Structure of protein O-mannose kinase reveals a unique active site architecture

Phosphorylation of N-glycans in the brain: The case for a non-canonical pathway?

Asparagine-linked glycosylation (N-glycosylation) is a common co- and post-translational modification that refers to the addition of complex carbohydrates, called N-linked glycans (N-glycans), to asparagine residues within defined sequons of polypeptide acceptors. Some N-glycans can be modified by the addition of phosphate moieties to their monosaccharide residues, thus forming phospho-N-glycans (PNGs). The most prominent such carbohydrate modification is mannose-6-phosphate (M6P) which plays a well-established role in trafficking of acid hydrolases to lysosomes. However, comparatively little is known about potential alternative types of glycan phosphorylation, particularly when it comes to the brain which is especially rich in phosphorylated oligosaccharides. Combining data from the literature and novel insights derived from our own analyses of published datasets, here we present what is currently known about PNGs in the brain and the glycoproteins they modify. We show that brain PNGs exhibit several distinctive features that don't completely align with our current understanding of the canonical M6P pathway. Furthermore, we demonstrate that there are numerous differences in the way that lysosomal and non-lysosomal neural glycoproteins are modified by PNGs. Based on these observations, we put forward the hypothesis that, in addition to the conventional M6P pathway, the brain employs an alternative oligosaccharide phosphorylation mechanism for the modification of a discrete set of glycoproteins. Here we examine the evidence underpinning this hypothesis and discuss the implications that it raises. Overall, our work suggests that phosphorylation of N-glycans in the brain may be more complex and more diverse than previously recognised.

A periodic table of monosaccharides

It is important to recognize the great diversity of monosaccharides commonly encountered in animals, plants, and microbes, as well as to organize them in a visually interesting style that also emphasizes their similarities and relatedness. This article discusses the nature of building blocks, monosaccharides, and monosaccharide derivatives-terms commonly used in discussing "glycomolecules" found in nature. To aid in awareness of monosaccharide diversity, here is presented a Periodic Table of Monosaccharides. The rationale is given for construction of the Table and the selection of 103 monosaccharides, which is largely based on those presented in the KEGG and SNFG websites of monosaccharides, and includes room to enlarge as new discoveries are made. The Table should have educational value and is intended to capture the attention and foster imagination of those not very familiar with glycosciences, and encourage researchers to delve deeper into this fascinating area.

N-terminal domain on dystroglycan enables LARGE1 to extend matriglycan on α-dystroglycan and prevents muscular dystrophy

Dystroglycan (DG) requires extensive post-translational processing and O-glycosylation to function as a receptor for extracellular matrix (ECM) proteins containing laminin-G (LG) domains. Matriglycan is an elongated polysaccharide of alternating xylose (Xyl) and glucuronic acid (GlcA) that binds with high affinity to ECM proteins with LG domains and is uniquely synthesized on α-dystroglycan (α-DG) by like-acetylglucosaminyltransferase-1 (LARGE1). Defects in the post-translational processing or O-glycosylation of α-DG that result in a shorter form of matriglycan reduce the size of α-DG and decrease laminin binding, leading to various forms of muscular dystrophy. Previously, we demonstrated that protein O-mannose kinase (POMK) is required for LARGE1 to generate full-length matriglycan on α-DG (~150-250 kDa) (Walimbe et al., 2020). Here, we show that LARGE1 can only synthesize a short, non-elongated form of matriglycan in mouse skeletal muscle that lacks the DG N-terminus (α-DGN), resulting in an ~100-125 kDa α-DG. This smaller form of α-DG binds laminin and maintains specific force but does not prevent muscle pathophysiology, including reduced force production after eccentric contractions (ECs) or abnormalities in the neuromuscular junctions. Collectively, our study demonstrates that α-DGN, like POMK, is required for LARGE1 to extend matriglycan to its full mature length on α-DG and thus prevent muscle pathophysiology.

