Biosynthesis of phosphatidylinositol-glycan (PI-G)-anchored membrane proteins in cell-free systems: cleavage of the nascent protein and addition of the PI-G moiety depend on the size of the COOH-terminal signal peptide

Structure and Function of the Glycosylphosphatidylinositol Transamidase, a Transmembrane Complex Catalyzing GPI Anchoring of Proteins

Glycosylphosphatidylinositol (GPI) anchoring of proteins is a ubiquitous posttranslational modification in eukaryotic cells. GPI-anchored proteins (GPI-APs) play critical roles in enzymatic, signaling, regulatory, and adhesion processes. Over 20 enzymes are involved in GPI synthesis, attachment to client proteins, and remodeling after attachment. The GPI transamidase (GPI-T), a large complex located in the endoplasmic reticulum membrane, catalyzes the attachment step by replacing a C-terminal signal peptide of proproteins with GPI. In the last three decades, extensive research has been conducted on the mechanism of the transamidation reaction, the components of the GPI-T complex, the role of each subunit, and the substrate specificity. Two recent studies have reported the three-dimensional architecture of GPI-T, which represent the first structures of the pathway. The structures provide detailed mechanisms for assembly that rationalizes previous biochemical results and subunit-dependent stability data. While the structural data confirm the catalytic role of PIGK, which likely uses a caspase-like mechanism to cleave the proproteins, they suggest that unlike previously proposed, GPAA1 is not a catalytic subunit. The structures also reveal a shared cavity for GPI binding. Somewhat unexpectedly, PIGT, a single-pass membrane protein, plays a crucial role in GPI recognition. Consistent with the assembly mechanisms and the active site architecture, most of the disease mutations occur near the active site or the subunit interfaces. Finally, the catalytic dyad is located ~22 Å away from the membrane interface of the GPI-binding site, and this architecture may confer substrate specificity through topological matching between the substrates and the elongated active site. The research conducted thus far sheds light on the intricate processes involved in GPI anchoring and paves the way for further mechanistic studies of GPI-T.

Structures of liganded glycosylphosphatidylinositol transamidase illuminate GPI-AP biogenesis

Many eukaryotic receptors and enzymes rely on glycosylphosphatidylinositol (GPI) anchors for membrane localization and function. The transmembrane complex GPI-T recognizes diverse proproteins at a signal peptide region that lacks consensus sequence and replaces it with GPI via a transamidation reaction. How GPI-T maintains broad specificity while preventing unintentional cleavage is unclear. Here, substrates- and products-bound human GPI-T structures identify subsite features that enable broad proprotein specificity, inform catalytic mechanism, and reveal a multilevel safeguard mechanism against its promiscuity. In the absence of proproteins, the catalytic site is invaded by a locally stabilized loop. Activation requires energetically unfavorable rearrangements that transform the autoinhibitory loop into crucial catalytic cleft elements. Enzyme-proprotein binding in the transmembrane and luminal domains respectively powers the conformational rearrangement and induces a competent cleft. GPI-T thus integrates various weak specificity regions to form strong selectivity and prevent accidental activation. These findings provide important mechanistic insights into GPI-anchored protein biogenesis.

Recent developments in the molecular, biochemical and functional characterization of GPI8 and the GPI-anchoring mechanism [review]

Glycoconjugates are utilized by eukaryotic organisms ranging from yeast to humans for the cell surface expression of a wide variety of proteins and lipids. These glycoconjugates are expressed as enzymes or receptors and serve a diversity of functions, including cell signaling and cell survival. In parasitic protozoans, glycoconjugates play roles in infectivity, survival, virulence and immune evasion. Among the alternate glycoconjugate structures that have been identified, glycosylphosphatidylinositols (GPIs) represent a universal structure for the anchorage of proteins, lipids, and phosphosaccharides to cellular membranes. Biosynthesis of the GPI is a multi-step process that culminates in the attachment of the assembled GPI to a precursor protein. This final step in the transfer of the GPI to a protein is catalyzed by GPI8 of the putative transamidase complex (TAM). GPI8 functions dually to perform the proteolytic cleavage of the C-terminal signal sequence of the precursor protein, followed by the formation of an amide bond between the protein and the ethanolamine phosphate of the GPI. This review summarizes the current aggregate of biochemical, gene-disruption and active site mutagenesis studies, which provide evidence that GPI8 is responsible for the protein-GPI anchoring reaction. We describe recently published studies that have identified other potential components of the TAM complex and that have elucidated their likely role in protein-GPI attachment. Further, we discuss the biochemical, molecular and functional differences between protozoan and mammalian GPI8 and the protein-GPI anchoring machinery. Finally, we will present the implications of these studies for the development of anti-parasite drug therapies.

