Biosynthesis of glycosylphosphatidylinositol anchors

The Re-Localization of Proteins to or Away from Membranes as an Effective Strategy for Regulating Stress Tolerance in Plants

The membranes of plant cells are dynamic structures composed of phospholipids and proteins. Proteins harboring phospholipid-binding domains or lipid ligands can localize to membranes. Stress perception can alter the subcellular localization of these proteins dynamically, causing them to either associate with or detach from membranes. The mechanisms behind the re-localization involve changes in the lipidation state of the proteins and interactions with membrane-associated biomolecules. The functional significance of such re-localization includes the regulation of molecular transport, cell integrity, protein folding, signaling, and gene expression. In this review, proteins that re-localize to or away from membranes upon abiotic and biotic stresses will be discussed in terms of the mechanisms involved and the functional significance of their re-localization. Knowledge of the re-localization mechanisms will facilitate research on increasing plant stress adaptability, while the study on re-localization of proteins upon stresses will further our understanding of stress adaptation strategies in plants.

Comprehensive Profiling of Glycosphingolipid Glycans Using a Novel Broad Specificity Endoglycoceramidase in a High-Throughput Workflow

The biological function of glycosphingolipids (GSLs) is largely determined by their glycan headgroup moiety. This has placed a renewed emphasis on detailed GSL headgroup structural analysis. Comprehensive profiling of GSL headgroups in biological samples requires the use of endoglycoceramidases with broad substrate specificity and a robust workflow that enables their high-throughput analysis. We present here the first high-throughput glyco-analytical platform for GSL headgroup profiling. The workflow features enzymatic release of GSL glycans with a novel broad-specificity endoglycoceramidase I (EGCase I) from Rhodococcus triatomea, selective glycan capture on hydrazide beads on a robotics platform, 2AB-fluorescent glycan labeling, and analysis by UPLC-HILIC-FLD. R. triatomea EGCase I displayed a wider specificity than known EGCases and was able to efficiently hydrolyze gangliosides, globosides, (n)Lc-type GSLs, and cerebrosides. Our workflow was validated on purified GSL standard lipids and was applied to the characterization of GSLs extracted from several mammalian cell lines and human serum. This study should facilitate the analytical workflow in functional glycomics studies and biomarker discovery.

In vivo incorporation of an azide-labeled sugar analog to detect mammalian glycosylphosphatidylinositol molecules isolated from the cell surface

N-Acetylgalactosamine (GalNAc) linked to the first mannose of glycosylphosphatidylinositol (GPI) core has been previously reported to be heterogeneously present on some mammalian GPI-anchored proteins. Here we present a method for profiling GalNAc-containing GPI-anchored proteins in mammalian cells by metabolic labeling with tetraacetylated N-azidoacetylgalactosamine (GalNAz) followed by biotinylation of the incorporated sugar analog. We have labeled both endogenous and recombinant GPI-anchored proteins with GalNAz, and demonstrated that the azide-activated sugar gets incorporated into the GPI glycan, likely as an unsubstituted side branch of the core structure. GalNAz was detected only on GPI molecules attached to proteins, and not on GPI precursors, indicating that GalNAc modification takes place after the GPI anchor is transferred to protein. We have highlighted the utility of this cell labeling approach by demonstrating the ability to examine specific GalNAc-containing GPI-anchored proteins isolated non-destructively from separate membrane domains (apical and basolateral) in polarized epithelial cells. This study represents the first demonstration of site-specific in vivo labeling of a GPI moiety with a synthetic sugar analog.

A Conserved Proline in the Last Transmembrane Segment of Gaa1 Is Required for Glycosylphosphatidylinositol (GPI) Recognition by GPI Transamidase

Glycosylphosphatidylinositol (GPI)-anchored proteins are synthesized as precursor proteins that are processed in the endoplasmic reticulum by GPI transamidase (GPIT). Human GPIT is a multisubunit membrane-bound protein complex consisting of Gaa1, Gpi8, phosphatidylinositol glycan (PIG)-S, PIG-T, and PIG-U. The enzyme recognizes a C-terminal signal sequence in the proprotein and replaces it with a preformed GPI lipid. The nature of the functional interaction of the GPIT subunits with each other and with the proprotein and GPI substrates is largely unknown. We recently analyzed the GPIT subunit Gaa1, a polytopic protein with seven transmembrane (TM) spans, to identify sequence determinants in the protein that are required for its interaction with other subunits and for function (Vainauskas, S., Maeda, Y., Kurniawan, H., Kinoshita, T., and Menon, A. K. (2002) J. Biol. Chem. 277, 30535-30542). We showed that elimination of the C-terminal TM segment of Gaa1 allows the protein to interact with Gpi8, PIG-S, and PIG-T but renders the resulting GPIT complex nonfunctional. We now show that GPIT complexes containing C-terminally truncated Gaa1 possess a full complement of subunits and are able to interact with a proprotein substrate but cannot co-immunoprecipitate GPI. We go on to show that mutation of a conserved proline residue centrally located within the C-terminal TM span of Gaa1 is sufficient to abrogate the ability of the resulting GPIT complex to co-immunoprecipitate GPI. We suggest that the putative dynamic hinge created by the proline residue provides a structural basis for the interaction of GPI with GPIT.

