Pathogenic variants in RNPC3 are associated with hypopituitarism and primary ovarian insufficiency.

PURPOSE: We aimed to investigate the molecular basis underlying a novel phenotype including hypopituitarism associated with primary ovarian insufficiency. METHODS: We used next-generation sequencing to identify variants in all pedigrees. Expression of Rnpc3/RNPC3 was analyzed by in situ hybridization on murine/human embryonic sections. CRISPR/Cas9 was used to generate mice carrying the p.Leu483Phe pathogenic variant in the conserved murine Rnpc3 RRM2 domain. RESULTS: We described 15 patients from 9 pedigrees with biallelic pathogenic variants in RNPC3, encoding a specific protein component of the minor spliceosome, which is associated with a hypopituitary phenotype, including severe growth hormone (GH) deficiency, hypoprolactinemia, variable thyrotropin (also known as thyroid-stimulating hormone) deficiency, and anterior pituitary hypoplasia. Primary ovarian insufficiency was diagnosed in 8 of 9 affected females, whereas males had normal gonadal function. In addition, 2 affected males displayed normal growth when off GH treatment despite severe biochemical GH deficiency. In both mouse and human embryos, Rnpc3/RNPC3 was expressed in the developing forebrain, including the hypothalamus and Rathke's pouch. Female Rnpc3 mutant mice displayed a reduction in pituitary GH content but with no reproductive impairment in young mice. Male mice exhibited no obvious phenotype. CONCLUSION: Our findings suggest novel insights into the role of RNPC3 in female-specific gonadal function and emphasize a critical role for the minor spliceosome in pituitary and ovarian development and function.

A homozygous Y443C variant in the RNPC3 is associated with severe syndromic congenital hypopituitarism and diffuse brain atrophy.

Biallelic RNPC3 variants have been reported in a few patients with growth hormone deficiency, either in isolation or in association with central hypothyroidism, congenital cataract, neuropathy, developmental delay/intellectual disability, hypogonadism, and pituitary hypoplasia. To describe a new patient with syndromic congenital hypopituitarism and diffuse brain atrophy due to RNPC3 mutations and to compare her clinical and molecular characteristics and pituitary functions with previously published patients. A 20-year-old female presented with severe growth, neuromotor, and developmental delay. Her weight, height, and head circumference were 5135 gr (-25.81 SDS), 68 cm (-16.17 SDS), and 34 cm (-17.03 SDS), respectively. She was prepubertal, and had dysmorphic facies, contractures, and spasticity in the extremities, and severe truncal hypotonia. There were no radiological signs of a skeletal dysplasia. The bone age was extremely delayed at 2 years. Investigation of pituitary function revealed growth hormone, prolactin, and thyroid-stimulating hormone deficiencies. Whole-exome sequencing revealed a novel homozygous missense (c.1328A > G; Y443C) variant in RNPC3. Cranial MRI revealed a hypoplastic anterior pituitary with diffuse cerebral and cerebellar atrophy. The Y443C variant in RNPC3 associated with syndromic congenital hypopituitarism and abnormal brain development. This report extends the RNPC3-related hypopituitarism phenotype with a severe neurodegenerative presentation.

Oncogenic RET kinase domain mutations perturb the autophosphorylation trajectory by enhancing substrate presentation in trans.

To decipher the molecular basis for RET kinase activation and oncogenic deregulation, we defined the temporal sequence of RET autophosphorylation by label-free quantitative mass spectrometry. Early autophosphorylation sites map to regions flanking the kinase domain core, while sites within the activation loop only form at later time points. Comparison with oncogenic RET kinase revealed that late autophosphorylation sites become phosphorylated much earlier than wild-type RET, which is due to a combination of an enhanced enzymatic activity, increased ATP affinity, and surprisingly, by providing a better intermolecular substrate. Structural analysis of oncogenic M918T and wild-type RET kinase domains reveal a cis-inhibitory mechanism involving tethering contacts between the glycine-rich loop, activation loop, and αC-helix. Tether mutations only affected substrate presentation but perturbed the autophosphorylation trajectory similar to oncogenic mutations. This study reveals an unappreciated role for oncogenic RET kinase mutations in promoting intermolecular autophosphorylation by enhancing substrate presentation.