Deletion of the GPI pre-anchor sequence in human p97—a general approach for generating the soluble form of GPI-linked proteins

Melanotransferrin, also named p97, belongs to the transferrin-like group of iron-binding proteins. Unlike the other members of this family, p97 exists in two forms-one soluble form and one attached to the cell membrane by a glycosylphosphatidylinositol (GPI) anchor. The GPI-linked form plays a role in the uptake of iron, while the soluble form of p97 has the unique ability of traversing the blood-brain barrier and may be utilized to deliver drug conjugates into the brain. To investigate these possibilities, a recombinant soluble form of p97 from the GPI-linked p97 protein is required. The approach involved sequential deletions of the p97 GPI pre-anchor sequence (PAS) up to the putative site of cleavage/attachment, releasing p97 from attachment to the GPI-anchor and rendering it soluble. Transfection of the p97 deletion constructs into both the CHO and BHK TK(-) cells was performed with the aim of optimizing the production of p97 by utilizing the cell characteristics unique to each cell line. Altering the GPI PAS resulted in the generation of a recombinant soluble form that was secreted at significantly higher rates than from the full-length expressing cell lines. Increases were from 22 x 10(-9) to 241 x 10(-9)microg/cell/h for expression in the CHO cell system and from 220 x 10(-9) to 4970 x 10(-9)microg/cell/h for the BHK system. Furthermore, there appeared to be differences in the secretion rates between the various deletions suggesting the need for closer examination of the C-terminus in achieving maximum production of the altered proteins. The results of this study are likely applicable for expressing soluble forms of other GPI-linked proteins.

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.

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.

Mutational analysis of the variant surface glycoprotein GPI-anchor signal sequence inTrypanosoma brucei

The variant surface glycoproteins (VSG) of Trypanosoma brucei are anchored to the cell surface via a glycosylphosphatidylinositol (GPI) anchor. All GPI-anchored proteins are synthesized with a C-terminal signal sequence,which is replaced by a GPI-anchor in a rapid post-translational transamidation reaction. VSG GPI signal sequences are extraordinarily conserved. They contain either 23 or 17 amino acids, a difference that distinguishes the two major VSG classes, and consist of a spacer sequence followed by a more hydrophobic region. The ω amino acid, to which GPI is transferred, is either Ser,Asp or Asn, the ω+2 amino acid is always Ser, and the ω+7 amino acid is almost always Lys. In order to determine whether this high conservation is necessary for GPI anchoring, we introduced several mutations into the signal peptide. Surprisingly, changing the most conserved amino acids, at positions ω+1, ω+2 and ω+7, had no detectable effect on the efficiency of GPI-anchoring or on protein abundance. Several more extensive changes also had no discernable impact on GPI-anchoring. Deleting the entire 23 amino-acid signal sequence or the 15 amino-acid hydrophobic region generated proteins that were not anchored. Instead of being secreted, these truncated proteins accumulated in the endoplasmic reticulum prior to lysosomal degradation. Replacing the GPI signal sequence with a proven cell-surface membrane-spanning domain reduced expression by about 99%and resulted not in cell surface expression but in accumulation close to the flagellar pocket and in non-lysosomal compartments. These results indicate that the high conservation of the VSG GPI signal sequence is not necessary for efficient expression and GPI attachment. Instead, the GPI anchor is essential for surface expression of VSG. However, because the VSG is a major virulence factor, it is possible that small changes in the efficiency of GPI anchoring,undetectable in our experiments, might have influenced the evolution of VSG GPI signal sequences.

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.

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.

Characterization of the ETSA-21 antigen, a glycosylphosphatidyl-inositol anchor glycoprotein identified in breast cancer cells using monoclonal antibody B21

Mab B21 is a monoclonal antibody (Mab) that recognizes an epithelial tumor surface antigen (ETSA-B21) from diverse human tumor cell lines including breast, ovary, uterus, and their cognate carcinoma tissues. A lower reactivity has been observed in normal breast tissue and benign hyperplesia. In this study, the characteristics of the ETSA-B21 antigen have been examined in greater detail in the MCF-7, SK-BR-3, and MDA-MB-453 breast cancer cell lines. Treatment with phosphatidylinositol-phospholipase C, but no neuraminidase were found to partially remove the ETSA-B21 signal from the cell surface as revealed by immunofluorescence microscopy. Inhibition of the N-glycosylation pathway by tunicamycin resulted in a decreased ETSA-B21 signal on the cell membrane. In addition, the antigen-antibody complex was internalized in breast cancer cells as demonstrated by an acidic was internalization assay evaluated using immunofluorescence. In conclusion, this study suggests that ETSA-B21 is a GPI anchor N-glycosylated protein promoting specific antibody internalization in breast cancer cells.

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.

