sp²)-H bond functionalizations for the synthesis of C-branched glycosides along with future directions.

RTN4/NoGo-receptor binding to BAI adhesion-GPCRs regulates neuronal development

RTN4-binding proteins were widely studied as "NoGo" receptors, but their physiological interactors and roles remain elusive. Similarly, BAI adhesion-GPCRs were associated with numerous activities, but their ligands and functions remain unclear. Using unbiased approaches, we observed an unexpected convergence: RTN4 receptors are high-affinity ligands for BAI adhesion-GPCRs. A single thrombospondin type 1-repeat (TSR) domain of BAIs binds to the leucine-rich repeat domain of all three RTN4-receptor isoforms with nanomolar affinity. In the 1.65 Å crystal structure of the BAI1/RTN4-receptor complex, C-mannosylation of tryptophan and O-fucosylation of threonine in the BAI TSR-domains creates a RTN4-receptor/BAI interface shaped by unusual glycoconjugates that enables high-affinity interactions. In human neurons, RTN4 receptors regulate dendritic arborization, axonal elongation, and synapse formation by differential binding to glial versus neuronal BAIs, thereby controlling neural network activity. Thus, BAI binding to RTN4/NoGo receptors represents a receptor-ligand axis that, enabled by rare post-translational modifications, controls development of synaptic circuits.

Thrombospondin type 1 repeat-derived C-mannosylated peptide attenuates synaptogenesis of cortical neurons induced by primary astrocytes via TGF-β

C-Mannosylation is a rare type of protein glycosylation and is reportedly critical for the proper folding and secretion of parental proteins. Still, the effects of C-mannosylation on the biological functions of these modified proteins remain to be elucidated. The Trp-x-x-Trp (WxxW) sequences, whose first tryptophan (Trp) can be C-mannosylated, constitute the consensus motifs for this glycosylation modification and are commonly found in thrombospondin type 1 repeats that regulate molecular functions of thrombospondin 1 in binding and activation of transforming growth factor β (TGF-β). TGF-β plays critical roles in the control of the central nervous system including synaptogenesis. Here, we investigated whether C-mannosylation of the synthetic Trp-Ser-Pro-Trp (WSPW) peptide may confer certain functions to this peptide in TGF-β-mediated synaptogenesis. By using primary cultured rat astrocytes and cortical neurons, we found that the C-mannosylated WSPW (C-Man-WSPW) peptide, but not non-mannosylated WSPW peptide, suppressed astrocyte-conditioned medium (ACM)-stimulated synaptogenesis. C-Man-WSPW peptide inhibited both ACM- and recombinant mature TGF-β1-induced activations of Smad 2, an important mediator in TGF-β signaling. Interactions of recombinant mature TGF-β with the C-Man-WSPW peptide were similar to those with non-C-mannosylated WSPW peptide. Taken together, our results reveal a novel function of C-mannosylation of the WxxW motif in signaling and synaptogenesis mediated by TGF-β. Molecular details of how C-mannosylation affects the biological functions of WxxW motifs deserve future study for clarification.

Loss of the AMD-associated B3GLCT gene affects glycosylation of TSP1 without impairing secretion in retinal pigment epithelial cells

Age-related macular degeneration (AMD) has been associated with protective genetic variants in the β1-3 glucosyltransferase (B3GLCT) locus through genome-wide association studies. B3GLCT mediates modification of proteins with thrombospondin type I repeats (TSR) that contain O-linked glucose β1-3 fucose and C-linked mannose glycosylation motifs. B3GLCT-mediated modification is required for proper secretion of TSR-containing proteins. We aimed to start understanding the role of B3GLCT in AMD by evaluating its effect on glycosylation and secretion of proteins from retinal pigment epithelium (RPE) cells. We generated B3GLCT knockout (KO) RPE cells and analyzed glycosylation and secretion of thrombospondin 1 (TSP1), a protein involved in cellular processes highly relevant to AMD. Glycopeptide analysis confirmed the presence of the glucose-β1,3-fucose product of B3GLCT on TSP1 in wildtype (WT) cells and its absence in KO cells. C-mannosylation was variably present on WT TSP1 and increased on TSR domains 1 and 3 in KO cells. Secretion of TSP1 was not affected by the absence of B3GLCT, even not when TSP1 was upregulated by TNFα treatment or when TSP1 was overexpressed in HEK293T cells. Future research is needed to elucidate the effect of the observed glycosylation defects in the context of AMD, which might involve functional loss of TSP1 or effects on other TSR proteins.