The Mechanism of Acetyl Transfer Catalyzed by Mycobacterium tuberculosis GlmU.

The biosynthetic pathway of peptidoglycan is essential for Mycobacterium tuberculosis. We report here the acetyltransferase substrate specificity and catalytic mechanism of the bifunctional N-acetyltransferase/uridylyltransferase from M. tuberculosis (GlmU). This enzyme is responsible for the final two steps of the synthesis of UDP- N-acetylglucosamine, which is an essential precursor of peptidoglycan, from glucosamine 1-phosphate, acetyl-coenzyme A, and uridine 5'-triphosphate. GlmU utilizes ternary complex formation to transfer an acetyl from acetyl-coenzyme A to glucosamine 1-phosphate to form N-acetylglucosamine 1-phosphate. Steady-state kinetic studies and equilibrium binding experiments indicate that GlmU follows a steady-state ordered kinetic mechanism, with acetyl-coenzyme A binding first, which triggers a conformational change in GlmU, followed by glucosamine 1-phosphate binding. Coenzyme A is the last product to dissociate. Chemistry is partially rate-limiting as indicated by pH-rate studies and solvent kinetic isotope effects. A novel crystal structure of a mimic of the Michaelis complex, with glucose 1-phosphate and acetyl-coenzyme A, helps us to propose the residues involved in deprotonation of glucosamine 1-phosphate and the loop movement that likely generates the active site required for glucosamine 1-phosphate to bind. Together, these results pave the way for the rational discovery of improved inhibitors against M. tuberculosis GlmU, some of which might become candidates for antibiotic discovery programs.

G-actin regulates the shuttling and PP1 binding of the RPEL protein Phactr1 to control actomyosin assembly.

The Phactr family of PP1-binding proteins is implicated in human diseases including Parkinson's, cancer and myocardial infarction. Each Phactr protein contains four G-actin binding RPEL motifs, including an N-terminal motif, abutting a basic element, and a C-terminal triple RPEL repeat, which overlaps a conserved C-terminus required for interaction with PP1. RPEL motifs are also found in the regulatory domains of the MRTF transcriptional coactivators, where they control MRTF subcellular localisation and activity by sensing signal-induced changes in G-actin concentration. However, whether G-actin binding controls Phactr protein function - and its relation to signalling - has not been investigated. Here, we show that Rho-actin signalling induced by serum stimulation promotes the nuclear accumulation of Phactr1, but not other Phactr family members. Actin binding by the three Phactr1 C-terminal RPEL motifs is required for Phactr1 cytoplasmic localisation in resting cells. Phactr1 nuclear accumulation is importin α-ß dependent. G-actin and importin α-ß bind competitively to nuclear import signals associated with the N- and C-terminal RPEL motifs. All four motifs are required for the inhibition of serum-induced Phactr1 nuclear accumulation when G-actin is elevated. G-actin and PP1 bind competitively to the Phactr1 C-terminal region, and Phactr1 C-terminal RPEL mutants that cannot bind G-actin induce aberrant actomyosin structures dependent on their nuclear accumulation and on PP1 binding. In CHL-1 melanoma cells, Phactr1 exhibits actin-regulated subcellular localisation and is required for stress fibre assembly, motility and invasiveness. These data support a role for Phactr1 in actomyosin assembly and suggest that Phactr1 G-actin sensing allows its coordination with F-actin availability.

Fluorescence-based incision assay for human XPF-ERCC1 activity identifies important elements of DNA junction recognition.

