Phosphatidylinositol-glycan (PI-G)-anchored membrane proteins: requirement of ATP and GTP for translation-independent COOH-terminal processing

Vascular Endothelial Cells Produce Coagulation Factors That Control Their Growth via Joint Protease-Activated Receptor and C5a Receptor 1 (CD88) Signaling

As per the classical view of the coagulation system, it functions solely in plasma to maintain hemostasis. An experimental approach modeling vascular reconstitution was used to show that vascular endothelial cells (ECs) endogenously synthesize coagulation factors during angiogenesis. Intracellular thrombin generated from this synthesis promotes the mitotic function of vascular endothelial cell growth factor A (VEGF-A). The thrombin concurrently cleaves C5a from EC-synthesized complement component C5 and unmasks the tethered ligand for EC-expressed protease-activated receptor 4 (PAR4). The two ligands jointly trigger EC C5a receptor-1 (C5ar1) and PAR4 signaling, which together promote VEGF receptor 2 growth signaling. C5ar1 is functionally associated with PAR4, enabling C5a or thrombin to elicit Gαi and/or Gαq signaling. EC coagulation factor and EC complement component synthesis concurrently down-regulate with contact inhibition. The connection of these processes with VEGF receptor 2 signaling provides new insights into mechanisms underlying angiogenesis. Knowledge of endogenous coagulation factor/complement component synthesis and joint PAR4/C5ar1 signaling could be applied to other cell types.

Further characterization of the cathepsin L-associated protein and its gene in two species of the brine shrimp, Artemia

The major cysteine protease in embryos and larvae of the brine shrimp Artemia franciscana is a heterodimer composed of a cathepsin L-like polypeptide of 28.5 kDa and a 31.5 kDa polypeptide called the cathepsin L-associated protein or CLAP. In a previous study, CLAP was shown to be a cell adhesion protein containing two Fas I domains and two GTP/ATP binding sites known as Walker A and B motifs. Here, we have characterized CLAP and its genes to better understand the role of this protein in Artemia development. The polymerase chain reaction was used to investigate the structure of the CLAP gene in two species of Artemia, the New World bisexual diploid A. franciscana and the Old World parthenogenetic tetraploid Artemia parthenogenetica. The protein coding region of the CLAP gene from each species was 99.5% identical for a protein of 332 amino acids, while the 3' non-coding region, representing nearly 45% of the gene, was only 86% identical between the two related species. However, while the CLAP gene is intronless in A. franciscana, in A. parthenogenetica the gene contained a mini-intron of 30 base pairs in the 3' non-coding region. The sequences representing the CLAP gene in A. franciscana and A. parthenogenetica have been entered into the NCBI database as AY757920 and DQ100385, respectively. Northern blot analysis showed that while the cathepsin L gene is expressed constitutively in Artemia franciscana embryos and young larvae, the CLAP gene is not expressed in late embryos and young larvae. In contrast, Western blots indicated that CLAP is present in developing embryos and young larvae, at least to the first larval molt, supporting results obtained previously showing CLAP's resistance to degradation by its dimeric partner, cathepsin L. At the protein level we showed that the GTP/ATP binding sites in CLAP are functional with rate constants of 0.024 and 0.022 for GTP and ATP hydrolase activity, respectively. GTP but not ATP also had a slight stimulatory effect on cathepsin L activity of the heterodimeric protease containing CLAP. Our results support the hypothesis that CLAP plays an important role in targeting and expression regulation of cathepsin L activity during early development of Artemia.