Glycosylphosphatidylinositol (GPI)-deficient Jurkat T cells as a model to study functions of GPI-anchored proteins

Many cell surface proteins attached to the membrane by GPI are involved in cell signalling. However, the role of the GPI membrane anchor itself remains poorly understood. GPI-defective cells from patients with paroxysmal nocturnal haemoglobinuria (PNH) are relatively resistant to apoptosis induction. We developed a Jurkat T cell model for GPI deficiency by isolating a GPI-negative mutant, which is defective in the GPI biosynthetic gene PIG-A. Using retroviral PIG-A gene transfer along with the transfer of a vector control, we obtained two genetically identical cell lines, distinguished only by expression of the PIG-A gene and, thus, their ability to produce GPI. Cell proliferation and survival were not affected by this difference. Apoptotic stimuli such as serum starvation and camptothecin exposure elicited similar responses. In contrast, GPI-defective Jurkat cells were more susceptible to Fas-mediated apoptosis than GPI-positive cells. These results indicate that a deficiency in GPI-anchored proteins, as is found in PNH, does not confer resistance to apoptosis.

Characterisation of the carbohydrate components of Taenia solium metacestode glycoprotein antigens

Human neurocysticercosis is caused by Taenia solium metacestodes. It usually affects the central nervous system of humans and can be confused with other brain pathologies. The Lens culinaris-binding glycoproteins from this parasite have been shown to be ideal targets for the development of a highly specific immunoassay for the diagnosis of neurocysticercosis. In the present study we characterised the carbohydrates associated with five antigenic glycoproteins of T. solium metacestodes in the range of 12-28 kilodaltons. Lectin-affinities and enzymatic deglycosylations suggested that each of the five antigens contain various glycoforms of asparagine-linked carbohydrates of the hybrid, complex and probably high mannose type. These carbohydrates accounted for at least 30-66% of the apparent molecular mass of the glycoconjugates. In contrast, there was no evidence for the presence of O-linked carbohydrates. Lectin affinity patterns suggested that the sugars are short and truncated in their biosynthetic route, and that some contain terminal galactose moieties. Elucidating the precise structure of the carbohydrates and establishing their role in antigenicity will be essential to design strategies to produce them in large and reproducible amounts for the development of improved immunoassays.

Purification and analysis of the neutral glycan moiety of glycosyl phosphatidylinositol from porcine thyroid cells

Metabolic labelling of inositolphosphate glycan with radioactive precursors is not sufficient to characterize and assess the involvement of the glycosyl phosphatidylinositol/inositolphosphate glycan (GPI/IPG) system in porcine thyroid cell signal transduction machinery. A protocol is described for the isolation and purification of free GPI using differential polarity of lipids and sequential thin layer chromatography. The purification until homogeneity of GPI constitutes a required step for gas chromatographic analysis. Next, successive chemical treatments allowed us to remove the neutral glycan moiety of thyroidal GPI, and its composition was obtained by gas chromatography. The proposed structure is consistent with data available for GPI anchor, but differs from compositional analysis data reported for insulin-sensitive GPI. Our results support the existence in porcine thyroid cells of the GPI/IPG system, which can take part in TSH-dependent signal transduction processes.

