DPM1 modulates desmosomal adhesion and epidermal differentiation through SERPINB5

Glycosylation is essential to facilitate cell-cell adhesion and differentiation. We determined the role of the dolichol phosphate mannosyltransferase (DPM) complex, a central regulator for glycosylation, for desmosomal adhesive function and epidermal differentiation. Deletion of the key molecule of the DPM complex, DPM1, in human keratinocytes resulted in weakened cell-cell adhesion, impaired localization of the desmosomal components desmoplakin and desmoglein-2, and led to cytoskeletal organization defects in human keratinocytes. In a 3D organotypic human epidermis model, loss of DPM1 caused impaired differentiation with abnormally increased cornification, reduced thickness of non-corneal layers, and formation of intercellular gaps in the epidermis. Using proteomic approaches, SERPINB5 was identified as a DPM1-dependent interaction partner of desmoplakin. Mechanistically, SERPINB5 reduced desmoplakin phosphorylation at serine 176, which was required for strong intercellular adhesion. These results uncover a novel role of the DPM complex in connecting desmosomal adhesion with epidermal differentiation.

Hydrolytic activity of yeast oligosaccharyltransferase is enhanced when misfolded proteins accumulate in the endoplasmic reticulum

It is known that oligosaccharyltransferase (OST) has hydrolytic activity toward dolichol-linked oligosaccharides (DLO), which results in the formation of free N-glycans (FNGs), i.e. unconjugated oligosaccharides with structural features similar to N-glycans. The functional importance of this hydrolytic reaction, however, remains unknown. In this study, the hydrolytic activity of OST was characterized in yeast. It was shown that the hydrolytic activity of OST is enhanced in ubiquitin ligase mutants that are involved in endoplasmic reticulum-associated degradation. Interestingly, this enhanced hydrolysis activity is completely suppressed in asparagine-linked glycosylation (alg) mutants, bearing mutations related to the biosynthesis of DLO, indicating that the effect of ubiquitin ligase on OST-mediated hydrolysis is context-dependent. The enhanced hydrolysis activity in ubiquitin ligase mutants was also found to be canceled upon treatment of the cells with dithiothreitol, a reagent that potently induces protein unfolding in the endoplasmic reticulum (ER). Our results clearly suggest that the hydrolytic activity of OST is enhanced under conditions in which the formation of unfolded proteins is promoted in the ER in yeast. The possible role of FNGs on protein folding is discussed.

Aptamer-Based Imaging of Polyisoprenoids in the Malaria Parasite

Dolichols are isoprenoid end-products of the mevalonate and 2C-methyl-D-erythritol-4-phosphate pathways. The synthesis of dolichols is initiated with the addition of several molecules of isopentenyl diphosphate to farnesyl diphosphate. This reaction is catalyzed by a cis-prenyltransferase and leads to the formation of polyprenyl diphosphate. Subsequent steps involve the dephosphorylation and reduction of the α-isoprene unit by a polyprenol reductase, resulting in the generation of dolichol. The size of the dolichol varies, depending on the number of isoprene units incorporated. In eukaryotes, dolichols are synthesized as a mixture of four or more different lengths. Their biosynthesis is predicted to occur in the endoplasmic reticulum, where dolichols play an essential role in protein glycosylation. In this study, we have developed a selection of aptamers targeting dolichols and enhanced their specificity by incorporating fatty acids for negative selection. One aptamer showed high enrichment and specificity for linear polyisoprenoids containing at least one oxygen atom, such as an alcohol or aldehyde, in the α-isoprene unit. The selected aptamer proved to be a valuable tool for the subcellular localization of polyisoprenoids in the malaria parasite. To the best of our knowledge, this is the first time that polyisoprenoids have been localized within a cell using aptamer-based imaging techniques.