Exploring the growth trait molecular markers in two sheep breeds based on Genome-wide association analysis

Growth traits are quantitative traits controlled by multiple micro-effect genes. we identified molecular markers related to sheep growth traits, which formed the basis of molecular breeding. In this study, we randomly selected 100 Qira Black sheep and 84 German Merino sheep for the blood collection the jugular vein to genotype by using the Illumina Ovine SNP 50K Bead Chip. quality control criteria for statistical analysis were: rejection detection rate < 90% and minimum allele frequency (MAF) < 5%. Then, we performed Genome-wide association studies (GWAS) on sheep body weight, body height, body length, and chest circumference using mixed linear models. After getting 55 SNPs with significant correlation, they were annotated by reference genome of Ovis aries genome (Oar_v4.0) and We obtained a total of 84 candidate genes associated with production traits (BMPR1B, HSD17B3, TMEM63C, etc.). We selected BMPR1B for population validation and found a correlation between the FecB locus and body weight traits. Therefore, this study not only supplements the existing knowledge of molecular markers of sheep growth traits, but also has important theoretical significance and reference value for the mining of functional genes of sheep growth traits.

Regulation of secretory pathway kinase or kinase-like proteins in human cancers

Secretory pathway kinase or kinase-like proteins (SPKKPs) are effective in the lumen of the endoplasmic reticulum (ER), Golgi apparatus (GA), and extracellular space. These proteins are involved in secretory signaling pathways and are distinctive from typical protein kinases. Various reports have shown that SPKKPs regulate the tumorigenesis and progression of human cancer via the phosphorylation of various substrates, which is essential in physiological and pathological processes. Emerging evidence has revealed that the expression of SPKKPs in human cancers is regulated by multiple factors. This review summarizes the current understanding of the contribution of SPKKPs in tumorigenesis and the progression of immunity. With the epidemic trend of immunotherapy, targeting SPKKPs may be a novel approach to anticancer therapy. This study briefly discusses the recent advances regarding SPKKPs.

Looking lively: emerging principles of pseudokinase signaling

Progress towards understanding catalytically 'dead' protein kinases - pseudokinases - in biology and disease has hastened over the past decade. An especially lively area for structural biology, pseudokinases appear to be strikingly similar to their kinase relatives, despite lacking key catalytic residues. Distinct active- and inactive-like conformation states, which are crucial for regulating bona fide protein kinases, are conserved in pseudokinases and appear to be essential for function. We discuss recent structural data on conformational transitions and nucleotide binding by pseudokinases, from which some common principles emerge. In both pseudokinases and bona fide kinases, a conformational toggle appears to control the ability to interact with signaling effectors. We also discuss how biasing this conformational toggle may provide opportunities to target pseudokinases pharmacologically in disease.

FAM69C, a kinase critical for synaptic function and memory, is defective in neurodegenerative dementia

Synapse loss and memory decline are the primary features of neurodegenerative dementia. However, the molecular underpinnings that drive memory loss remain largely unknown. Here, we report that FAM69C is a kinase critically involved in neurodegenerative dementia. Biochemical analyses uncover that FAM69C is a serine/threonine kinase. We generate the Fam69c knockout mice and show by single-cell RNA sequencing that FAM69C deficiency drives cell-type-specific transcriptional changes relevant to synapse dysfunction. Electrophysiological, morphological, and behavioral experiments demonstrate impairments in synaptic plasticity, dendritic spine density, and memory in Fam69c knockout mice, as well as stress-induced neuronal death. Phosphoproteomic characterizations reveal that FAM69C substrates are involved in synaptic structure and function. Finally, reduced levels of FAM69C are found in postmortem brains of Alzheimer's disease patients. Our study demonstrates that FAM69C is a protective regulator of memory and suggests FAM69C as a potential therapeutic target for memory loss in neurodegenerative dementia.

Co-expression of recombinant RIPK3:MLKL complexes using the baculovirus-insect cell system

Pseudokinase domains are found throughout the kingdoms of life and serve myriad roles in cell signaling. These domains, which resemble protein kinases but are catalytically-deficient, have been described principally as protein interaction domains. Broadly, pseudokinases have been reported to function as: allosteric regulators of conventional enzymes; scaffolds to nucleate assembly and/or localization of signaling complexes; molecular switches; or competitors of signaling complex assembly. From detailed structural and biochemical studies of individual pseudokinases, a picture of how they mediate protein interactions is beginning to emerge. Many such studies have relied on recombinant protein production in insect cells, where endogenous chaperones and modifying enzymes favor bona fide folding of pseudokinases. Here, we describe methods for co-expression of pseudokinases and their interactors in insect cells, as exemplified by the MLKL pseudokinase, which is the terminal effector in the necroptosis cell death pathway, and its upstream regulator kinase RIPK3. These methods are broadly applicable to co-expression of other pseudokinases with their interaction partners from bacmids using the baculovirus-insect cell expression system.