Characterization of a cathepsin L-associated protein in Artemia and its relationship to the FAS-I family of cell adhesion proteins

We reported previously that the major cysteine protease in embryos and larvae of the brine shrimp, Artemia franciscana, is a heterodimeric protein consisting of a catalytic subunit (28.5 kDa) with a high degree of homology with cathepsin L, and a noncatalytic subunit (31.5 kDa) of unknown function. In the study reported here the noncatalytic subunit, or cathepsin L-associated protein (CLAP), was separated from cathepsin L by chromatography on Mono S and found to contain multiple isoforms with pIs ranging from 5.9 to 6.1. Heterodimeric and monomeric cathepsin L showed similar activity between pH 5 and 6.5, while the heterodimer was about twice as active as monomeric cathepsin L below pH 5. The heterodimer was more stable than the monomer between pH 6 and 7.4 and at 30-50 degrees C. Artemia CLAP and cathepsin L are present in nearly equimolar amounts at all stages in the life cycle and most abundant in encysted eggs and embyros. Moreover, CLAP, either free or as a complex with cathepsin L, was resistant to hydrolysis by cathepsin L. Two clones coding for CLAP were isolated from an Artemia embryo cDNA library and sequenced. Both clones have nearly identical open reading frames, but show differences at the 5'- and 3'-termini. Each cDNA clone has an extensive 3'-untranslated region containing 70-72% A+T. The deduced amino acid sequence of CLAP cDNA revealed two domains which were very similar to domains in fasciclin I and other cell adhesion proteins. The nucleotide sequences of clones 1 and 2 have been entered into the NCBI database (AY307377 and AY462276). This study supports the view that the noncatalytic subunit of the heterodimeric cysteine protease in Artemia stabilizes cathepsin L at various pH and temperatures normally inconsistent with cathepsin L from other organisms, and that CLAP serves as a docking mechanism for cathepsin L at nonlysosomal sites in Artemia embryos.

Proprotein interaction with the GPI transamidase

For characterizing how the glycosylphosphatidylinositol (GPI) transamidase complex functions, we exploited a two-step miniPLAP (placental alkaline phosphatase) in vitro translation system. With this system, rough microsomal membranes (RM) containing either [(35)S]-labeled Gaa1p or epitope-tagged Gpi8p, alternative components of the enzymatic complex, were first prepared. In a second translation, unmodified or mutant miniPLAP mRNA was used such that [(35)S]-labeled native or variant miniPLAP nascent protein was introduced. Following this, the RM were solubilized and anti-PLAP or anti-epitope immunoprecipitates were analyzed. With transamidase competent HeLa cell RM, anti-PLAP or anti-epitope antibody coprecipitated both Gaa1p and Gpi8p consistent with the assembly of the proprotein into a Gaa1p:Gpi8p-containing complex. When RM from K562 mutant K cells which lack Gpi8p were used, anti-PLAP antibody coprecipitated Gaa1p. The proprotein coprecipitation of Gaa1p increased with a nonpermissive GPI anchor addition (omega) site. In contrast, if a miniPLAP mutant devoid of its C-terminal signal was used, no coprecipitation occurred. During the transamidation reaction, a transient high Mr band forms. To definitively characterize this product, RM from K cells transfected with FLAG-tagged GPI8 were employed. Western blots of anti-FLAG bead isolates of solubilized RM from the cells showed that the high Mr band corresponded to Gpi8p covalently bound to miniPLAP. Loss of the band following hydrazinolysis demonstrated that the two components were associated in a thioester linkage. The data indicate that recognition of the proprotein involves Gaa1p, that the interaction with the complex does not depend on a permissive omega site, and that Gpi8p forms a thioester intermediate with the proprotein. The method could be useful for rapid analysis of nascent protein interactions with transamidase components, and possibly for helping to prepare a functional in vitro transamidase system.