Dissecting the role of dolichol in cell wall assembly in the yeast mutants impaired in early glycosylation reactions

Evidence is presented that temperature-sensitive Saccharomyces cerevisiae mutants, impaired in dolichol kinase (Sec59p) or dolichyl phosphate mannose synthase (Dpm1p) activity have an aberrant cell wall composition and ultrastructure. The mutants were oversensitive to Calcofluor white, an agent interacting with the cell wall chitin. In accordance with this, chemical analysis of the cell wall alkali-insoluble fraction indicated an increased amount of chitin and changes in the quantity of beta1,6- and beta1,3-glucan in sec59-1 and dpm1-6 mutants. In order to unravel the link between the formation of dolichyl phosphate and dolichyl phosphate mannose and the cell wall assembly, we screened a yeast genomic library for a multicopy suppressors of the thermosensitive phenotype. The RER2 and SRT1 genes, encoding cis-prenyltransferases, were isolated. In addition, the ROT1 gene, encoding protein involved in beta1,6-glucan synthesis (Machi et al., 2004) and protein folding (Takeuchi et al., 2006) acted as a multicopy suppressor of the temperature-sensitive phenotype of the sec59-1 mutant. The cell wall of the mutants and of mutants bearing the multicopy suppressors was analysed for carbohydrate and mannoprotein content. We also examined the glycosylation status of the plasma membrane protein Gas1p, a beta1,3-glucan elongase, and the degree of phosphorylation of the Mpk1/Slt2 protein, involved in the cell wall integrity pathway.

Glycoproteins and their relationship to human disease

Glycoproteins are proteins that carry N- and O-glycosidically-linked carbohydrate chains of complex structures and functions. N-glycan chains are assembled in the endoplasmic reticulum and the Golgi by a controlled sequence of glycosyltransferase and glycosidase processing reactions involving dolichol intermediates. The assembly of O-glycans occurs in the Golgi and does not involve dolichol. For most reactions, families of glycosyltransferases exist; the expression of the individual enzymes within a family is often subject to complex regulation. The biosynthesis of N- and O-glycan is controlled at the level of gene expression, mRNA, enzyme protein activity and localization, and through substrate and cofactor concentrations at the site of synthesis. This complex regulation results in many hundreds of structures, the range of which varies in different species, cell types, tissue types, states of development and differentiation. In diseased cells, the relative proportions of these structures are often characteristically different from normal, and may be useful for the assessment of the stage of the disease and for diagnosis. Knowledge of disease-specific glycoprotein structures and their functions may be used therapeutically, in immunotherapy, in blocking cell adhesion or interfering with other binding or biological processes. Recently, some of the mechanisms underlying glycoprotein alterations in disease have been elucidated. This opens the possibility of an active interference in the disease process. The functions of glycans in diseased cells will become more clear with the tools of molecular biology and transgenic animal models.

Transient N-acetylgalactosaminylation of mannosyl phosphate side chain in Paramecium primaurelia glycosylphosphatidylinositols

The surface antigens of the free-living protozoan Paramecium primaurelia belong to the family of glycosylphosphatidylinositol (GPtdIns)-anchored proteins. Using a cell-free system prepared from P. primaurelia, we have described the structure and biosynthetic pathway for GPtdIns glycolipids. The core glycans of the polar glycolipids are modified by a mannosyl phosphate side chain. The data suggest that the mannosyl phosphate side chain is added onto the core glycan in two steps. The first step involves the phosphorylation of the GPtdIns trimannosyl conserved core glycan via an ATP-dependent kinase, prior to the addition of the mannose linked to the phosphate group. We show that dolichol phosphate mannose is the donor of all mannose residues including the mannose linked to phosphate. Furthermore, we were able to identify in vitro a hydrophilic intermediate containing an additional N-acetylgalactosamine linked to the mannosyl phosphate side chain. The addition of this purified hydrophilic radiolabelled intermediate into the cell-free system leads to a loss of the GalNAc residue and its conversion to the penultimate intermediate having only mannosyl phosphate as a side chain. Together the data indicate that the GalNAc-containing intermediate is a transitional intermediate. We suggest that the GalNAc-containing intermediate is essential for biosynthesis and maturation of GPtdIns precursors. It is hypothesized that this oligosaccharide processing in the course of GPtdIns biosynthesis is required for the translocation of GPtdIns from the cytoplasmic side of the endoplasmic reticulum to the luminal side.

