Proteins containing an uncleaved signal for glycophosphatidylinositol membrane anchor attachment are retained in a post-ER compartment

Altered GPI modification of insect AChE improves tolerance to organophosphate insecticides

The olive fruit fly Bactrocera oleae is the most destructive and intractable pest of olives. The management of B. oleae has been based on the use of organophosphate (OP) insecticides, a practice that induced resistance. OP-resistance in the olive fly was previously shown to be associated with two mutations in the acetylcholinesterase (AChE) enzyme that, apparently, hinder the entrance of the OP into the active site. The search for additional mutations in the ace gene that encodes AChE revealed a short deletion of three glutamines (Δ3Q) from a stretch of five glutamines, in the C-terminal peptide that is normally cleaved and substituted by a GPI anchor. We verified that AChEs from B. oleae and other Dipterans are actually GPI-anchored, although this is not predicted by the "big-PI" algorithm. The Δ3Q mutation shortens the unusually long hydrophilic spacer that follows the predicted GPI attachment site and may thus improve the efficiency of GPI anchor addition. We expressed the wild type B. oleae AChE, the natural mutant Δ3Q and a constructed mutant lacking all 5 consecutive glutamines (Δ5Q) in COS cells and compared their kinetic properties. All constructs presented identical K(m) and k(cat) values, in agreement with the fact that the mutations did not affect the catalytic domain of the enzyme. In contrast, the mutants produced higher AChE activity, suggesting that a higher proportion of the precursor protein becomes GPI-anchored. An increase in the number of GPI-anchored molecules in the synaptic cleft may reduce the sensitivity to insecticides.

Exit of GPI-anchored proteins from the ER differs in yeast and mammalian cells

Previous studies have shown that yeast glycosylphosphatidylinositol-anchored proteins (GPI-APs) and other secretory proteins are preferentially incorporated into distinct coat protein II (COPII) vesicle populations for their transport from the endoplasmic reticulum (ER) to the Golgi apparatus, and that incorporation of yeast GPI-APs into COPII vesicles requires specific lipid interactions. We compared the ER exit mechanism and segregation of GPI-APs from other secretory proteins in mammalian and yeast cells. We find that, unlike yeast, ER-to-Golgi transport of GPI-APs in mammalian cells does not depend on sphingolipid synthesis. Whereas ER exit of GPI-APs is tightly dependent on Sar1 in mammalian cells, it is much less so in yeast. Furthermore, in mammalian cells, GPI-APs and other secretory proteins are not segregated upon COPII vesicle formation, in contrast to the remarkable segregation seen in yeast. These findings suggest that GPI-APs use different mechanisms to concentrate in COPII vesicles in the two organisms, and the difference might explain their propensity to segregate from other secretory proteins upon ER exit.

Requirement of glycosylphosphatidylinositol anchor of Cripto-1 for trans activity as a Nodal co-receptor

Cripto-1 (CR-1) has an indispensable role as a Nodal co-receptor for patterning of body axis in embryonic development. CR-1 is reported to have a paracrine activity as a Nodal co-receptor, although CR-1 is primarily produced as a glycosylphosphatidylinositol (GPI)-anchored membrane protein. Regulation of cis and trans function of CR-1 should be important to establish the precise body patterning. However, the mechanism by which GPI-anchored CR-1 can act in trans is not well known. Here we confirmed the paracrine activity of CR-1 by fluorescent cell-labeling and immunofluorescent staining. We generated COOH-terminal-truncated soluble forms of CR-1 based on the attachment site for the GPI moiety (omega-site), which we identified in the present study. GPI-anchored CR-1 has a significantly higher activity than COOH-terminal-truncated soluble forms to induce Nodal signal in trans as well as in cis. Moreover, transmembrane forms of CR-1 partially retained their ability to induce Nodal signaling only when type I receptor Activin-like kinase 4 was overexpressed. NTERA2/D1 cells, which express endogenous CR-1, lost the cell-surface expression of CR-1 after phosphatidylinositol-phospholipase C treatment and became refractory to stimulation of Nodal. These observations suggest that GPI attachment of CR-1 is required for the paracrine activity as a Nodal co-receptor.