Glycosylphosphatidylinositol (GPI)-anchored proteins

From the numerous studies developed at the last quarter of the 20th century, glycosylphosphatidylinositol (GPI) anchor has been established as a unique mode of protein binding to the plasma membrane. The core structure of this anchor consists of ethanolamine phosphate, trimannoside, glucosamine and inositol phospholipid in this order. The anchor is combined with the carboxyl-terminal of protein by the ethanolamine head. GPI-anchored proteins are ubiquitously distributed among Eucarya from vertebrates to protozoa, and also shown to be present in some of Archaeobacteria such as Sulfolobus. There is no evidence for the presence of GPI-anchored protein in Eubacteria. In the eucaryotic cells, both biosynthesis of GPI precursors and posttranslational protein modification with GPI proceed in the endoplasmic reticulum. On GPI modification, the carboxyl-terminal signal peptide is split off from the protein and the resulting new carboxyl-terminal is then combined with the amino group of ethanolamine residue in the GPI precursors. The whole process of cleavage and GPI attachment is catalyzed by GPI-transamidase complex. Many genes concerning GPI biosynthesis and protein modification have been cloned and sequenced. The carboxyl-terminal signal peptide containing hydrophobic tail is characterized by genetic analysis and shown to be essential for GPI modification of protein. Recent computational analysis further clarified the detailed requirement of the carboxyl-terminal regions for GPI-anchoring. GPI-anchored proteins are assumed to be transported from Golgi to the plasma membrane in the form of "lipid rafts", and expressed as the clusters in the cell surface.

Comparative efficiencies of C-terminal signals of native glycophosphatidylinositol (GPI)-anchored proproteins in conferring GPI-anchoring

Every protein fated to receive the glycophosphatidylinositol (GPI) anchor post-translational modification has a C-terminal GPI-anchor attachment signal sequence. This signal peptide varies with respect to length, content, and hydrophobicity. With the exception of predictions based on an upstream amino acid triplet termed omega-->omega + 2 which designates the site of GPI uptake, there is no information on how the efficiencies of different native signal sequences compare in the transamidation reaction that catalyzes the substitution of the GPI anchor for the C-terminal peptide. In this study we utilized the placental alkaline phosphatase (PLAP) minigene, miniPLAP, and replaced its native 3' end-sequence encoding omega-2 to the C-terminus with the corresponding C-terminal sequences of nine other human GPI-anchored proteins. The resulting chimeras then were fed into an in vitro processing microsomal system where the cleavages leading to mature product from the nascent preproprotein could be followed by resolution on an SDS-PAGE system after immunoprecipitation. The results showed that the native signal of each protein differed markedly with respect to transamidation efficiency, with the signals of three proteins out-performing the others in GPI-anchor addition and those of two proteins being poorer substrates for the GPI transamidase. The data additionally indicated that the hierarchical order of efficiency of transamidation did not depend solely on the combination of permissible residues at omega-->omega + 2.

Early events in glycosylphosphatidylinositol anchor addition. substrate proteins associate with the transamidase subunit gpi8p

The addition of glycosylphosphatidylinositol (GPI) anchors to proteins occurs by a transamidase-catalyzed reaction mechanism soon after completion of polypeptide synthesis and translocation. We show that placental alkaline phosphatase becomes efficiently GPI-anchored when translated in the presence of semipermeabilized K562 cells but is not GPI-anchored in cell lines defective in the transamidase subunit hGpi8p. By studying the synthesis of placental alkaline phosphatase, we demonstrate that folding of the protein is not influenced by the addition of a GPI anchor and conversely that GPI anchor addition does not require protein folding. These results demonstrate that folding of the ectodomain and GPI addition are two distinct processes and can be mutually exclusive. When GPI addition is prevented, either by synthesis of the protein in the presence of cell lines defective in GPI addition or by mutation of the GPI carboxyl-terminal signal sequence cleavage site, the substrate forms a prolonged association with the transamidase subunit hGpi8p. The ability of the transamidase to recognize and associate with GPI anchor signal sequences provides an explanation for the retention of GPI-anchored protein within the ER in the absence of GPI anchor addition.