The structure-specific endonuclease activity of the human XPF-ERCC1 complex is essential for a number of DNA processing mechanisms that help to maintain genomic integrity. XPF-ERCC1 cleaves DNA structures such as stem-loops, bubbles or flaps in one strand of a duplex where there is at least one downstream single strand. Here, we define the minimal substrate requirements for cleavage of stem-loop substrates allowing us to develop a real-time fluorescence-based assay to measure endonuclease activity. Using this assay, we show that changes in the sequence of the duplex upstream of the incision site results in up to 100-fold variation in cleavage rate of a stem-loop substrate by XPF-ERCC1. XPF-ERCC1 has a preference for cleaving the phosphodiester bond positioned on the 3'-side of a T or a U, which is flanked by an upstream T or U suggesting that a T/U pocket may exist within the catalytic domain. In addition to an endonuclease domain and tandem helix-hairpin-helix domains, XPF has a divergent and inactive DEAH helicase-like domain (HLD). We show that deletion of HLD eliminates endonuclease activity and demonstrate that purified recombinant XPF-HLD shows a preference for binding stem-loop structures over single strand or duplex alone, suggesting a role for the HLD in initial structure recognition. Together our data describe features of XPF-ERCC1 and an accepted model substrate that are important for recognition and efficient incision activity.

Structural basis for morpheein-type allosteric regulation of Escherichia coli glucosamine-6-phosphate synthase: equilibrium between inactive hexamer and active dimer.

The amino-terminal cysteine of glucosamine-6-phosphate synthase (GlmS) acts as a nucleophile to release and transfer ammonia from glutamine to fructose 6-phosphate through a channel. The crystal structure of the C1A mutant of Escherichia coli GlmS, solved at 2.5 Å resolution, is organized as a hexamer, where the glutaminase domains adopt an inactive conformation. Although the wild-type enzyme is active as a dimer, size exclusion chromatography, dynamic and quasi-elastic light scattering, native polyacrylamide gel electrophoresis, and ultracentrifugation data show that the dimer is in equilibrium with a hexameric state, in vitro and in cellulo. The previously determined structures of the wild-type enzyme, alone or in complex with glucosamine 6-phosphate, are also consistent with a hexameric assembly that is catalytically inactive because the ammonia channel is not formed. The shift of the equilibrium toward the hexameric form in the presence of cyclic glucosamine 6-phosphate, together with the decrease of the specific activity with increasing enzyme concentration, strongly supports product inhibition through hexamer stabilization. Altogether, our data allow us to propose a morpheein model, in which the active dimer can rearrange into a transiently stable form, which has the propensity to form an inactive hexamer. This would account for a physiologically relevant allosteric regulation of E. coli GlmS. Finally, in addition to cyclic glucose 6-phosphate bound at the active site, the hexameric organization of E. coli GlmS enables the binding of another linear sugar molecule. Targeting this sugar-binding site to stabilize the inactive hexameric state is therefore suggested for the development of specific antibacterial inhibitors.

Atg8 family proteins, LIR/AIM motifs and other interaction modes.

The Atg8 family of ubiquitin-like proteins play pivotal roles in autophagy and other processes involving vesicle fusion and transport where the lysosome/vacuole is the end station. Nuclear roles of Atg8 proteins are also emerging. Here, we review the structural and functional features of Atg8 family proteins and their protein-protein interaction modes in model organisms such as yeast, Arabidopsis, C. elegans and Drosophila to humans. Although varying in number of homologs, from one in yeast to seven in humans, and more than ten in some plants, there is a strong evolutionary conservation of structural features and interaction modes. The most prominent interaction mode is between the LC3 interacting region (LIR), also called Atg8 interacting motif (AIM), binding to the LIR docking site (LDS) in Atg8 homologs. There are variants of these motifs like "half-LIRs" and helical LIRs. We discuss details of the binding modes and how selectivity is achieved as well as the role of multivalent LIR-LDS interactions in selective autophagy. A number of LIR-LDS interactions are known to be regulated by phosphorylation. New methods to predict LIR motifs in proteins have emerged that will aid in discovery and analyses. There are also other interaction surfaces than the LDS becoming known where we presently lack detailed structural information, like the N-terminal arm region and the UIM-docking site (UDS). More interaction modes are likely to be discovered in future studies.

Molecular basis for G-actin binding to RPEL motifs from the serum response factor coactivator MAL.