FKRP directed fibronectin glycosylation: A novel mechanism giving insights into muscular dystrophies?

The recently uncovered role of Fukutin-related protein (FKRP) in fibronectin glycosylation has challenged our understanding of the basis of disease pathogenesis in the muscular dystrophies. FKRP is a Golgi-resident glycosyltransferase implicated in a broad spectrum of muscular dystrophy (MD) pathologies that are not fully attributable to the well-described α-Dystroglycan hypoglycosylation. By revealing a new role for FKRP in the glycosylation of fibronectin, a modification critical for the development of the muscle basement membrane (MBM) and its associated muscle linkages, new possibilities for understanding clinical phenotype arise. This modification involves an interaction between FKRP and myosin-10, a protein involved in the Golgi organization and function. These observations suggest a FKRP nexus exists that controls two critical aspects to muscle fibre integrity, both fibre stability at the MBM and its elastic properties. This review explores the new potential disease axis in the context of our current knowledge of muscular dystrophies.

Protein O-mannosylation across kingdoms and related diseases: From glycobiology to glycopathology

The post-translational glycosylation of proteins by O-linked α-mannose is conserved from bacteria to humans. Due to advances in high-throughput mass spectrometry-based approaches, a variety of glycoproteins are identified to be O-mannosylated. Various proteins with O-mannosylation are involved in biological processes, providing essential necessity for proper growth and development. In this review, we summarize the process and regulation of O-mannosylation. The multi-step O-mannosylation procedures are quite dynamic and complex, especially when considering the structural and functional inspection of the involved enzymes. The widely studied O-mannosylated proteins in human include α-Dystroglycan (α-DG), cadherins, protocadherins, and plexin, and their aberrant O-mannosylation are associated with many diseases. In addition, O-mannosylation also contributes to diverse functions in lower eukaryotes and prokaryotes. Finally, we present the relationship between O-mannosylation and gut microbiota (GM), and elucidate that O-mannosylation in microbiome is of great importance in the dynamic balance of GM. Our study provides an overview of the processes of O-mannosylation in mammalian cells and other organisms, and also associated regulated enzymes and biological functions, which could contribute to the understanding of newly discovered O-mannosylated glycoproteins.

High degree of conservation of the enzymes synthesizing the laminin-binding glycoepitope of α-dystroglycan

The dystroglycan (DG) complex plays a pivotal role for the stabilization of muscles in Metazoa. It is formed by two subunits, extracellular α-DG and transmembrane β-DG, originating from a unique precursor via a complex post-translational maturation process. The α-DG subunit is extensively glycosylated in sequential steps by several specific enzymes and employs such glycan scaffold to tightly bind basement membrane molecules. Mutations of several of these enzymes cause an alteration of the carbohydrate structure of α-DG, resulting in severe neuromuscular disorders collectively named dystroglycanopathies. Given the fundamental role played by DG in muscle stability, it is biochemically and clinically relevant to investigate these post-translational modifying enzymes from an evolutionary perspective. A first phylogenetic history of the thirteen enzymes involved in the fabrication of the so-called 'M3 core' laminin-binding epitope has been traced by an overall sequence comparison approach, and interesting details on the primordial enzyme set have emerged, as well as substantial conservation in Metazoa. The optimization along with the evolution of a well-conserved enzymatic set responsible for the glycosylation of α-DG indicate the importance of the glycosylation shell in modulating the connection between sarcolemma and surrounding basement membranes to increase skeletal muscle stability, and eventually support movement and locomotion.

Fic and non-Fic AMPylases: protein AMPylation in metazoans

Protein AMPylation refers to the covalent attachment of an AMP moiety to the amino acid side chains of target proteins using ATP as nucleotide donor. This process is catalysed by dedicated AMP transferases, called AMPylases. Since this initial discovery, several research groups have identified AMPylation as a critical post-translational modification relevant to normal and pathological cell signalling in both bacteria and metazoans. Bacterial AMPylases are abundant enzymes that either regulate the function of endogenous bacterial proteins or are translocated into host cells to hijack host cell signalling processes. By contrast, only two classes of metazoan AMPylases have been identified so far: enzymes containing a conserved filamentation induced by cAMP (Fic) domain (Fic AMPylases), which primarily modify the ER-resident chaperone BiP, and SelO, a mitochondrial AMPylase involved in redox signalling. In this review, we compare and contrast bacterial and metazoan Fic and non-Fic AMPylases, and summarize recent technological and conceptual developments in the emerging field of AMPylation.