C-Mannosylated tryptophan-containing WSPW peptide binds to actinin-4 and alters E-cadherin subcellular localization in lung epithelial-like A549 cells

The Trp-x-x-Trp (W-x-x-W) peptide motif, a consensus site for C-mannosylation, is the functional motif in cytokine type I receptors or thrombospondin type I repeat (TSR) superfamily proteins. W-x-x-W motifs are important for physiological and pathological functions of their parental proteins, but effects of C-mannosylation on protein functions remain to be elucidated. By using chemically synthesized WSPW peptides and C-mannosylated WSPW peptides (C-Man-WSPW), we herein investigated whether C-mannosylation of WSPW peptides confer additional biological functions to WSPW peptides. C-Man-WSPW peptide, but not non-mannosylated WSPW, reduced E-cadherin levels in A549 cells. Via peptide mass fingerprinting analysis, we identified actinin-4 as a C-Man-WSPW-binding protein in A549 cells. Actinin-4 partly co-localized with E-cadherin or β-catenin, despite no direct interaction between actinin-4 and E-cadherin. C-Man-WSPW reduced co-localization of E-cadherin and actinin-4; non-mannosylated WSPW had no effect on localization. In actinin-4-knockdown cells, E-cadherin was upregulated and demonstrated a punctate staining pattern in the cytoplasm, which suggests that actinin-4 regulated cell-surface E-cadherin localization. Thus, C-mannosylation of WSPW peptides is required for interaction with actinin-4 that subsequently alters expression and subcellular localization of E-cadherin and morphology of epithelial-like cells. Our results therefore suggest a regulatory role of C-mannosylation of the W-x-x-W motif in interactions between the motif and its binding partner and will thereby enhance understanding of protein C-mannosylation.

Other Types of Glycosylation

O-Linked glycosylation such as O-fucose, O-glucose, and O-N-acetylglucosamine are considered to be unusual. As suggested by the high levels of evolutional conservation, these O-glycans are fundamentally important for life. In the last two decades, our understanding of the importance of these glycans has greatly advanced. In particular, identification of the glycosyltransferases responsible for the biosynthesis of these glycans has accelerated basic research on the functional significance and molecular mechanisms by which these O-glycans regulate protein functions as well as clinical research on human diseases due to changes in these types of O-glycosylation. Notably, Notch receptor signaling is modified with and regulated by these types of O-glycans. Here, we summarize the current view of the structures and the significance of these O-glycans mainly in the context of Notch signaling regulation and human diseases.

Protein C-Mannosylation and C-Mannosyl Tryptophan in Chemical Biology and Medicine

C-Mannosylation is a post-translational modification of proteins in the endoplasmic reticulum. Monomeric α-mannose is attached to specific Trp residues at the first Trp in the Trp-x-x-Trp/Cys (W-x-x-W/C) motif of substrate proteins, by the action of C-mannosyltransferases, DPY19-related gene products. The acceptor substrate proteins are included in the thrombospondin type I repeat (TSR) superfamily, cytokine receptor type I family, and others. Previous studies demonstrated that C-mannosylation plays critical roles in the folding, sorting, and/or secretion of substrate proteins. A C-mannosylation-defective gene mutation was identified in humans as the disease-associated variant affecting a C-mannosylation motif of W-x-x-W of ADAMTSL1, which suggests the involvement of defects in protein C-mannosylation in human diseases such as developmental glaucoma, myopia, and/or retinal defects. On the other hand, monomeric C-mannosyl Trp (C-Man-Trp), a deduced degradation product of C-mannosylated proteins, occurs in cells and extracellular fluids. Several studies showed that the level of C-Man-Trp is upregulated in blood of patients with renal dysfunction, suggesting that the metabolism of C-Man-Trp may be involved in human kidney diseases. Together, protein C-mannosylation is considered to play important roles in the biosynthesis and functions of substrate proteins, and the altered regulation of protein C-manosylation may be involved in the pathophysiology of human diseases. In this review, we consider the biochemical and biomedical knowledge of protein C-mannosylation and C-Man-Trp, and introduce recent studies concerning their significance in biology and medicine.