A cell-free assay for glycosylphosphatidylinositol anchoring in African trypanosomes. Demonstration of a transamidation reaction mechanism

We established an in vitro assay for the addition of glycosyl-phosphatidylinositol (GPI) anchors to proteins using procyclic trypanosomes engineered to express GPI-anchored variant surface glycoprotein (VSG). The assay is based on the premise that small nucleophiles, such as hydrazine, can substitute for the GPI moiety and effect displacement of the membrane anchor of a GPI-anchored protein or pro-protein causing release of the protein into the aqueous medium. Cell membranes containing pulse-radiolabeled VSG were incubated with hydrazine, and the VSG released from the membranes was measured by carbonate extraction, immunoprecipitation, and SDS-polyacrylamide gel electrophoresis/fluorography. Release of VSG was time- and temperature-dependent, was stimulated by hydrazine, and occurred only for VSG molecules situated in early compartments of the secretory pathway. No nucleophile-induced VSG release was seen in membranes prepared from cells expressing a VSG variant with a conventional transmembrane anchor (i.e. a nonfunctional GPI signal sequence). Pro-VSG was shown to be a substrate in the reaction by assaying membranes prepared from cells treated with mannosamine, a GPI biosynthesis inhibitor. When a biotinylated derivative of hydrazine was used instead of hydrazine, the released VSG could be precipitated with streptavidin-agarose, indicating that the biotin moiety was covalently incorporated into the protein. Hydrazine was shown to block the C terminus of the released VSG hydrazide because the released material, unlike a truncated form of VSG lacking a GPI signal sequence, was not susceptible to proteolysis by carboxypeptidases. These results firmly establish that the released material in our assay is VSG hydrazide and strengthen the proof that GPI anchoring proceeds via a transamidation reaction mechanism. The reaction could be inhibited with sulfhydryl alkylating reagents, suggesting that the transamidase enzyme contains a functionally important sulfhydryl residue.

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.

The affected gene underlying the class K glycosylphosphatidylinositol (GPI) surface protein defect codes for the GPI transamidase

The final step in glycosylphosphatidylinositol (GPI) anchoring of cell surface proteins consists of a transamidation reaction in which preassembled GPI donors are substituted for C-terminal signal sequences in nascent polypeptides. In previous studies we described a human K562 cell mutant, termed class K, that accumulates fully assembled GPI units but is unable to transfer them to N-terminally processed proproteins. In further work we showed that, unlike wild-type microsomes, microsomes from these cells are unable to support C-terminal interaction of proproteins with the small nucleophiles hydrazine or hydroxylamine, and that the cells thus are defective in transamidation. In this study, using a modified recombinant vaccinia transient transfection system in conjunction with a composite cDNA prepared by 5' extension of an existing GenBank sequence, we found that the genetic element affected in these cells corresponds to the human homolog of yGPI8, a gene affected in a yeast mutant strain exhibiting similar accumulation of GPI donors without transfer. hGPI8 gives rise to mRNAs of 1.6 and 1.9 kb, both encoding a protein of 395 amino acids that varies in cells with their ability to couple GPIs to proteins. The gene spans approximately 25 kb of DNA on chromosome 1. Reconstitution of class K cells with hGPI8 abolishes their accumulation of GPI precursors and restores C-terminal processing of GPI-anchored proteins. Also, hGPI8 restores the ability of microsomes from the mutant cells to yield an active carbonyl in the presence of a proprotein which is considered to be an intermediate in catalysis by a transamidase.

COOH-terminal processing of nascent polypeptides by the glycosylphosphatidylinositol transamidase in the presence of hydrazine is governed by the same parameters as glycosylphosphatidylinositol addition