Nuclear magnetic resonance studies of polyisoprenols in model membranes

2H-NMR investigation of polyisoprenols (PIs) in model membranes has revealed information about their motions, relative order, and locale within the membrane. Initial 2H-NMR studies of the organization of the shorter chain homologues geraniol (C10), farnesol (C15), and solanesol (C45) were carried out by incorporating 2H-acetyl esters of the alcohol or the di-perdeuterome-thylated derivatives of the omega-labeled prenols into multilamellar phosphatidylcholine (PC) vesicles. 2H-NMR powder patterns interpretable in terms of quadrupole splittings and spin-lattice relaxation times were obtained. Similar experiments have now been carried out with the labeled free alcohol, acetyl ester, and phosphate ester of dolichol (C95) and undecaprenol (C55). 2H-NMR results show that the head and tail 2H-labeled sites of C55 and C95 exhibit a fast motion isotropic signal only; no slower motion anisotropy, as exhibited by the short chain PIs, was observed. These data suggest that C55 and C95 either have substantially different (faster) motions and/or conformations relative to the shorter chain PIs within the membrane, and that the longer PIs alter the membrane host packing matrix. This conclusion was supported by 31P-NMR studies of C55 and C95 derivatives in PC and PE/PC membranes, which showed new pronounced spectral changes relative to the results obtained with the shorter chain PIs. These spectral changes indicate that undecaprenol and dolichol derivatives appear to induce a non-bilayer (isotropic) organization of phospholipid molecules in PE/PC (2:1) vesicles. The possible physiological consequences of this perturbation remains to be determined.

The conformation of dolichol

An understanding of the natural conformation of dolichol is important for the elucidation of the mechanism of protein glycosylation and dolichol's other as yet undisclosed biological functions. Since the molecular mechanics method has been shown to be well suited for the prediction of alcohol and alkene conformations, we have employed it to study the conformations of apparent least energy of dolichol-19 and smaller polymers of isoprene, namely, squalene, trans,trans-farnesol, and cis,cis-farnesol. Additionally, the small-angle X-ray scattering (SAXS) method was employed to determine the validity of the apparent least energy conformer of dolichol-19 derived by the molecular mechanics method. The results indicate that the solution conformation of dolichol-19 is comprised of a central coiled region flanked by two arms. The central coiled region has two and a half turns of dimensions 9.84 x 16.55 x 51.66 A3. The arms of dimensions 3.99 x 5.89 x 17.47 A3 and 4.49 x 9.23 x 11.14 A3 are approximately diametrically opposed. Measurement of the intrinsic viscosity of dolichol in both isopentyl alcohol and oleyl alcohol showed that the natural conformation of dolichol is capable of increasing solution fluidity (i.e., lowering solution viscosity). Thus, while examination of the conformation of dolichol in a membrane-mimetic solvent by SAXS is not possible, the quantitative measure of the effect of dolichol on solution viscosity (and thus solution fluidity) is possible. The results are consistent with dolichol acting as a membrane-fluidizing agent and provide the first quantitative measure of the effect of dolichol on solution fluidity of a membrane-mimetic solvent.

Determination of arrangement of isoprene units in pig liver dolichol by 13C-n.m.r. spectroscopy

The arrangement of isoprene units in pig liver dolichol-18, -19 and -20 was determined by 1H- and 13C-n.m.r. spectroscopies. The alignment of trans and cis isoprene units was found to be in the order: dimethylallyl unit, two trans units, a sequence of 14-16 cis units, and a saturated isoprene unit terminated with a hydroxyl group, which verified the presumed chemical structure of dolichol. The absence of geometric isomers was confirmed. A slight amount of impurity was detected in each reversed-phase h.p.l.c. fraction of dolichol obtained by a conventional method. Detailed assignments of the 13C-n.m.r. spectrum were given for these dolichols by using model compounds and INEPT (insensitive nuclei enhanced by polarization transfer) measurement. The chemical structure of synthetic dolichol-19, which was prepared by the addition of a saturated isoprene unit to the polyprenol-18 isolated from Ginkgo biloba, was confirmed to be identical with that of pig liver dolichol-19.

2H NMR investigation of the organization and dynamics of polyisoprenols in membranes