Expression and immunogenicity of the Plasmodium falciparum circumsporozoite protein: the role of GPI signal sequence

Previous studies have shown that the immunogenicity of rodent malaria parasite-derived circumsporozoite protein (CS) can be improved by deleting the glycosyl-phosphatidyl-inositol (GPI) signal sequence. To study whether GPI signal sequence deletion would also improve immunogenicity of CS derived from the major plasmodium species causing mortality in humans (P. falciparum), we tested different variants of the P. falciparum CS protein in the context of a live vector-based vaccine carrier (rAd35). We demonstrate that deletion of the GPI signal sequence from CS did not result in altered expression or secretion. In contrast, cellular localization was clearly altered, which perhaps helps to explain the significant improvement of anti-CS antibody and T-cell responses observed in mice using deletion variants in the context of the rAd35 carrier. Our results show that rational design of antigens is warranted for further development of malaria vaccines.

The C-terminal peptides of acetylcholinesterase: Cellular trafficking, oligomerization and functional anchoring

In vertebrates, the catalytic domain of acetylcholinesterase (AChE) may be associated with several C-terminal peptides generated by alternative splicing in the 3' region of transcripts. The "readthrough" (R) variant results from a lack of splicing after the last exon encoding the catalytic domain. Such a variant has been observed in Torpedo and in mammals; its C-terminal r peptide, also called "AChE Related Peptide" (ARP), is poorly conserved between rodents and humans. In rodents, it is significantly expressed in embryonic tissues and at a very low level in the brain of adult mice; it may be increased under various stress conditions, but remains very low. The "hydrophobic" (H) variant generates glycolipid (GPI)-anchored dimers, which are expressed in muscles of Torpedo, and in blood cells of mammals; H variants exist in Torpedo and in mammals, but apparently not in other vertebrate classes, suggesting that they were lost during evolution of early vertebrates and re-appeared independently in mammals. The "tailed" (T) variant exists in all vertebrate cholinesterases and their C-terminal t peptides are strongly conserved; in mammals, AChE(T) subunits represent the major type of acetylcholinesterase in cholinergic tissues. They produce a wide variety of oligomeric forms, ranging from monomers to heteromeric assemblies containing the anchoring proteins ColQ (collagen-tailed forms) and PRiMA (membrane-bound tetramers), which constitute the major functional enzyme species in mammalian muscles and brain, respectively. The oligomerization of AChE(T) subunits depends largely on the properties of their C-terminal t peptide. These peptides contain seven conserved aromatic residues, including three tryptophans, and are organized in an amphiphilic alpha helix in which these residues form a hydrophobic cluster. The presence of a cysteine is required for dimerization, while aromatic residues are necessary for tetramerization. In the collagen-tailed molecules, four t peptides form a coiled coil around a proline-rich motif (PRAD) located in the N-terminal region of ColQ. The t peptide also strongly influences the folding and cellular trafficking of AChE(T) subunits: the presence of hydrophobic residues induces partial misfolding leading to inactive protein, while aromatic residues, organized or not in an amphiphilic helix, induce intracellular degradation through the "Endoplasmic Reticulum Associated Degradation" (ERAD) pathway, rather than secretion. It has been proposed that the r and t C-terminal peptides, or fragments of these peptides, may exert independent, non cholinergic biological functions: this interesting possibility still needs to be documented, especially in view of their various degrees of evolutionary conservation.

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.

Minimal mutations are required to effect a radical change in function in CEA family members of the Ig superfamily

GPI anchorage in the CEA family results in the acquisition of radically different functions relative to TM anchorage, including inhibition of differentiation and anoikis, disruption of tissue architecture and promotion of tumorigenicity. CEA GPI anchors, as determined by the carboxy-terminal exon of CEA, demonstrate biological specificity in their ability to confer these functional changes. CEA family GPI anchorage appears to have evolved twice independently during the primate radiation, in a manner suggestive of evolution from more primitive TM-anchored CEACAM1. We show here that very few mutations in the TM exon of present-day human CEACAM1 are required to give efficient GPI anchorage and the biological specificity of CEA GPI anchors, i.e., to give the differentiation-blocking function of GPI-anchored CEA. Such a change in anchorage could therefore represent a relatively facile means for producing radical change in molecular function of Ig superfamily members during evolution.