Serum response factor transcriptional activity is controlled through interactions with regulatory cofactors such as the coactivator MAL/MRTF-A (myocardin-related transcription factor A). MAL is itself regulated in vivo by changes in cellular actin dynamics, which alter its interaction with G-actin. The G-actin-sensing mechanism of MAL/MRTF-A resides in its N-terminal domain, which consists of three tandem RPEL repeats. We describe the first molecular insights into RPEL function obtained from structures of two independent RPEL(MAL) peptide:G-actin complexes. Both RPEL peptides bind to the G-actin hydrophobic cleft and to subdomain 3. These RPEL(MAL):G-actin structures explain the sequence conservation defining the RPEL motif, including the invariant arginine. Characterisation of the RPEL(MAL):G-actin interaction by fluorescence anisotropy and cell reporter-based assays validates the significance of actin-binding residues for proper MAL localisation and regulation in vivo. We identify important differences in G-actin engagement between the two RPEL(MAL) structures. Comparison with other actin-binding proteins reveals an unexpected similarity to the vitamin-D-binding protein, extending the G-actin-binding protein repertoire.

Dynamics of glucosamine-6-phosphate synthase catalysis.

Glucosamine-6P synthase, which catalyzes glucosamine-6P synthesis from fructose-6P and glutamine, channels ammonia over 18Å between its glutaminase and synthase active sites. The crystal structures of the full-length Escherichia coli enzyme have been determined alone, in complex with the first bound substrate, fructose-6P, in the presence of fructose-6P and a glutamine analog, and in the presence of the glucosamine-6P product. These structures represent snapshots of reaction intermediates, and their comparison sheds light on the dynamics of catalysis. Upon fructose-6P binding, the C-terminal loop and the glutaminase domains get ordered, leading to the closure of the synthase site, the opening of the sugar ring and the formation of a "closed" ammonia channel. Then, glutamine binding leads to the closure of the Q-loop to protect the glutaminase site, the activation of the catalytic residues involved in glutamine hydrolysis, the swing of the side chain of Trp74, which allows the communication between the two active sites through an "open" channel, and the rotation of the glutaminase domains relative to the synthase domains dimer. Therefore, binding of the substrates at the appropriate reaction time is responsible for the formation and opening of the ammonia channel and for the activation of the enzyme glutaminase function.

SAMM50 is a receptor for basal piecemeal mitophagy and acts with SQSTM1/p62 in OXPHOS-induced mitophagy.

Mitophagy, the clearance of surplus or damaged mitochondria or mitochondrial parts by autophagy, is important for maintenance of cellular homeostasis. Whereas knowledge on programmed and stress-induced mitophagy is increasing, much less is known about mechanisms of basal mitophagy. Recently, we identified SAMM50 (SAMM50 sorting and assembly machinery component) as a receptor for piecemeal degradation of components of the sorting and assembly machinery (SAM) complex and mitochondrial contact site and cristae organizing system (MICOS) complexes. SAMM50 interacts directly with Atg8-family proteins through a canonical LIR motif and with SQSTM1/p62 to mediate basal piecemeal mitophagy. During a metabolic switch to oxidative phosphorylation (OXPHOS), SAMM50 cooperates with SQSTM1 to mediate efficient piecemeal mitophagy.

SAMM50 acts with p62 in piecemeal basal- and OXPHOS-induced mitophagy of SAM and MICOS components.

Mitophagy is the degradation of surplus or damaged mitochondria by autophagy. In addition to programmed and stress-induced mitophagy, basal mitophagy processes exert organelle quality control. Here, we show that the sorting and assembly machinery (SAM) complex protein SAMM50 interacts directly with ATG8 family proteins and p62/SQSTM1 to act as a receptor for a basal mitophagy of components of the SAM and mitochondrial contact site and cristae organizing system (MICOS) complexes. SAMM50 regulates mitochondrial architecture by controlling formation and assembly of the MICOS complex decisive for normal cristae morphology and exerts quality control of MICOS components. To this end, SAMM50 recruits ATG8 family proteins through a canonical LIR motif and interacts with p62/SQSTM1 to mediate basal mitophagy of SAM and MICOS components. Upon metabolic switch to oxidative phosphorylation, SAMM50 and p62 cooperate to mediate efficient mitophagy.