The ABCs of the atypical Fam20 secretory pathway kinases

The study of extracellular phosphorylation was initiated in late 19th century when the secreted milk protein, casein, and egg-yolk protein, phosvitin, were shown to be phosphorylated. However, it took more than a century to identify Fam20C, which phosphorylates both casein and phosvitin under physiological conditions. This kinase, along with its family members Fam20A and Fam20B, defined a new family with altered amino acid sequences highly atypical from the canonical 540 kinases comprising the kinome. Fam20B is a glycan kinase that phosphorylates xylose residues and triggers peptidoglycan biosynthesis, a role conserved from sponges to human. The protein kinase, Fam20C, conserved from nematodes to humans, phosphorylates well over 100 substrates in the secretory pathway with overall functions postulated to encompass endoplasmic reticulum homeostasis, nutrition, cardiac function, coagulation, and biomineralization. The preferred phosphorylation motif of Fam20C is SxE/pS, and structural studies revealed that related member Fam20A allosterically activates Fam20C by forming a heterodimeric/tetrameric complex. Fam20A, a pseudokinase, is observed only in vertebrates. Loss-of-function genetic alterations in the Fam20 family lead to human diseases such as amelogenesis imperfecta, nephrocalcinosis, lethal and nonlethal forms of Raine syndrome with major skeletal defects, and altered phosphate homeostasis. Together, these three members of the Fam20 family modulate a diverse network of secretory pathway components playing crucial roles in health and disease. The overarching theme of this review is to highlight the progress that has been made in the emerging field of extracellular phosphorylation and the key roles secretory pathway kinases play in an ever-expanding number of cellular processes.

There's more to death than life: Noncatalytic functions in kinase and pseudokinase signaling

Protein kinases are present in all domains of life and play diverse roles in cellular signaling. Whereas the impact of substrate phosphorylation by protein kinases has long been appreciated, it is becoming increasingly clear that protein kinases also play other, noncatalytic, functions. Here, we review recent developments in understanding the noncatalytic functions of protein kinases. Many noncatalytic activities are best exemplified by protein kinases that are devoid of enzymatic activity altogether-known as pseudokinases. These dead proteins illustrate that, beyond conventional notions of kinase function, catalytic activity can be dispensable for biological function. Through key examples we illustrate diverse mechanisms of noncatalytic kinase activity: as allosteric modulators; protein-based switches; scaffolds for complex assembly; and as competitive inhibitors in signaling pathways. In common, these noncatalytic mechanisms exploit the nature of the protein kinase fold as a versatile protein-protein interaction module. Many examples are also intrinsically linked to the ability of the protein kinase to switch between multiple states, a function shared with catalytic protein kinases. Finally, we consider the contemporary landscape of small molecules to modulate noncatalytic functions of protein kinases, which, although challenging, has significant potential given the scope of noncatalytic protein kinase function in health and disease.

POMK regulates dystroglycan function via LARGE1-mediated elongation of matriglycan

Matriglycan [-GlcA-β1,3-Xyl-α1,3-]n serves as a scaffold in many tissues for extracellular matrix proteins containing laminin-G domains including laminin, agrin, and perlecan. Like-acetyl-glucosaminyltransferase 1 (LARGE1) synthesizes and extends matriglycan on α-dystroglycan (α-DG) during skeletal muscle differentiation and regeneration; however, the mechanisms which regulate matriglycan elongation are unknown. Here, we show that Protein O-Mannose Kinase (POMK), which phosphorylates mannose of core M3 (GalNAc-β1,3-GlcNAc-β1,4-Man) preceding matriglycan synthesis, is required for LARGE1-mediated generation of full-length matriglycan on α-DG (~150 kDa). In the absence of Pomk gene expression in mouse skeletal muscle, LARGE1 synthesizes a very short matriglycan resulting in a ~ 90 kDa α-DG which binds laminin but cannot prevent eccentric contraction-induced force loss or muscle pathology. Solution NMR spectroscopy studies demonstrate that LARGE1 directly interacts with core M3 and binds preferentially to the phosphorylated form. Collectively, our study demonstrates that phosphorylation of core M3 by POMK enables LARGE1 to elongate matriglycan on α-DG, thereby preventing muscular dystrophy.