Synthesis of C-Mannosylated Glycopeptides Enabled by Ni-Catalyzed Photoreductive Cross-Coupling Reactions

The biological functions of tryptophan C-mannosylation are poorly understood, in part, due to a dearth of methods for preparing pure glycopeptides and glycoproteins with this modification. To address this issue, efficient and scalable methods are required for installing this protein modification. Here, we describe unique Ni-catalyzed cross-coupling conditions that utilize photocatalysis or a Hantzsch ester photoreductant to couple glycosyl halides with (hetero)aryl bromides, thereby enabling the α-C-mannosylation of 2-bromo-tryptophan, peptides thereof, and (hetero)aryl bromides more generally. We also report that 2-(α-d-mannopyranosyl)-L-tryptophan undergoes facile anomerization in the presence of acid: something that must be considered when preparing and handling peptides with this modification. These developments enabled the first automated solid-phase peptide syntheses of C-mannosylated glycopeptides, which we used to map the epitope of an antibody, as well as providing the first verified synthesis of Carmo-HrTH-I, a C-mannosylated insect hormone. To complement this approach, we also performed late-stage tryptophan C-mannosylation on a diverse array of peptides, demonstrating the broad scope and utility of this methodology for preparing glycopeptides.

The C-Mannosylome of Human Induced Pluripotent Stem Cells Implies a Role for ADAMTS16 C-Mannosylation in Eye Development

C-mannosylation is a modification of tryptophan residues with a single mannose and can affect protein folding, secretion, and/or function. To date, only a few proteins have been demonstrated to be C-mannosylated, and studies that globally assess protein C-mannosylation are scarce. To interrogate the C-mannosylome of human induced pluripotent stem cells, we compared the secretomes of CRISPR–Cas9 mutants lacking either the C-mannosyltransferase DPY19L1 or DPY19L3 to WT human induced pluripotent stem cells using MS-based quantitative proteomics. The secretion of numerous proteins was reduced in these mutants, including that of A Disintegrin And Metalloproteinase with ThromboSpondin Motifs 16 (ADAMTS16), an extracellular protease that was previously reported to be essential for optic fissure fusion in zebrafish eye development. To test the functional relevance of this observation, we targeted dpy19l1 or dpy19l3 in embryos of the Japanese rice fish medaka (Oryzias latipes) by CRISPR–Cas9. We observed that targeting of dpy19l3 partially caused defects in optic fissure fusion, called coloboma. We further showed in a cellular model that DPY19L1 and DPY19L3 mediate C-mannosylation of a recombinantly expressed thrombospondin type 1 repeat of ADAMTS16 and thereby support its secretion. Taken together, our findings imply that DPY19L3-mediated C-mannosylation is involved in eye development by assisting secretion of the extracellular protease ADAMTS16.

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TSR1 of ADAMTS16 can be C-mannosylated.

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Deletion of DPY19L1 or DPY19L3 in hiPSCs caused reduced secretion of ADAMTS16.

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Targeting of dpy19l3 in medaka occasionally led to coloboma.