Phosphorylation of the LIR Domain of SCOC Modulates ATG8 Binding Affinity and Specificity.

Autophagy is a highly conserved degradative pathway, essential for cellular homeostasis and implicated in diseases including cancer and neurodegeneration. Autophagy-related 8 (ATG8) proteins play a central role in autophagosome formation and selective delivery of cytoplasmic cargo to lysosomes by recruiting autophagy adaptors and receptors. The LC3-interacting region (LIR) docking site (LDS) of ATG8 proteins binds to LIR motifs present in autophagy adaptors and receptors. LIR-ATG8 interactions can be highly selective for specific mammalian ATG8 family members (LC3A-C, GABARAP, and GABARAPL1-2) and how this specificity is generated and regulated is incompletely understood. We have identified a LIR motif in the Golgi protein SCOC (short coiled-coil protein) exhibiting strong binding to GABARAP, GABARAPL1, LC3A and LC3C. The residues within and surrounding the core LIR motif of the SCOC LIR domain were phosphorylated by autophagy-related kinases (ULK1-3, TBK1) increasing specifically LC3 family binding. More distant flanking residues also contributed to ATG8 binding. Loss of these residues was compensated by phosphorylation of serine residues immediately adjacent to the core LIR motif, indicating that the interactions of the flanking LIR regions with the LDS are important and highly dynamic. Our comprehensive structural, biophysical and biochemical analyses support and provide novel mechanistic insights into how phosphorylation of LIR domain residues regulates the affinity and binding specificity of ATG8 proteins towards autophagy adaptors and receptors.

Conformational changes in ammonia-channeling glutamine amidotransferases.

Glutamine amidotransferases (GATs), which catalyze the synthesis of different aminated products, channel ammonia over 10-40 A from a glutamine substrate at the glutaminase site to an acceptor substrate at the synthase site. Ammonia production usually uses a cysteine-histidine-glutamate triad or a N-terminal cysteine residue. Crystal structures of several amidotransferase ligand complexes, mimicking intermediates along the catalytic cycle, have now been determined. In most cases, acceptor binding triggers glutaminase activation through domain-hinged movements and other conformational changes. Structural information shows how flexible loops of the synthase and glutaminase domains move to shield the two catalytic sites and anchor the substrates, and how the ammonia channel forms and opens or closes.

Molecular basis for substrate specificity of the Phactr1/PP1 phosphatase holoenzyme.

PPP-family phosphatases such as PP1 have little intrinsic specificity. Cofactors can target PP1 to substrates or subcellular locations, but it remains unclear how they might confer sequence-specificity on PP1. The cytoskeletal regulator Phactr1 is a neuronally enriched PP1 cofactor that is controlled by G-actin. Structural analysis showed that Phactr1 binding remodels PP1's hydrophobic groove, creating a new composite surface adjacent to the catalytic site. Using phosphoproteomics, we identified mouse fibroblast and neuronal Phactr1/PP1 substrates, which include cytoskeletal components and regulators. We determined high-resolution structures of Phactr1/PP1 bound to the dephosphorylated forms of its substrates IRSp53 and spectrin αII. Inversion of the phosphate in these holoenzyme-product complexes supports the proposed PPP-family catalytic mechanism. Substrate sequences C-terminal to the dephosphorylation site make intimate contacts with the composite Phactr1/PP1 surface, which are required for efficient dephosphorylation. Sequence specificity explains why Phactr1/PP1 exhibits orders-of-magnitude enhanced reactivity towards its substrates, compared to apo-PP1 or other PP1 holoenzymes.