Prospects for pharmacological targeting of pseudokinases

Pseudokinases are members of the protein kinase superfamily but signal primarily through noncatalytic mechanisms. Many pseudokinases contribute to the pathologies of human diseases, yet they remain largely unexplored as drug targets owing to challenges associated with modulation of their biological functions. Our understanding of the structure and physiological roles of pseudokinases has improved substantially over the past decade, revealing intriguing similarities between pseudokinases and their catalytically active counterparts. Pseudokinases often adopt conformations that are analogous to those seen in catalytically active kinases and, in some cases, can also bind metal cations and/or nucleotides. Several clinically approved kinase inhibitors have been shown to influence the noncatalytic functions of active kinases, providing hope that similar properties in pseudokinases could be pharmacologically regulated. In this Review, we discuss known roles of pseudokinases in disease, their unique structural features and the progress that has been made towards developing pseudokinase-directed therapeutics.

N-glycosylated SGK196 suppresses the metastasis of basal-like breast cancer cells

SGK196 is a protein O-mannose kinase involved in an indispensable phosphorylation step during laminin-binding glycan synthesis on alpha-dystroglycan (α-DG). However, the function of SGK196 in cancer diseases remains elusive. In the current study, we demonstrated that SGK196 is primarily modified by N-glycosylation in breast cancer (BC) cells. Furthermore, gain and loss-of-function studies showed that N-glycosylated SGK196 suppresses cell migration, invasion, and metastasis in BC, particularly in the basal-like breast cancer (BLBC) type. In addition, we found that SGK196 N-glycosylation performs the regulatory function through the PI3K/AKT/GSK3β signaling pathway. Collectively, our results show that N-glycosylated SGK196 plays suppression roles in BLBC metastases, therefore providing new insights into SGK196 function in BC.

Emerging concepts in pseudoenzyme classification, evolution, and signaling

The 21st century is witnessing an explosive surge in our understanding of pseudoenzyme-driven regulatory mechanisms in biology. Pseudoenzymes are proteins that have sequence homology with enzyme families but that are proven or predicted to lack enzyme activity due to mutations in otherwise conserved catalytic amino acids. The best-studied pseudoenzymes are pseudokinases, although examples from other families are emerging at a rapid rate as experimental approaches catch up with an avalanche of freely available informatics data. Kingdom-wide analysis in prokaryotes, archaea and eukaryotes reveals that between 5 and 10% of proteins that make up enzyme families are pseudoenzymes, with notable expansions and contractions seemingly associated with specific signaling niches. Pseudoenzymes can allosterically activate canonical enzymes, act as scaffolds to control assembly of signaling complexes and their localization, serve as molecular switches, or regulate signaling networks through substrate or enzyme sequestration. Molecular analysis of pseudoenzymes is rapidly advancing knowledge of how they perform noncatalytic functions and is enabling the discovery of unexpected, and previously unappreciated, functions of their intensively studied enzyme counterparts. Notably, upon further examination, some pseudoenzymes have previously unknown enzymatic activities that could not have been predicted a priori. Pseudoenzymes can be targeted and manipulated by small molecules and therefore represent new therapeutic targets (or anti-targets, where intervention should be avoided) in various diseases. In this review, which brings together broad bioinformatics and cell signaling approaches in the field, we highlight a selection of findings relevant to a contemporary understanding of pseudoenzyme-based biology.

Life is sweet: the cell biology of glycoconjugates

Cells are dazzling in their diversity, both within and across organisms. And yet, throughout this variety runs at least one common thread: sugars. All cells on Earth, in all domains of life, are literally covered in glycans, a term referring to the carbohydrate portion of glycoproteins and glycolipids. In spite of (or, perhaps, because of) their tremendous structural and functional complexity, glycans have historically been underexplored compared with other areas of cell biology. Recently, however, advances in experimental systems and analytical methods have ushered in a renaissance in glycobiology, the study of the biosynthesis, structures, interactions, functions, and evolution of glycans. Today, glycobiology is poised to make major new contributions to cell biology and become more fully integrated into our understanding of cell and organismal physiology.