Hydrocephalus in mouse B3glct mutants is likely caused by defects in multiple B3GLCT substrates in ependymal cells and subcommissural organ

Peters plus syndrome, characterized by defects in eye and skeletal development with isolated cases of ventriculomegaly/hydrocephalus, is caused by mutations in the β3-glucosyltransferase (B3GLCT) gene. In the endoplasmic reticulum, B3GLCT adds glucose to O-linked fucose on properly folded Thrombospondin Type 1 Repeats (TSRs). The resulting glucose-fucose disaccharide is proposed to stabilize the TSR fold and promote secretion of B3GLCT substrates, with some substrates more sensitive than others to loss of glucose. Mouse B3glct mutants develop hydrocephalus at high frequency. In this study, we demonstrated that B3glct mutant ependymal cells had fewer cilia basal bodies and altered translational polarity compared to controls. Localization of mRNA encoding A Disintegrin and Metalloproteinase with ThromboSpondin type 1 repeat 20 (ADAMTS20) and ADAMTS9, suggested that reduced function of these B3GLCT substrates contributed to ependymal cell abnormalities. In addition, we showed that multiple B3GLCT substrates (Adamts3, Adamts9, and Adamts20) are expressed by the subcommissural organ, that subcommissural organ-spondin (SSPO) TSRs were modified with O-linked glucose-fucose, and that loss of B3GLCT reduced secretion of SSPO in cultured cells. In the B3glct mutant subcommissural organ intracellular SSPO levels were reduced and BiP levels increased, suggesting a folding defect. Secreted SSPO colocalized with BiP, raising the possibility that abnormal extracellular assembly of SSPO into Reissner's fiber also contributed to impaired CSF flow in mutants. Combined, these studies underscore the complexity of the B3glct mutant hydrocephalus phenotype and demonstrate that impaired cerebrospinal fluid (CSF) flow likely stems from the collective effects of the mutation on multiple processes.

Roles of glycoconjugates in neural patterning in C. elegans

Establishment of neural circuits requires reproducible and precise interactions between growing axons, dendrites and their tissue environment. Cell adhesion molecules and guidance factors are involved in the process, but how specificity is achieved remains poorly understood. Glycans are the third major class of biopolymers besides nucleic acids and proteins, and are usually covalently linked to proteins to form glycoconjugates. Common to most glycans is an extraordinary level of molecular diversity, making them attractive candidates to contribute specificity during neural development. Indeed, many genes important for neural development encode glycoproteins, or enzymes involved in synthesizing or modifying glycans. Glycoconjugates are classified based on both the types of glycans and type of attachment that link them to proteins. Here I discuss progress in understanding the function of glycans, glycan modifications and glycoconjugates during neural development in Caenorhabditis elegans. I will also highlight relevance to human disease and known roles of glycoconjugates in regeneration.

Yeast- and antibody-based tools for studying tryptophan C-mannosylation

Tryptophan C-mannosylation is an unusual co-translational protein modification performed by metazoans and apicomplexan protists. The prevalence and biological functions of this modification are poorly understood, with progress in the field hampered by a dearth of convenient tools for installing and detecting the modification. Here, we engineer a yeast system to produce a diverse array of proteins with and without tryptophan C-mannosylation and interrogate the modification's influence on protein stability and function. This system also enabled mutagenesis studies to identify residues of the glycosyltransferase and its protein substrates that are crucial for catalysis. The collection of modified proteins accrued during this work facilitated the generation and thorough characterization of monoclonal antibodies against tryptophan C-mannosylation. These antibodies empowered proteomic analyses of the brain C-glycome by enriching for peptides possessing tryptophan C-mannosylation. This study revealed many new modification sites on proteins throughout the secretory pathway with both conventional and non-canonical consensus sequences.

Residue-Selective Protein C-Formylation via Sequential Difluoroalkylation-Hydrolysis