Phosphorylation of a major GPI-anchored surface protein of Trypanosoma brucei during transport to the plasma membrane

The surface coat of procyclic forms of Trypanosoma brucei consists of related, internally repetitive glycoproteins known as EP and GPEET procyclins. Previously we showed that the extracellular domain of GPEET is phosphorylated. We now show that phosphorylation of this glycosylphosphatidylinositol-anchored surface protein can be induced in vitro using a procyclic membrane extract. Using antibodies that recognize either the phosphorylated or unphosphorylated form of GPEET, we analyzed their expression during differentiation of bloodstream forms to procyclic forms. Unphosphorylated GPEET, together with EP, was detected in cell lysates 2–4 hours after initiating differentiation whereas phosphorylated GPEET only appeared after 24 hours. Surface expression of EP and both forms of GPEET occurred after 24–48 hours and correlated with the detection of phosphorylated GPEET on immuno-blots. Electron micrographs showed that unphosphorylated GPEET was predominantly in the flagellar pocket whereas the phosphorylated form was distributed over the cell surface. In contrast, expression of a membrane-bound human placental alkaline phosphatase in procyclic forms caused the accumulation of dephosphorylated GPEET on the cell surface, while the phosphorylated form was restricted to the flagellar pocket. A GPEET-Fc fusion protein, which was retained intracellularly, was not phosphorylated. We propose that unphosphorylated GPEET procyclin is transported to a location close to or at the cell surface, most probably the flagellar pocket, where it becomes phosphorylated. To the best of our knowledge, this study represents the first localization of phosphorylated and unphosphorylated forms of a GPI-anchored protein within a cell.

Segregation of glycosylphosphatidylinositol biosynthetic reactions in a subcompartment of the endoplasmic reticulum

Glycosylphosphatidylinositols (GPIs) are synthesized in the endoplasmic reticulum (ER) via the sequential addition of monosaccharides, fatty acid, and phosphoethanolamine(s) to phosphatidylinositol (PI). While attempting to establish a mammalian cell-free system for GPI biosynthesis, we found that the assembly of mannosylated GPI species was impaired when purified ER preparations were substituted for unfractionated cell lysates as the enzyme source. To explore this problem we analyzed the distribution of the various GPI biosynthetic reactions in subcellular fractions prepared from homogenates of mammalian cells. The results indicate the following: (i) the initial reaction of GPI assembly, i.e. the transfer of GlcNAc to PI to form GlcNAc-PI, is uniformly distributed in the ER; (ii) the second step of the pathway, i.e. de-N-acetylation of GlcNAc-PI to yield GlcN-PI, is largely confined to a subcompartment of the ER that appears to be associated with mitochondria; (iii) the mitochondria-associated ER subcompartment is enriched in enzymatic activities involved in the conversion of GlcN-PI to H5 (a singly mannosylated GPI structure containing one phosphoethanolamine side chain; and (iv) the mitochondria-associated ER subcompartment, unlike bulk ER, is capable of the de novo synthesis of H5 from UDP-GlcNAc and PI. The confinement of these GPI biosynthetic reactions to a domain of the ER provides another example of the compositional and functional heterogeneity of the ER. The implications of these findings for GPI assembly are discussed.

Surface proteins of gram-positive bacteria and mechanisms of their targeting to the cell wall envelope

The cell wall envelope of gram-positive bacteria is a macromolecular, exoskeletal organelle that is assembled and turned over at designated sites. The cell wall also functions as a surface organelle that allows gram-positive pathogens to interact with their environment, in particular the tissues of the infected host. All of these functions require that surface proteins and enzymes be properly targeted to the cell wall envelope. Two basic mechanisms, cell wall sorting and targeting, have been identified. Cell well sorting is the covalent attachment of surface proteins to the peptidoglycan via a C-terminal sorting signal that contains a consensus LPXTG sequence. More than 100 proteins that possess cell wall-sorting signals, including the M proteins of Streptococcus pyogenes, protein A of Staphylococcus aureus, and several internalins of Listeria monocytogenes, have been identified. Cell wall targeting involves the noncovalent attachment of proteins to the cell surface via specialized binding domains. Several of these wall-binding domains appear to interact with secondary wall polymers that are associated with the peptidoglycan, for example teichoic acids and polysaccharides. Proteins that are targeted to the cell surface include muralytic enzymes such as autolysins, lysostaphin, and phage lytic enzymes. Other examples for targeted proteins are the surface S-layer proteins of bacilli and clostridia, as well as virulence factors required for the pathogenesis of L. monocytogenes (internalin B) and Streptococcus pneumoniae (PspA) infections. In this review we describe the mechanisms for both sorting and targeting of proteins to the envelope of gram-positive bacteria and review the functions of known surface proteins.