Divergent kinase regulates membrane ultrastructure of the Toxoplasma parasitophorous vacuole

Apicomplexan parasites replicate within a protective organelle, called the parasitophorous vacuole (PV). The Toxoplasma gondii PV is filled with a network of tubulated membranes, which are thought to facilitate trafficking of effectors and nutrients. Despite being critical to parasite virulence, there is scant mechanistic understanding of the network's functions. Here, we identify the parasite-secreted kinase WNG1 (With-No-Gly-loop) as a critical regulator of tubular membrane biogenesis. WNG1 family members adopt an atypical protein kinase fold lacking the glycine rich ATP-binding loop that is required for catalysis in canonical kinases. Unexpectedly, we find that WNG1 is an active protein kinase that localizes to the PV lumen and phosphorylates PV-resident proteins, several of which are essential for the formation of a functional intravacuolar network. Moreover, we show that WNG1-dependent phosphorylation of these proteins is required for their membrane association, and thus their ability to tubulate membranes. Consequently, WNG1 knockout parasites have an aberrant PV membrane ultrastructure. Collectively, our results describe a unique family of Toxoplasma kinases and implicate phosphorylation of secreted proteins as a mechanism of regulating PV development during parasite infection.

Protein AMPylation by an Evolutionarily Conserved Pseudokinase

Approximately 10% of human protein kinases are believed to be inactive and named pseudokinases because they lack residues required for catalysis. Here, we show that the highly conserved pseudokinase selenoprotein-O (SelO) transfers AMP from ATP to Ser, Thr, and Tyr residues on protein substrates (AMPylation), uncovering a previously unrecognized activity for a member of the protein kinase superfamily. The crystal structure of a SelO homolog reveals a protein kinase-like fold with ATP flipped in the active site, thus providing a structural basis for catalysis. SelO pseudokinases localize to the mitochondria and AMPylate proteins involved in redox homeostasis. Consequently, SelO activity is necessary for the proper cellular response to oxidative stress. Our results suggest that AMPylation may be a more widespread post-translational modification than previously appreciated and that pseudokinases should be analyzed for alternative transferase activities.

New tools for carbohydrate sulfation analysis: heparan sulfate 2-O-sulfotransferase (HS2ST) is a target for small-molecule protein kinase inhibitors

Sulfation of carbohydrate residues occurs on a variety of glycans destined for secretion, and this modification is essential for efficient matrix-based signal transduction. Heparan sulfate (HS) glycosaminoglycans control physiological functions ranging from blood coagulation to cell proliferation. HS biosynthesis involves membrane-bound Golgi sulfotransferases, including HS 2-O-sulfotransferase (HS2ST), which transfers sulfate from the cofactor PAPS (3'-phosphoadenosine 5'-phosphosulfate) to the 2-O position of α-l-iduronate in the maturing polysaccharide chain. The current lack of simple non-radioactive enzyme assays that can be used to quantify the levels of carbohydrate sulfation hampers kinetic analysis of this process and the discovery of HS2ST inhibitors. In the present paper, we describe a new procedure for thermal shift analysis of purified HS2ST. Using this approach, we quantify HS2ST-catalysed oligosaccharide sulfation using a novel synthetic fluorescent substrate and screen the Published Kinase Inhibitor Set, to evaluate compounds that inhibit catalysis. We report the susceptibility of HS2ST to a variety of cell-permeable compounds in vitro, including polyanionic polar molecules, the protein kinase inhibitor rottlerin and oxindole-based RAF kinase inhibitors. In a related study, published back-to-back with the present study, we demonstrated that tyrosyl protein sulfotranferases are also inhibited by a variety of protein kinase inhibitors. We propose that appropriately validated small-molecule compounds could become new tools for rapid inhibition of glycan (and protein) sulfation in cells, and that protein kinase inhibitors might be repurposed or redesigned for the specific inhibition of HS2ST.