The carbonyl group is now a widely useful, nonproteinogenic functional group in chemical biology, yet methods for its generation in proteins have relied upon either cotranslational incorporation of unnatural amino acids bearing carbonyls or oxidative conversion (chemical or enzymatic) of existing natural amino acids. If available, alternative strategies for directly adding the C=O group through C-C bond-forming C-carbonylation, particularly at currently inaccessible amino acid sites, would provide a powerful method for adding valuable reactivity and expanding possible function in proteins. Here, following a survey of methods for HCF₂· generation, we show that reductive photoredox catalysis enables mild radical-mediated difluoromethylation-hydrolysis of native protein residues as an effective method for carbonylation. Inherent selectivity of HCF₂· allowed preferential modification of Trp residues. The resulting C-2-difluoromethylated Trp undergoes Reimer-Tiemann-type dehalogenation providing highly effective spontaneous hydrolytic collapse in proteins to carbonylated HC(O)-Trp (C-formyl-Trp = CfW) residues. This new, unnatural protein residue CfW not only was found to be effective in bioconjugation, ligation, and labeling reactions but also displayed strong "red-shifting" of its absorption and fluorescent emission maxima, allowing direct use of Trp sites as UV-visualized fluorophores in proteins and even cells. In this way, this method for the effective generation of masked formyl-radical "HC(O)·" equivalents enables first examples of C-C bond-forming carbonylation in proteins, thereby expanding the chemical reactivity and spectroscopic function that may be selectively and post-translationally "edited" into biology.

Properdin oligomers adopt rigid extended conformations supporting function

Properdin stabilizes convertases formed upon activation of the complement cascade within the immune system. The biological activity of properdin depends on the oligomerization state, but whether properdin oligomers are rigid and how their structure links to function remains unknown. We show by combining electron microscopy and solution scattering, that properdin oligomers adopt extended rigid and well-defined conformations which are well approximated by single models of apparent n-fold rotational symmetry with dimensions of 230–360 Å. Properdin monomers are pretzel-shaped molecules with limited flexibility. In solution, properdin dimers are curved molecules, whereas trimers and tetramers are close to being planar molecules. Structural analysis indicates that simultaneous binding through all binding sites to surface-linked convertases is unlikely for properdin trimer and tetramers. We show that multivalency alone is insufficient for full activity in a cell lysis assay. Hence, the observed rigid extended oligomer structure is an integral component of properdin function.

Glycomics, Glycoproteomics, and Glycogenomics: An Inter-Taxa Evolutionary Perspective

Glycosylation is a highly diverse set of co- and posttranslational modifications of proteins. For mammalian glycoproteins, glycosylation is often site-, tissue-, and species-specific and diversified by microheterogeneity. Multitudinous biochemical, cellular, physiological, and organismic effects of their glycans have been revealed, either intrinsic to the carrier proteins or mediated by endogenous reader proteins with carbohydrate recognition domains. Furthermore, glycans frequently form the first line of access by or defense from foreign invaders, and new roles for nucleocytoplasmic glycosylation are blossoming. We now know enough to conclude that the same general principles apply in invertebrate animals and unicellular eukaryotes-different branches of which spawned the plants or fungi and animals. The two major driving forces for exploring the glycomes of invertebrates and protists are (i) to understand the biochemical basis of glycan-driven biology in these organisms, especially of pathogens, and (ii) to uncover the evolutionary relationships between glycans, their biosynthetic enzyme genes, and biological functions for new glycobiological insights. With an emphasis on emerging areas of protist glycobiology, here we offer an overview of glycan diversity and evolution, to promote future access to this treasure trove of glycobiological processes.

Regulation of N-glycosylation and secretion of Isthmin-1 by its C-mannosylation

BACKGROUND

C-mannosylation is a type of protein glycosylation. Human Isthmin-1 (ISM1) is a 52-kDa secreted protein with a thrombospondin type 1 repeat (TSR) domain, containing two consensus C-mannosylation sequences at Trp²²³ and Trp²²⁶. In this study, we sought to examine the role of C-mannosylation in the secretion of ISM1.

METHODS

We established and cultured an ISM1-overexpressing HT1080 cell line and purified recombinant ISM1 for analysis from the conditioned medium by LC-MS/MS. Subcellular localization of ISM1 was observed by confocal fluorescence microscopy.

RESULTS

We found that ISM1 is C-mannosylated at Trp²²³ and Trp²²⁶ in the TSR domain. To determine the functions of the C-mannosylation of ISM1, we established a C-mannosylation-defective mutant ISM1-overexpressing HT1080 cell line and measured its secretion of ISM1. The secretion of ISM1 decreased significantly in this mutant ISM1-overexpressing line compared with wild-type cells. Furthermore, ISM1 was N-glycosylated only in these C-mannosylation-defective cells.