Expression of intracellular and GPI-anchored forms of GPI-specific phospholipase D in COS-1 cells

Glycosylphosphatidylinositol (GPI)-specific phospholipase D (GPI-PLD) is a secretory protein present in high amounts in mammalian body fluids. Its cDNA has been isolated and encodes a signal peptide of 23 amino acids and the mature protein of 816 amino acids. We generated cDNAs encoding a signal peptide-deficient and a GPI-anchored form of GPI-PLD and transiently transfected these constructs into COS-1 cells. The signal peptide-deficient form of GPI-PLD was expressed as a 90-kDa protein that was catalytically active and was localized intracellularly. Cells transfected with cDNA encoding the GPI-anchored form of GPI-PLD expressed a catalytically active enzyme of 100 kDa that could be labelled with [3H]ethanolamine demonstrating its modification by a GPI structure. Expression of the GPI-anchored form of GPI-PLD resulted in the release of endogenous GPI-anchored alkaline phosphatase from COS-1 cells, whereas expression of the intracellular form of GPI-PLD had no effect on membrane attachment of endogenous alkaline phosphatase. Similarly, in cells cotransfected with GPI-anchored placental alkaline phosphatase (PLAP) and the GPI-anchored form of GPI-PLD, PLAP was released into the cell culture supernatant while expression of the signal peptide-deficient form of GPI-PLD did not affect the amount of cell-associated PLAP.

Phosphatidylinositol hydrolysis by Trypanosoma brucei glycosylphosphatidylinositol phospholipase C

Detergent-solubilized glycosylphosphatidylinositol (GPI)-anchored structures can be cleaved by C-type phospholipases isolated from peanuts and bloodstream cells of the African trypanosome, Trypanosoma brucei. The two enzymes differ in their reported ability to hydrolyze phosphatidylinositol (PI); while the peanut enzyme readily hydrolyzes PI in vitro, the T. brucei enzyme was reported to be virtually inactive against PI and consequently named GPI-specific phospholipase C (GPI-PLC). In this paper, we describe experiments in which we reinvestigated the substrate specificity of T. brucei GPI-PLC by incubating the purified enzyme with Triton X-100/PI-mixed micelles and by studying PI hydrolysis. We found that PI hydrolysis occurred in a detergent-dependent fashion over the range of concentrations tested (5 microM to 1 mM PI). At 5 microM PI, hydrolysis was maximal at 0.005% Triton X-100, whereas at 1 mM PI, maximal hydrolysis required 0.05% Triton X-100. Hydrolysis of both PI and GPI was strongly affected by the presence of phospholipids. Endogenous PI was hydrolyzed during osmotic and detergent lysis of trypanosomes under conditions used to obtain quantitative hydrolysis of the GPI-anchored trypanosome variant surface glycoprotein. PI hydrolysis in the lysates was inhibited by sodium p-chloromercuriphenylsulfonate but unaffected by EGTA, consistent with the proposal that hydrolysis is due to GPI-PLC. These results suggest that the function of T. brucei GPI-PLC may be to regulate PI as well as (or instead of) GPI levels.

Glucose-induced sequential processing of a glycosyl-phosphatidylinositol-anchored ectoprotein in Saccharomyces cerevisiae

Transfer of spheroplasts from the yeast Saccharomyces cerevisiae to glucose leads to the activation of an endogenous (glycosyl)-phosphatidylinositol-specific phospholipase C ([G]PI-PLC), which cleaves the anchor of at least one glycosyl-phosphatidylinositol (GPI)-anchored protein, the cyclic AMP (cAMP)-binding ectoprotein Gce1p (G. Müller and W. Bandlow, J. Cell Biol. 122:325-336, 1993). Analyses of the turnover of two constituents of the anchor, myo-inositol and ethanolamine, relative to the protein label as well as separation of the two differently processed versions of Gce1p by isoelectric focusing in spheroplasts demonstrate the glucose-induced conversion of amphiphilic Gce1p first into a lipolytically cleaved hydrophilic intermediate, which is then processed into another hydrophilic version lacking both myo-inositol and ethanolamine. When incubated with unlabeled spheroplasts, the lipolytically cleaved intermediate prepared in vitro is converted into the version lacking all anchor constituents, whereby the anchor glycan is apparently removed as a whole. The secondary cleavage ensues independently of the carbon source, attributing the key role in glucose-induced anchor processing to the endogenous (G)PI-PLC. The secondary processing of the lipolytically cleaved intermediate of Gce1p at the plasma membrane is correlated with the emergence of a covalently linked high-molecular-weight form of a cAMP-binding protein at the cell wall. This protein lacks anchor components, and its protein moiety appears to be identical with double-processed Gce1p detectable at the plasma membrane in spheroplasts. The data suggest that glucose-induced double processing of GPI anchors represents part of a mechanism of regulated cell wall expression of proteins in yeast cells.