Proteins anchored to the cell membrane via a glycosylphosphatidylinositol (GPI) moiety are found in all eukaryotes. After NH2-terminal peptide cleavage of the nascent protein by the signal peptidase, a second COOH-terminal signal peptide is cleaved with the concomitant addition of the GPI unit. The proposed mechanism of the GPI transfer is a transamidation reaction that involves the formation of an activated carbonyl intermediate (enzyme-substrate complex) with the ethanolamine moiety of the preassembled GPI unit serving as a nucleophile. Other nucleophilic acceptors like hydrazine (HDZ) and hydroxylamine have been shown to be possible alternate substrates for GPI. Since GPI has yet to be purified, the use of readily available nucleophilic substitutes such as HDZ and hydroxylamine is a viable alternative to study COOH-terminal processing by the putative transamidase. As a first step in developing a soluble system to study this process, we have examined the amino acid requirements at the COOH terminus for the transamidation reaction using HDZ as the nucleophilic acceptor instead of GPI. The hydrazide-forming reaction shows identical amino acid requirement profiles to that of GPI anchor addition. Additionally, we have studied other parameters relating to the kinetics of the transamidation reaction in the context of rough microsomal membranes. The findings with HDZ provide further evidence for the transamidase nature of the enzyme and also provide a starting point for development of a soluble assay.

An active carbonyl formed during glycosylphosphatidylinositol addition to a protein is evidence of catalysis by a transamidase

Glycosylphosphatidylinositol (GPI) substitution is now recognized to be a ubiquitous method of anchoring a protein to membranes in eukaryotes. The structure of GPI and its biosynthetic pathways are known and the signals in a nascent protein for GPI addition have been elucidated. The enzyme(s) responsible for GPI addition with release of a COOH-terminal signal peptide has been considered to be a transamidase but has yet to be isolated, and evidence that it is a transamidase is indirect. The experiments reported here show that hydrazine and hydroxylamine, in the presence of rough microsomal membranes, catalyze the conversion of the pro form of the engineered protein miniplacental alkaline phosphatase (prominiPLAP) to mature forms from which the COOH-terminal signal peptide has been cleaved, apparently at the same site but without the addition of GPI. The products, presumable the hydrazide or hydroxamate of miniPLAP, have yet to be characterized definitively. However, our demonstration of enzyme-catalyzed cleavage of the signal peptide in the presence of the small nucleophiles, even in the absence of an energy source, is evidence of an activated carbonyl intermediate which is the hallmark of a transamidase.

The GPI anchor of cell-surface proteins is synthesized on the cytoplasmic face of the endoplasmic reticulum

Glycosylphosphatidylinositol (GPI) membrane protein anchors are synthesized from sugar nucleotides and phospholipids in the ER and transferred to newly synthesized proteins destined for the cell surface. The topology of GPI synthesis in the ER was investigated using sealed trypanosome microsomes and the membrane-impermeant probes phosphatidylinositol-specific phospholipase C, Con A, and proteinase K. All the GPI biosynthetic intermediates examined were found to be located on the external face of the microsomal vesicles suggesting that the principal steps of GPI assembly occur in the cytoplasmic leaflet of the ER. Protease protection experiments showed that newly GPI-modified trypanosome variant surface glycoprotein was primarily oriented towards the ER lumen, consistent with eventual expression at the cell surface. The unusual topographical arrangement of the GPI assembly pathway suggests that a biosynthetic intermediate, possibly the phosphoethanolamine-containing anchor precursor, must be translocated across the ER membrane bilayer in the process of constructing a GPI anchor.

The structure, biosynthesis and function of glycosylated phosphatidylinositols in the parasitic protozoa and higher eukaryotes

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Evidence that the putative COOH-terminal signal transamidase involved in glycosylphosphatidylinositol protein synthesis is present in the endoplasmic reticulum

Nascent proteins destined to be processed to a glycosylphosphatidylinositol (GPI)-anchored membrane form contain NH2-terminal and COOH-terminal signal peptides. The first directs a nascent protein into the endoplasmic reticulum; the second peptide targets the protein to a putative COOH-terminal signal transamidase where cleavage of the peptide and addition of the GPI anchor occur. We recently showed that ATP hydrolysis is required for maturation of GPI proteins at a stage prior to transamidation. Here we show that one of the ATP-requiring proteins involved in processing of GPI-anchored proteins in the endoplasmic reticulum is the immunoglobulin heavy chain binding protein (BiP; GRP 78). This and related findings indicate that GPI transamidase is localized in the endoplasmic reticulum.