Specific arrangements of atoms such as bulky phosphate groups can change the activity of a protein and how it interacts with other molecules. Enzymes called kinases are responsible for adding these groups onto a protein, while phosphatases remove them. Kinases are generally specific for a small number of proteins, adding phosphate groups only at sites embedded in a particular sequence in the target protein. Phosphatases, however, are generalists: only a few different types exist, which exhibit little target sequence specificity. Partner proteins can attach to phosphatases to bring the enzymes to specific locations in the cell, or to deliver target proteins to them; yet, it is unclear whether partner binding could also change the structure of the enzyme so the phosphatase can recognise only a restricted set of targets. To investigate this, Fedoryshchak, Prechová et al. studied a phosphatase called PP1 and its partner, Phactr1. First, the structure of the Phactr1/PP1 complex was examined using biochemistry approaches and X-ray crystallography. This showed that binding of Phactr1 to PP1 creates a new surface pocket, which comprised elements of both proteins. In particular, this composite pocket is located next to the part of the PP1 enzyme responsible for phosphate removal. Next, mass spectrometry and genetics methods were harnessed to identify and characterise the targets of the Phactr1/PP1 complex. Structural analysis of the proteins most susceptible to Phactr1/PP1 activity showed that they had particular sequences that could interact with Phactr1/PP1's composite pocket. Further experiments revealed that, compared to PP1 acting alone, the pocket increased the binding efficiency and reactivity of the complex 100-fold. This work demonstrates that a partner protein can make phosphatases more sequence-specific, suggesting that future studies could adopt a similar approach to examine how other enzymes in this family perform their role. In addition, the results suggest that it will be possible to design Phactr1/PP1-specific drugs that act on the composite pocket. This would represent an important proof of principle, since current phosphatase-specific drugs do not target particular phosphatase complexes.

RPEL-family rhoGAPs link Rac/Cdc42 GTP loading to G-actin availability.

RPEL proteins, which contain the G-actin-binding RPEL motif, coordinate cytoskeletal processes with actin dynamics. We show that the ArhGAP12- and ArhGAP32-family GTPase-activating proteins (GAPs) are RPEL proteins. We determine the structure of the ArhGAP12/G-actin complex, and show that G-actin contacts the RPEL motif and GAP domain sequences. G-actin inhibits ArhGAP12 GAP activity, and this requires the G-actin contacts identified in the structure. In B16 melanoma cells, ArhGAP12 suppresses basal Rac and Cdc42 activity, F-actin assembly, invadopodia formation and experimental metastasis. In this setting, ArhGAP12 mutants defective for G-actin binding exhibit more effective downregulation of Rac GTP loading following HGF stimulation and enhanced inhibition of Rac-dependent processes, including invadopodia formation. Potentiation or disruption of the G-actin/ArhGAP12 interaction, by treatment with the actin-binding drugs latrunculin B or cytochalasin D, has corresponding effects on Rac GTP loading. The interaction of G-actin with RPEL-family rhoGAPs thus provides a negative feedback loop that couples Rac activity to actin dynamics.

Molecular determinants regulating selective binding of autophagy adapters and receptors to ATG8 proteins.

Autophagy is an essential recycling and quality control pathway. Mammalian ATG8 proteins drive autophagosome formation and selective removal of protein aggregates and organelles by recruiting autophagy receptors and adaptors that contain a LC3-interacting region (LIR) motif. LIR motifs can be highly selective for ATG8 subfamily proteins (LC3s/GABARAPs), however the molecular determinants regulating these selective interactions remain elusive. Here we show that residues within the core LIR motif and adjacent C-terminal region as well as ATG8 subfamily-specific residues in the LIR docking site are critical for binding of receptors and adaptors to GABARAPs. Moreover, rendering GABARAP more LC3B-like impairs autophagy receptor degradation. Modulating LIR binding specificity of the centriolar satellite protein PCM1, implicated in autophagy and centrosomal function, alters its dynamics in cells. Our data provides new mechanistic insight into how selective binding of LIR motifs to GABARAPs is achieved, and elucidate the overlapping and distinct functions of ATG8 subfamily proteins.

Members of the autophagy class III phosphatidylinositol 3-kinase complex I interact with GABARAP and GABARAPL1 via LIR motifs.