Quality Control and Protein Folding in the Secretory Pathway

The biosynthesis of secretory and membrane proteins in the endoplasmic reticulum (ER) yields mostly properly folded and assembled structures with full biological activity. Such fidelity is maintained by quality control (QC) mechanisms that avoid the production of nonnative structures. QC relies on chaperone systems in the ER that monitor and assist in the folding process. When folding promotion is not sufficient, proteins are retained in the ER and eventually retranslocated to the cytosol for degradation by the ubiquitin proteasome pathway. Retention of proteins that fail QC can sometimes occur beyond the ER, and degradation can take place in lysosomes. Several diseases are associated with proteins that do not pass QC, fail to be degraded efficiently, and accumulate as aggregates. In other cases, pathology arises from the downregulation of mutated but potentially functional proteins that are retained and degraded by the QC system.

Effect of mutation of two critical glutamic acid residues on the activity and stability of human carboxypeptidase M and characterization of its signal for glycosylphosphatidylinositol anchoring

Human carboxypeptidase (CP) M was expressed in baculovirus-infected insect cells in a glycosylphosphatidylinositol-anchored form, whereas a truncated form, lacking the putative signal sequence for glycosylphosphatidylinositol anchoring, was secreted at high levels into the medium. Both forms had lower molecular masses (50 kDa) than native placental CPM (62 kDa), indicating minimal glycosylation. The predicted glycosylphosphatidylinositol-anchor attachment site was investigated by mutation of Ser(406) to Ala, Thr or Pro and expression in HEK-293 and COS-7 cells. The wild-type and S406A and S406T mutants were expressed on the plasma membrane in glycosylphosphatidylinositol-anchored form, but the S406P mutant was not and was retained in a perinuclear location. The roles of Glu(260) and Glu(264) in CPM were investigated by site-directed mutagenesis. Mutation of Glu(260) to Gln had minimal effects on kinetic parameters, but decreased heat stability, whereas mutation to Ala reduced the k(cat)/ K(m) by 104-fold and further decreased stability. In contrast, mutation of Glu(264) to Gln resulted in a 10000-fold decrease in activity, but the enzyme still bound to p-aminobenzoylarginine-Sepharose and was resistant to trypsin treatment, indicating that the protein was folded properly. These results show that Glu(264) is the critical catalytic glutamic acid and that Glu(260) probably stabilizes the conformation of the active site.

GPI anchor transamidase of Trypanosoma brucei: in vitro assay of the recombinant protein and VSG anchor exchange

GPI8 from Trypanosoma brucei was cloned and expressed in Escherichia coli. TbGPI8 encodes a 37 kDa protein (35 kDa after removal of the putative signal sequence) with a pI of 5.5. It contains one potential N-glycosylation site near the N-terminus but no C-terminal hydrophobic region. Enzyme activity assays using trypanosomal lysates or recombinant TbGpi8 exhibited cleavage of the synthetic peptide acetyl-S-V-L-N-aminomethyl-coumarine, indicating that TbGpi8 is indeed directly involved in the proteolytic processing of the GPI anchoring signal. Intracellular localization of TbGpi8 within tubular structures, such as the endoplasmic reticulum, was observed by using specific anti-TbGpi8 antibodies.

The transamidase mechanism of GPI anchoring was studied in bloodstream forms of Trypanosoma brucei using media containing hydrazine or biotinylated hydrazine. In the presence of the latter nucleophile, part of the newly formed VSG was linked to this instead of the GPI anchor and was not transferred to the cell surface. VSG-hydrazine-biotin was detected by streptavidin in western blots and intracellularly in Golgi-like compartments.

Addition of a glycophosphatidylinositol to acetylcholinesterase. Processing, degradation, and secretion