CONCLUSIONS

ISM1 is C-mannosylated in its TSR domain, and the status of the C-mannosylation of ISM1 affects its N-glycosylation.

GENERAL SIGNIFICANCE

The C-mannosylation of ISM1 regulates its N-glycosylation status.

Requirement for C-mannosylation to be secreted and activated a disintegrin and metalloproteinase with thrombospondin motifs 4 (ADAMTS4)

BACKGROUND

C-mannosylation is a unique type of glycosylation. A disintegrin and metalloproteinase with thrombospondin motifs 4 (ADAMTS4) is a multidomain extracellular metalloproteinase that contains several potential C-mannosylation sites. Although some ADAMTS family proteins have been reported to be C-mannosylated proteins, whether C-mannosylation affects the activation and protease activity of these proteins is unclear.

METHODS

We established wild-type and mutant ADAMTS4-overexpressing HT1080 cell lines. Recombinant ADAMTS4 was purified from the conditioned medium of the wild-type ADAMTS4-overexpressing cells, and the C-mannosylation sites of ADAMTS4 were identified by LC-MS/MS. The processing, secretion, and intracellular localization of ADAMTS4 were examined by immunoblot and immunofluorescence analyses. ADAMTS4 enzymatic activity was evaluated by assessing the cleavage of recombinant aggrecan.

RESULTS

We identified that ADAMTS4 is C-mannosylated at Trp⁴⁰⁴ in the metalloprotease domain and at Trp⁵²³, Trp⁵²⁶, and Trp⁵²⁹ in the thrombospondin type 1 repeat (TSR). The replacement of Trp⁴⁰⁴ with Phe affected ADAMTS4 processing, without affecting secretion and intracellular localization. In contrast, the substitution of Trp⁵²³, Trp⁵²⁶, and Trp⁵²⁹ with Phe residues suppressed ADAMTS4 secretion, processing, intracellular trafficking, and enzymatic activity.

CONCLUSIONS

Our results demonstrated that the C-mannosylation of ADAMTS4 plays important roles in protein processing, intracellular trafficking, secretion, and enzymatic activity.

GENERAL SIGNIFICANCE

Because C-mannosylation appears to regulate many ADAMTS4 functions, C-mannosylation may also affect other members of the ADAMTS superfamily.

Meta-heterogeneity: Evaluating and Describing the Diversity in Glycosylation Between Sites on the Same Glycoprotein

Mass spectrometry-based glycoproteomics has gone through some incredible developments over the last few years. Technological advances in glycopeptide enrichment, fragmentation methods, and data analysis workflows have enabled the transition of glycoproteomics from a niche application, mainly focused on the characterization of isolated glycoproteins, to a mature technology capable of profiling thousands of intact glycopeptides at once. In addition to numerous biological discoveries catalyzed by the technology, we are also observing an increase in studies focusing on global protein glycosylation and the relationship between multiple glycosylation sites on the same protein. It has become apparent that just describing protein glycosylation in terms of micro- and macro-heterogeneity, respectively, the variation and occupancy of glycans at a given site, is not sufficient to describe the observed interactions between sites. In this perspective we propose a new term, meta-heterogeneity, to describe a higher level of glycan regulation: the variation in glycosylation across multiple sites of a given protein. We provide literature examples of extensive meta-heterogeneity on relevant proteins such as antibodies, erythropoietin, myeloperoxidase, and a number of serum and plasma proteins. Furthermore, we postulate on the possible biological reasons and causes behind the intriguing meta-heterogeneity observed in glycoproteins.

NMR Spectroscopic Characterization of the C-Mannose Conformation in a Thrombospondin Repeat Using a Selective Labeling Approach

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 13 C-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 1 C4 state and the B03 / 1 S3 state. This flexibility may be essential for protein folding and stabilization. We foresee that this versatile tool to produce proteins with selectively 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.