Structure and evolution of the Fam20 kinases

The Fam20 proteins are novel kinases that phosphorylate secreted proteins and proteoglycans. Fam20C phosphorylates hundreds of secreted proteins and is activated by the pseudokinase Fam20A. Fam20B phosphorylates a xylose residue to regulate proteoglycan synthesis. Despite these wide-ranging and important functions, the molecular and structural basis for the regulation and substrate specificity of these kinases are unknown. Here we report molecular characterizations of all three Fam20 kinases, and show that Fam20C is activated by the formation of an evolutionarily conserved homodimer or heterodimer with Fam20A. Fam20B has a unique active site for recognizing Galβ1-4Xylβ1, the initiator disaccharide within the tetrasaccharide linker region of proteoglycans. We further show that in animals the monomeric Fam20B preceded the appearance of the dimeric Fam20C, and the dimerization trait of Fam20C emerged concomitantly with a change in substrate specificity. Our results provide comprehensive structural, biochemical, and evolutionary insights into the function of the Fam20 kinases.

Recent advancements in understanding mammalian O-mannosylation

The post-translational glycosylation of select proteins by O-linked mannose (O-mannose or O-man) is a conserved modification from yeast to humans and has been shown to be necessary for proper development and growth. The most well studied O-mannosylated mammalian protein is α-dystroglycan (α-DG). Hypoglycosylation of α-DG results in varying severities of congenital muscular dystrophies, cancer progression and metastasis, and inhibited entry and infection of certain arenaviruses. Defects in the gene products responsible for post-translational modification of α-DG, primarily glycosyltransferases, are the basis for these diseases. The multitude of clinical phenotypes resulting from defective O-mannosylation highlights the biomedical significance of this unique modification. Elucidation of the various O-mannose biosynthetic pathways is imperative to understanding a broad range of human diseases and for the development of novel therapeutics. In this review, we will focus on recent discoveries delineating the various enzymes, structures and functions associated with O-mannose-initiated glycoproteins. Additionally, we discuss current gaps in our knowledge of mammalian O-mannosylation, discuss the evolution of this pathway, and illustrate the utility and limitations of model systems to study functions of O-mannosylation.

Metal coordination in kinases and pseudokinases

Protein phosphorylation, mediated by protein kinases, is a key event in the regulation of eukaryotic signal transduction. The majority of eukaryotic protein kinases perform phosphoryl transfer, assisted by two divalent metal ions. About 10% of all human protein kinases are, however, thought to be catalytically inactive. These kinases lack conserved residues of the kinase core and are classified as pseudokinases. Yet, it has been demonstrated that pseudokinases are critically involved in biological functions. Here, we show how pseudokinases have developed strategies by modifying amino acid residues in order to achieve stable, active-like conformations. This includes binding of the co-substrate ATP in a two metal-, one metal- or even no metal-binding mode. Examples of the respective pseudokinases are provided on a structural basis and compared with a canonical protein kinase, Protein Kinase A. Moreover, the functional roles of both independent metal-binding sites, Me1 and Me2, are discussed. Lack of phosphotransferase activity does not implicate a loss of function and can easily point to alternative roles of pseudokinases, i.e. acting as switches or scaffolds, and having evolved as components crucial for cellular cross-talk and signaling. Interestingly, pseudokinases are present in all kingdoms of life and their specific roles remain enigmatic. More studies are needed to unravel the crucial functions of those interesting proteins.

Structure of Fam20A reveals a pseudokinase featuring a unique disulfide pattern and inverted ATP-binding

Mutations in FAM20A cause tooth enamel defects known as Amelogenesis Imperfecta (AI) and renal calcification. We previously showed that Fam20A is a secretory pathway pseudokinase and allosterically activates the physiological casein kinase Fam20C to phosphorylate secreted proteins important for biomineralization (Cui et al., 2015). Here we report the nucleotide-free and ATP-bound structures of Fam20A. Fam20A exhibits a distinct disulfide bond pattern mediated by a unique insertion region. Loss of this insertion due to abnormal mRNA splicing interferes with the structure and function of Fam20A, resulting in AI. Fam20A binds ATP in the absence of divalent cations, and strikingly, ATP is bound in an inverted orientation compared to other kinases. Fam20A forms a dimer in the crystal, and residues in the dimer interface are critical for Fam20C activation. Together, these results provide structural insights into the function of Fam20A and shed light on the mechanism by which Fam20A mutations cause disease.

DOI:http://dx.doi.org/10.7554/eLife.23990.001