Autophagosome formation depends on a carefully orchestrated interplay between membrane-associated protein complexes. Initiation of macroautophagy/autophagy is mediated by the ULK1 (unc-51 like autophagy activating kinase 1) protein kinase complex and the autophagy-specific class III phosphatidylinositol 3-kinase complex I (PtdIns3K-C1). The latter contains PIK3C3/VPS34, PIK3R4/VPS15, BECN1/Beclin 1 and ATG14 and phosphorylates phosphatidylinositol to generate phosphatidylinositol 3-phosphate (PtdIns3P). Here, we show that PIK3C3, BECN1 and ATG14 contain functional LIR motifs and interact with the Atg8-family proteins with a preference for GABARAP and GABARAPL1. High resolution crystal structures of the functional LIR motifs of these core components of PtdIns3K-C1were obtained. Variation in hydrophobic pocket 2 (HP2) may explain the specificity for the GABARAP family. Mutation of the LIR motif in ATG14 did not prevent formation of the PtdIns3K-C1 complex, but blocked colocalization with MAP1LC3B/LC3B and impaired mitophagy. The ULK-mediated phosphorylation of S29 in ATG14 was strongly dependent on a functional LIR motif in ATG14. GABARAP-preferring LIR motifs in PIK3C3, BECN1 and ATG14 may, via coincidence detection, contribute to scaffolding of PtdIns3K-C1 on membranes for efficient autophagosome formation. Abbreviations: ATG: autophagy-related; BafA1: bafilomycin A1; GABARAP: GABA type A receptor-associated protein; GABARAPL1: GABA type A receptor associated protein like 1; GFP: enhanced green fluorescent protein; KO: knockout; LDS: LIR docking site; LIR: LC3-interacting region; MAP1LC3/LC3: microtubule associated protein 1 light chain 3; PIK3C3: phosphatidylinositol 3-kinase catalytic subunit type 3; PIK3R4: phosphoinositide-3-kinase regulatory subunit 4; PtdIns3K: phosphatidylinositol 3-kinase; PtdIns3P: phosphatidylinositol-3-phosphate; SQSTM1/p62: sequestosome 1; VPS: Vacuolar protein sorting; ULK: unc-51 like autophagy activating kinase.

ASPP proteins discriminate between PP1 catalytic subunits through their SH3 domain and the PP1 C-tail.

Serine/threonine phosphatases such as PP1 lack substrate specificity and associate with a large array of targeting subunits to achieve the requisite selectivity. The tumour suppressor ASPP (apoptosis-stimulating protein of p53) proteins associate with PP1 catalytic subunits and are implicated in multiple functions from transcriptional regulation to cell junction remodelling. Here we show that Drosophila ASPP is part of a multiprotein PP1 complex and that PP1 association is necessary for several in vivo functions of Drosophila ASPP. We solve the crystal structure of the human ASPP2/PP1 complex and show that ASPP2 recruits PP1 using both its canonical RVxF motif, which binds the PP1 catalytic domain, and its SH3 domain, which engages the PP1 C-terminal tail. The ASPP2 SH3 domain can discriminate between PP1 isoforms using an acidic specificity pocket in the n-Src domain, providing an exquisite mechanism where multiple motifs are used combinatorially to tune binding affinity to PP1.

Highlights of glucosamine-6P synthase catalysis.

L-Glutamine:d-fructose-6-phosphate amidotransferase, also known as glucosamine-6-phosphate synthase (GlcN6P synthase), which catalyzes the first step in a pathway leading to the formation of uridine 5'-diphospho-N-acetyl-d-glucosamine (UDP-GlcNAc), is a key point in the metabolic control of the biosynthesis of amino sugar-containing macromolecules. The molecular mechanism of the reaction catalyzed by GlcN6P synthase is complex and involves amide bond cleavage followed by ammonia channeling and sugar isomerization. This article provides a comprehensive overview of the present knowledge on this multi-faceted enzyme emphasizing the progress made during the last five years.