Sialic acid esterase (SIAE) catalyzes the removal of O-acetyl groups from sialic acids found on cell surface glycoproteins to regulate cellular processes such as B cell receptor signalling and apoptosis. Loss-of-function mutations in SIAE are associated with several common autoimmune diseases including Crohn's, ulcerative colitis, and arthritis. To gain a better understanding of the function and regulation of this protein, we determined crystal structures of SIAE from three mammalian homologs, including an acetate bound structure.
View Article and Find Full Text PDFMucopolysaccharidoses (MPS) are lysosomal storage diseases caused by defects in catabolism of glycosaminoglycans. MPS I, II, III and VII are associated with lysosomal accumulation of heparan sulphate and manifest with neurological deterioration. Most of these neurological MPS currently lack effective treatments.
View Article and Find Full Text PDFSialic acids linked to glycoproteins and glycolipids are important mediators of cell and protein recognition events. These sugar residues are removed by neuraminidases (sialidases). Neuraminidase-1 (sialidase-1 or NEU1) is a ubiquitously expressed mammalian sialidase located in lysosomes and on the cell membrane.
View Article and Find Full Text PDFProc Natl Acad Sci U S A
August 2022
The mannose-6-phosphate (M6P) pathway is responsible for the transport of hydrolytic enzymes to lysosomes. N-acetylglucosamine-1-phosphotransferase (GNPT) catalyzes the first step of tagging these hydrolases with M6P, which when recognized by receptors in the Golgi diverts them to lysosomes. Genetic defects in the GNPT subunits, GNPTAB and GNPTG, cause the lysosomal storage diseases mucolipidosis types II and III.
View Article and Find Full Text PDFThe enzymes β-galactosidase (GLB1) and neuraminidase 1 (NEU1; sialidase 1) participate in the degradation of glycoproteins and glycolipids in the lysosome. To remain active and stable, they associate with PPCA [protective protein cathepsin A (CTSA)] into a high-molecular weight lysosomal multienzyme complex (LMC), of which several forms exist. Genetic defects in these three proteins cause the lysosomal storage diseases GM1-gangliosidosis/mucopolysaccharidosis IV type B, sialidosis, and galactosialidosis, respectively.
View Article and Find Full Text PDFThe ecto-nucleoside triphosphate diphosphohydrolases (NTPDases) are a family of enzymes found on the cell surface and in the lumen of certain organelles, that are major regulators of purinergic signaling. Their intracellular roles, however, have not been clearly defined. NTPDase4 (UDPase, ENTPD4) is a Golgi protein potentially involved in nucleotide recycling as part of protein glycosylation, and is also found in lysosomes, where its purpose is unknown.
View Article and Find Full Text PDFMost lysosomal hydrolytic enzymes reach their destination via the mannose-6-phosphate (M6P) pathway. The enzyme N-acetylglucosamine-1-phosphodiester α-N-acetylglucosaminidase (NAGPA, or "uncovering enzyme") catalyzes the second step in the M6P tag formation, namely the removal of the masking N-acetylglucosamine (GlcNAc) portion. Defects in this protein are associated with non-syndromic stuttering.
View Article and Find Full Text PDFCaspase-6 is a cysteine protease that plays essential roles in programmed cell death, axonal degeneration, and development. The excess neuronal activity of Caspase-6 is associated with Alzheimer disease neuropathology and age-dependent cognitive impairment. Caspase-6 inhibition is a promising strategy to stop early stage neurodegenerative events, yet finding potent and selective Caspase-6 inhibitors has been a challenging task due to the overlapping structural and functional similarities between caspase family members.
View Article and Find Full Text PDFβ-Mannosidase is a lysosomal enzyme from the glycosyl hydrolase family 2 that cleaves the single β(1-4)-linked mannose at the nonreducing end of N-glycosylated proteins, and plays an important role in the polysaccharide degradation pathway. Mutations in the MANBA gene, which encodes the β-mannosidase, can lead to the lysosomal storage disease β-mannosidosis, as well as nystagmus, an eye condition characterized by involuntary eye movements. Here, we present the first structures of a mammalian β-mannosidase in both the apo- and mannose-bound forms.
View Article and Find Full Text PDFHuman acid ceramidase (AC) is a lysosomal cysteine amidase, which has received a great deal of interest in recent years as a potential target for the development of new therapeutics against melanoma and glioblastoma tumors. Despite the strong interest in obtaining structural information, only the structures of the apo-AC enzyme in its zymogen and activated conformations are available. In this work, the crystal structure of AC in complex with the covalent carmofur inhibitor is presented.