The polyisoprenols (PIs) dolichol and undecaprenol function as chemical carriers of glycosyl residues in the membrane-directed synthesis of glycoconjugates in prokaryotic and eukaryotic cells. The molecular details of how these lipid cofactors function is unknown. Presented here are results of deuterium NMR investigations of site specifically 2H-labeled PIs incorporated into model membranes. To complement previous omega-terminal PI labeling schemes, a simple synthesis of head group 2H-labeled PIs is presented in which a PI alcohol is esterified with deuterated acetyl chloride. The 2H-labeled PIs, when incorporated into multilamellar membranes composed of phosphatidylcholine, gave rise to 2H NMR powder patterns interpretable in terms of quadrupole splittings (delta vQ) and spin-lattice relaxation times (T1s). Pure isomers of head group 2H-labeled geraniol (C10) and solanesol (C45) gave rise to single splittings while farnesol (C15) gave rise to two sets of splittings due to cis-trans isomerization at the polar terminal double bond. Membranes containing C45 solanesol exhibited a large isotropic component, indicative of limited partitioning of this poly trans PI into the membrane. T1 measurements revealed high rates of motion for PIs relative to cholesterol in similar membrane hosts and revealed correlation times close to the fatty acyl methyl termini in phosphatidylcholine. The smaller PIs showed higher rates of motion but the T1s of head and tail labels were similar. These data indicate that both ends of the esterified PI molecules see similar environments, probably in the bilayer interior, and suggest that the esterified PIs studied here do not appear to adopt a conventional head group-at-interface orientation of lipids within the bilayer.

Absolute configuration of dolichol

A derivative of dolichol was formed and then chemically degraded to a small fragment containing the sole centre of asymmetry of the original molecule. Polarimetric comparison of this derivative with a standard prepared from (R)-citronellol showed dolichol to have an S-configuration at C-3. To determine the optical purity of dolichol a diastereoisomeric derivative was prepared and compared with standard diastereoisomers, which could be resolved by high-pressure liquid chromatography. Dolichols from pig liver, human liver and hen oviduct were analysed by this procedure and were all found to be greater than 95% S-configuration.

Paramagnetic isoprenoid carrier lipids. 1. Chemical synthesis and incorporation into model membranes

The synthesis, purification, and characterization of two types of spin-labeled glycosyl carrier lipids and shorter chain isoprenols are described. As models for phosphorylated lipids intermediates, phosphodiesters of tempol and the prenols dolichol, ficaprenol, solanesol, phytol, and farnesol were prepared. For analogues of neutral species each prenol was esterified with a pyrrolidinecarboxylic acid based label. Tripropylbenzenesulfonyl chloride was used as the condensing agent in both cases. Phosphodiester yields ranged from 36% for the 55-carbon compound to greater than 66% for the 95-carbon prenol. Both types of probes were incorporated into phospholipid bilayers, where each became oriented with the artificial head group at, or very close to, the water--hydrocarbon interface. Electron spin resonance spectra of the phosphodiesters are matrix dependent, indicating rapid isotropic tumbling in chloroform but highly anisotropic reorientation in unsaturated phosphatidylcholine (PC) hosts. Rotation or large amplitude oscillation about either or both the tempo C4--O linkage and the P--O (chain) bond as well as whole molecule rotation within the bilayer could account for the observed x-axis anisotropy. Segmental motion within the polyprene chain does not appear to be a determinant.

A mannosyl-carrier lipid of bovine adrenal meddulla and rat parotid

1. The transfer of mannose from GDP-(U-14-C)mannose into endogenous acceptors of bovine adrenal medullla and rat parotid was studied. The rapidly labelled product, a glycolipid, was partially purified and characterized. 2. It was stable to mild alkaline hydrolysis but yielded (14-C)mannose on mild acid hydrolysis. It co-chromatographed with mannosyl phosphoryl dolichol in four t.l.c. systems and on DEAE-cellulose acetate. Addition of dolichol phosphate or a dolichol phosphate-enriched fraction prepared from pig liver stimulated mannolipid synthesis. 3. The formation of mammolipid appeared reversible, since addition of GDP to a system synthesizing the mannolipid caused a rapid loss of label from the mannolipid. UDP-N-acetylglucosamine did not inhibit mannolipid synthesis except at high concentrations (2 mM), even though in the absence of GDP-mannose, N-acetylglucosamine was incorporated into a lipid having the properties of a glycosylated polyprenyl phosphate. 4. Mannose from GDP-mannose was also incorporated into two other acceptors, (2y being insoluble in chloroform-methanol (2:1, v/v) but soluble in choloroform-methanol-water (10:10:3, by vol.) and (ii) protein. These are formed much more slowly than the mannolipid. 5. Exogenous mannolipid served as a mannose donor for acceptors (i) and (ii), and it is suggested that transfer of mannose from GDP-mannose to mannosylated protein occurs via two intermediates, the mannolipid and acceptor (i).