Yeast Gaa1p is required for attachment of a completed GPI anchor onto proteins

Anchoring of proteins to membranes by glycosylphosphatidylinositols (GPIs) is ubiquitous among all eukaryotes and heavily used by parasitic protozoa. GPI is synthesized and transferred en bloc to form GPI-anchored proteins. The key enzyme in this process is a putative GPI:protein transamidase that would cleave a peptide bond near the COOH terminus of the protein and attach the GPI by an amide linkage. We have identified a gene, GAA1, encoding an essential ER protein required for GPI anchoring. gaal mutant cells synthesize the complete GPI anchor precursor at nonpermissive temperatures, but do not attach it to proteins. Overexpression of GAA1 improves the ability of cells to attach anchors to a GPI-anchored protein with a mutant anchor attachment site. Therefore, Gaa1p is required for a terminal step of GPI anchor attachment and could be part of the putative GPI:protein transamidase.

Cleavage without anchor addition accompanies the processing of a nascent protein to its glycosylphosphatidylinositol-anchored form

Rough microsomal membranes from most mammalian cells, in the presence of a translation system, process nascent proteins with appropriate COOH-terminal signal peptides to their mature glycosylphosphatidylinositol (GPI)-linked forms. The present study, using preprominiplacental alkaline phosphatase as substrate, shows that as much as 10% of the mature product is cleaved correctly but is not linked to GPI. Some of the factors that influence the relative proportions of GPI linked to free mini-placental alkaline phosphatase are the amounts of GPI in the cells and the amino acid substituent at the omega site of the nascent protein. A mechanism for explaining cleavage both with and without GPI addition is presented, which supports a transamidase type of enzyme as the catalyst.

Conversion of secretory proteins into membrane proteins by fusing with a glycosylphosphatidylinositol anchor signal of alkaline phosphatase

Placental alkaline phosphatase (PLAP) is initially synthesized as a precursor (proPLAP) with a C-terminal extension. We constructed a recombinant cDNA which encodes a chimeric protein (alpha GL-PLAP) comprising rat alpha 2u-globulin (alpha GL) and the C-terminal extension of PLAP. Two molecular species (25 kDa and 22 kDa) were expressed in the COS-1 cell transfected with the cDNA for alpha GL-PLAP. Only the 22 kDa form was labelled with both [3H]stearic acid and [3H]ethanolamine. Upon digestion with phosphatidylinositol-specific phospholipase C the 22 kDa form was released into the medium, indicating that this form is anchored on the cell surface via glycosylphosphatidylinositol (GPI). A specific IgG raised against a C-terminal nonapeptide of proPLAP precipitated the 25 kDa form but not the 22 kDa form, suggesting that the 25 kDa form is a precursor retaining the C-terminal propeptide. When a mutant alpha GL-PLAP, in which the aspartic acid residue is replaced with tryptophan at a putative cleavage/attachment site, was expressed in COS-1 cells, the 25 kDa precursor was the only form found inside the cell and retained in the endoplasmic reticulum, as judged by immunofluorescence microscopy. In vitro translation programmed with mRNAs coding for the wild-type and mutant forms of alpha GL-PLAP demonstrated that the C-terminal propeptide was cleaved from the wild-type chimeric protein, but not from the mutant one. This gave rise to the 22 kDa form attached with a GPI anchor, suggesting that GPI is covalently linked to the aspartic acid residue (Asp159) of alpha GL-PLAP. Taken together, these results indicate that the C-terminal propeptide of PLAP functions as a signal to render alpha GL a GPI-linked membrane protein in vitro and in vivo in cultured cells, and that the chimeric protein constructed in this study may be useful for elucidating the mechanism underlying the cleavage of the propeptide and attachment of GPI, which occur in the endoplasmic reticulum.

Biosynthesis of glycosylphosphatidylinositol-anchored human placental alkaline phosphatase: evidence for a phospholipase C-sensitive precursor and its post-attachment conversion into a phospholipase C-resistant form

Previous studies have shown that some cells (e.g. SKG3a) express human placental alkaline phosphatase (AP) in a form which can be released from the membrane by bacterial PtdIns-specific phospholipase C (PI-PLC) while others (e.g. HeLa) are relatively resistant to this enzyme. Chemical and enzymic degradation studies have suggested that the PI-PLC resistance of AP is due to inositol acylation of its glycosylphosphatidylinositol (GPI) anchor. In order to identify the biosynthetic origin of PI-PLC resistance we determined the PI-PLC sensitivity of AP in 35S-labelled cells (10 min pulse; 0-60 min chase) by Triton X-114 phase separation. At the beginning of the chase period, the majority of the AP synthesized was hydrophilic, indicating that it had not acquired a GPI anchor. The concentration of hydrophilic AP species decreased with a t1/2 of 30-60 min but was not processed to an endoglycosidase H-resistant species or secreted into the medium. In both SKG3a and HeLa cells all of the hydrophobic, GPI-anchored AP detectable at the beginning of the chase was PI-PLC sensitive. PI-PLC-resistant species of AP were only observed in HeLa cells and these only appeared after about 30 min. The delayed appearance of PI-PLC resistance was unexpected as previous studies have suggested that candidate GPI-anchor precursors are PI-PLC-resistant as a result of inositol acylation. This work reveals unanticipated complexities in the biosynthesis of AP and its GPI anchor.