We introduced various mutations and modifications in the GPI anchoring signal of rat acetylcholinesterase (AChE). 1) The resulting mutants, expressed in transiently transfected COS cells, were initially produced at the same rate, in an active form, but the fraction of GPI-anchored AChE and the steady state level of AChE activity varied over a wide range. 2) Productive interaction with the GPI addition machinery led to GPI anchoring, secretion of uncleaved protein, and secretion of a cleaved protein, in variable proportions. Unproductive interaction led to degradation; poorly processed molecules were degraded rather than retained intracellularly or secreted. 3) An efficient glypiation appeared necessary but not sufficient for a high level of secretion; the cleaved, secreted protein was possibly generated as a by-product of transamidation. 4) Glypiation was influenced by a wider context than the triplet omega/omega + 1/omega + 2, particularly omega - 1. 5) Glypiation was not affected by the closeness of the omega site to the alpha(10) helix of the catalytic domain. 6) A cysteine could simultaneously form a disulfide bond and serve as an omega site; however, there was a mutual interference between glypiation and the formation of an intercatenary disulfide bond, at a short distance upstream of omega. 7) Glypiation was not affected by the presence of an N-glycosylation site at omega or in its vicinity or by the addition of a short hydrophilic, highly charged peptide (FLAG; DYKDDDDK) at the C terminus of the hydrophobic region.

The carboxyl terminus of Dictyostelium discoideum protein 1I encodes a functional glycosyl-phosphatidylinositol signal sequence

The 1I gene is expressed in the prespore cells of culminating Dictyostelium discoideum. The open reading frame of 1I cDNA encodes a protein of 155 amino acids with hydrophobic segments at both its NH(2)- and COOH-termini that are indicative of a glycosyl-phosphatidylinositol (GPI)-anchored protein. A hexaHis-tagged form of 1I expressed in D. discoideum cells appeared on Western blot analysis as a doublet of 27 and 24 kDa, with a minor polypeptide of 22 kDa. None of the polypeptides were released from the cell surface with bacterial phosphatidylinositol-specific phospholipase C, although all three were released upon nitrous acid treatment, indicating the presence of a phospholipase-resistant GPI anchor. Further evidence for the C-terminal sequence of 1I acting as a GPI attachment signal was obtained by replacing the GPI anchor signal sequence of porcine membrane dipeptidase with that from 1I. Two constructs of dipeptidase with the 1I GPI signal sequence were constructed, one of which included an additional six amino acids in the hydrophilic spacer. Both of the resultant constructs were targeted to the surface of COS cells and were GPI-anchored as shown by digestion with phospholipase C, indicating that the Dictyostelium GPI signal sequence is functional in mammalian cells. Site-specific antibodies recognising epitopes either side of the expected GPI anchor attachment site were used to determine the site of GPI anchor attachment in the constructs. These parallel approaches show that the C-terminal signal sequence of 1I can direct the addition of a GPI anchor.

Targeting of the zymogen-granule protein syncollin in AR42J and AtT-20 cells

Syncollin is a 13-kDa protein associated with the membranes of pancreatic zymogen granules. Here we determine the in situ localization of syncollin in pancreatic acinar cells from adult and neonatal rats, and study the targeting of green fluorescent protein-(GFP-) and His(6)-tagged syncollin chimaeras in model exocrine and endocrine secretory cells. Immunocytochemical analysis of the distribution of syncollin in fully differentiated and neonatal acinar cells revealed a granular pattern that corresponded with that of the zymogen-granule markers synaptobrevin 2 and amylase. In fully differentiated acinar cells syncollin-positive vesicles were detected in the apical region of the cells, whereas in neonatal acinar cells they were found clustered near the cell nucleus. Both GFP- and His(6)-tagged syncollin entered the secretory pathway when transiently expressed in AR42J or AtT-20 cells. Syncollin-GFP was found predominantly in amylase-positive granules in AR42J cells and in adrenocorticotrophic hormone- (ACTH-) positive granules in AtT-20 cells. Syncollin-GFP was also present in the Golgi complex in AR42J cells. Syncollin-His(6) became localized in ACTH-containing granules in the neuritic processes of AtT-20 cells. In AR42J cells syncollin-His(6) did not co-localize with amylase, but was detected in acidic vesicles. These results show that the exocrine protein syncollin contains intrinsic cell-type-independent targeting information that is retained in both exocrine and endocrine cells after fusion to the GFP tag. In contrast, His(6)-tagged syncollin is efficiently targeted to secretory granules only in AtT-20 cells and not in AR42J cells.