View Article and Find Full Text PDFProc Natl Acad Sci U S A
October 2018
Palmitoylethanolamide is a bioactive lipid that strongly alleviates pain and inflammation in animal models and in humans. Its signaling activity is terminated through degradation by -acylethanolamine acid amidase (NAAA), a cysteine hydrolase expressed at high levels in immune cells. Pharmacological inhibitors of NAAA activity exert profound analgesic and antiinflammatory effects in rodent models, pointing to this protein as a potential target for therapeutic drug discovery.
View Article and Find Full Text PDFSaposins are accessory proteins that aid in the degradation of sphingolipids by hydrolytic enzymes. Their structure usually comprises four α-helices arranged in various conformations including an open, V-shaped form that is generally associated with the ability to interact with membranes and/or enzymes to accentuate activity. Saposin D is required by the lysosomal hydrolase, acid ceramidase, which breaks down ceramide into sphingosine and free fatty acid, to display optimal activity.
View Article and Find Full Text PDFUnlabelled: The ecto-nucleotide pyrophosphatase/phosphodiesterase (NPP) enzyme family modulates purinergic signaling by degrading extracellular nucleotides. CD203c (NPP3, ENPP3) regulates the inflammatory response of basophils via ATP hydrolysis and is a marker for allergen sensitivity on the surface of these cells. Multiple other roles and substrates have also been proposed for this protein.
View Article and Find Full Text PDFAcid ceramidase (aCDase, ASAH1) hydrolyzes lysosomal membrane ceramide into sphingosine, the backbone of all sphingolipids, to regulate many cellular processes. Abnormal function of aCDase leads to Farber disease, spinal muscular atrophy with progressive myoclonic epilepsy, and is associated with Alzheimer's, diabetes, and cancer. Here, we present crystal structures of mammalian aCDases in both proenzyme and autocleaved forms.
View Article and Find Full Text PDFProc Natl Acad Sci U S A
January 2018
LPS is a potent bacterial endotoxin that triggers the innate immune system. Proper recognition of LPS by pattern-recognition receptors requires a full complement of typically six acyl chains in the lipid portion. Acyloxyacyl hydrolase (AOAH) is a host enzyme that removes secondary (acyloxyacyl-linked) fatty acids from LPS, rendering it immunologically inert.
View Article and Find Full Text PDFUnlabelled: The ecto-nucleotide pyrophosphatase/phosphodiesterase (NPP) family of proteins mediates purinergic signaling by degrading extracellular nucleotides and also participates in phospholipid metabolism. NPP5 (ENPP5) is the least characterized member of this group and its specific role is unknown. This enzyme does not display activity on certain nucleotides and on other typical NPP substrates.
View Article and Find Full Text PDFAbsorption of dietary sphingomyelin (SM) requires its initial degradation into ceramide, a process catalyzed by the intestinal enzyme alkaline sphingomyelinase (alk-SMase, NPP7, ). alk-SMase belongs to the nucleotide pyrophosphatase/phosphodiesterase (NPP) family, the members of which hydrolyze nucleoside phosphates, phospholipids, and other related molecules. NPP7 is the only paralog that can cleave SM, and its activity requires the presence of bile salts, a class of physiological anionic detergents.
View Article and Find Full Text PDFThe enzyme acid sphingomyelinase-like phosphodiesterase 3B (SMPDL3B) was shown to act as a negative regulator of innate immune signaling, affecting cellular lipid composition and membrane fluidity. Furthermore, several reports identified this enzyme as an off target of the therapeutic antibody rituximab, with implications in kidney disorders. However, structural information for this protein is lacking.
View Article and Find Full Text PDFAcid sphingomyelinase (ASMase, ASM, SMPD1) converts sphingomyelin into ceramide, modulating membrane properties and signal transduction. Inactivating mutations in ASMase cause Niemann-Pick disease, and its inhibition is also beneficial in models of depression and cancer. To gain a better understanding of this critical therapeutic target, we determined crystal structures of mammalian ASMase in various conformations.
View Article and Find Full Text PDFSphingomyelin phosphodiesterase, acid-like 3A (SMPDL3A) is a member of a small family of proteins founded by the well characterized lysosomal enzyme, acid sphingomyelinase (ASMase). ASMase converts sphingomyelin into the signaling lipid, ceramide. It was recently discovered that, in contrast to ASMase, SMPDL3A is inactive against sphingomyelin and, surprisingly, can instead hydrolyze nucleoside diphosphates and triphosphates, which may play a role in purinergic signaling.
View Article and Find Full Text PDFThe calnexin cycle is a process by which glycosylated proteins are subjected to folding cycles in the endoplasmic reticulum lumen via binding to the membrane protein calnexin (CNX) or to its soluble homolog calreticulin (CRT). CNX and CRT specifically recognize monoglucosylated Glc(1)Man(9)GlcNAc(2) glycans, but the structural determinants underlying this specificity are unknown. Here, we report a 1.
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