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

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Glucose induces lipolytic cleavage of a glycolipidic plasma membrane anchor in yeast

In the yeast Saccharomyces cerevisiae an amphiphilic cAMP-binding protein has been found recently to be anchored to plasma membranes by virtue of a glycolipid structure (Müller and Bandlow, 1991a, 1992). The cAMP-binding parameters of this protein are affected by the lipolytic removal of the glycosylphosphatidylinositol (GPI) membrane anchor by exogenous (G)PI-specific phospholipases C or D (PLC or PLD) (Müller and Bandlow, 1993) suggesting a regulatory role of glycolipidic membrane anchorage. Here we report that transfer of yeast cells from lactate to glucose medium results in the conversion of the amphiphilic form of the cAMP receptor protein into a hydrophilic version accompanied by the rapid loss of fatty acids from the GPI anchor of the [14C]palmitic acid-labeled protein. Analysis of the cleavage site identifies [14C]inositol phosphate as the major product after treatment of the soluble, [14C]inositol-labeled protein with nitrous acid which destroys the glucosamine constituent of the anchor. Together with the observed cross-reactivity of the hydrophilic fragment with antibodies directed against the cross-reacting determinant of soluble trypanosomal variable surface glycoproteins (i.e., myo-inositol-1,2-cyclic phosphate) this demonstrates that, in membrane release, the initial cleavage event is catalyzed by an intrinsic GPI-PLC activated upon transfer of cells to glucose medium. Release from the plasma membrane in soluble form requires, in addition, the presence of high salt or alpha-methyl mannopyranoside, or the removal of the carbohydrate moieties, because otherwise the protein remains associated with the membrane presumably at least in part via its N-glycosidic carbohydrate side chains. The data point to the possibility that cleavage of the anchor could play a role in the transfer of the signal for the nutritional situation to the interior of the cell.

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.

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

Placental alkaline phosphatase (PLAP) belongs to a class of proteins that are anchored to the plasma membrane by a COOH-terminal phosphatidylinositol-glycan (PI-G) moiety. Nascent forms of such proteins undergo NH2- and COOH-terminal processing to yield the mature PI-G-tailed proteins. We previously introduced a shortened engineered form of preproPLAP (preprominiPLAP) that permits monitoring in cell-free preparations its sequential processing to the pro form and then to the mature PI-G-tailed form. Previous studies were carried out by synthesizing the preproprotein cotranslationally in the presence of rough microsomal membranes (RM). Because of the complexity of the cotranslational system it was not possible to determine whether cofactors were required for processing. We have now prepared RM that are preloaded with prominiPLAP but contain little mature PI-G-tailed miniPLAP. Maximal processing requires supplementation with both ATP and GTP. Inhibitors of PI-G biosynthesis do not affect processing. Since cleavage and PI-G addition are presumably catalyzed by a transamidase, the nucleoside triphosphate requirements suggest that there are additional steps in prominiPLAP processing prior to transamidation with PI-G. These may involve translocation of the pro protein in a proper conformational state to the transamidase site.

Biosynthesis of phosphatidylinositol-glycan (PI-G)-anchored membrane proteins in cell-free systems: PI-G is an obligatory cosubstrate for COOH-terminal processing of nascent proteins

It is generally recognized that nascent proteins destined to be processed to a phosphatidylinositol-glycan (PI-G)-anchored membrane form contain a hydrophobic signal peptide at both their NH2 and COOH termini. In previous studies we showed that rough microsomal membranes (RM) prepared from CHO cells can carry out COOH-terminal processing. We have now investigated RM prepared from many additional cell types, including frog oocytes, B cells, and T cells, and found that all are competent with respect to COOH-terminal processing. Exceptions were certain mutant T cells that had been shown to be defective at various steps of PI-G anchor biosynthesis [Sugiyama, E., De Gasperi, R., Urakaze, M., Chang, H.-M., Thomas, L. J., Hyman, R., Warren, C. D. & Yeh, E. T. H. (1991) J. Biol. Chem. 266, 12119-12122]. In one such defective mutant, COOH-terminal processing activity of RM could be restored either by transfecting the intact cells with the gene for the deficient step in PI-G synthesis or by adding PI-G extracts to the RM in vitro. Cleavage of the COOH-terminal signal peptide in the RM is therefore dependent on the presence of intact PI-G incorporated into the mature protein.