Acylation-dependent protein export in Leishmania

The surface of the protozoan parasite Leishmania is unusual in that it consists predominantly of glycosylphosphatidylinositol-anchored glycoconjugates and proteins. Additionally, a family of hydrophilic acylated surface proteins (HASPs) has been localized to the extracellular face of the plasma membrane in infective parasite stages. These surface polypeptides lack a recognizable endoplasmic reticulum secretory signal sequence, transmembrane spanning domain, or glycosylphosphatidylinositol-anchor consensus sequence, indicating that novel mechanisms are involved in their transport and localization. Here, we show that the N-terminal domain of HASPB contains primary structural information that directs both N-myristoylation and palmitoylation and is essential for correct localization of the protein to the plasma membrane. Furthermore, the N-terminal 18 amino acids of HASPB, encoding the dual acylation site, are sufficient to target the heterologous Aequorea victoria green fluorescent protein to the cell surface of Leishmania. Mutagenesis of the predicted acylated residues confirms that modification by both myristate and palmitate is required for correct trafficking. These data suggest that HASPB is a representative of a novel class of proteins whose translocation onto the surface of eukaryotic cells is dependent upon a "non-classical" pathway involving N-myristoylation/palmitoylation. Significantly, HASPB is also translocated on to the extracellular face of the plasma membrane of transfected mammalian cells, indicating that the export signal for HASPB is recognized by a higher eukaryotic export mechanism.

Comparison of the glycosyl-phosphatidylinositol cleavage/attachment site between mammalian cells and parasitic protozoa

It was previously hypothesised that the requirements for glycosyl-phosphatidylinositol (GPI) anchoring in mammalian cells and parasitic protozoa are similar but not identical. We have investigated this by converting the GPI cleavage/attachment site in porcine membrane dipeptidase to that found in the trypanosomal variant surface glycoprotein 117 and expressing the resulting mutants in COS-1 cells. Changing the entire (omega), (omega)+1 and (omega)+2 triplet in membrane dipeptidase from Ser-Ala-Ala to Asp-Ser-Ser resulted in efficient GPI anchoring of the mutant proteins, as assessed by cell-surface activity assays and susceptibility to release by phosphatidylinositol-specific phospholipase C. Immunoelectrophoretic blot analysis with antibodies recognising epitopes either side of the native (omega) residue in porcine membrane dipeptidase, and expression of a mutant in which potential alternative cleavage/attachment sites were disrupted, indicated that alternative GPI cleavage/attachment sites had not been used. These results indicate that the requirements for GPI anchoring between mammalian and protozoal cells are not as different as previously suggested, and that rules for predicting the probability of a sequence acting as a GPI cleavage/attachment site need to be applied with caution.

Glycosyl-phosphatidylinositol-anchor addition signals are processed in Nicotiana tabacum

Recent studies have demonstrated the existence of glycosyl-phosphatidylinositol (GPI)-anchored proteins in higher plants. In this study we tested whether GPI-addition signals from diverse evolutionary sources would function to link a GPI-anchor to a reporter protein in plant cells. Tobacco protoplasts were transiently transfected with a truncated form of the Clostridium thermocellum endoglucanase E reporter gene (celE') fused with a tobacco secretion signal (PR-1a) at the N-terminus and either a yeast (GAS1), mammalian (Thy-1) or putative plant (LeAGP-1) GPI-anchor addition signal at the C-terminus. The yeast and plant C-terminal signals were found to be capable of directing the addition of a GPI-anchor to the endoglucanase protein (EGE') as shown by the sensitivity of the lipid component of GPI to phosphatidylinositol-specific phospholipase C (PI-PLC) digestion. In contrast, the mammalian signal was poorly processed for anchor addition. When EGE' was fused to a truncated form of the LeAGP-1 signal (missing three amino acids predicted to be critical to signal cleavage and anchor addition), a GPI-anchor was not linked to the EGE' protein indicating the necessity for the missing amino acids. Our results show the conservation of the properties of GPI-signals in plant cells and that there may be some similar preferences in GPI-addition signal sequences for yeast and plant cells.

Glycosylation and proteolytic processing of 70 kDa C-terminal recombinant polypeptides of Plasmodium falciparum merozoite surface protein 1 expressed in mammalian cells

The cDNAs that encode the 70 kDa C-terminal portion of Plasmodium falciparum merozoite surface protein 1 (MSP-1), with or without an N-terminal signal peptide sequence and C-terminal glycosylphosphatidylinositol (GPI) signal sequence of MSP-1, were expressed in mammalian cell lines via recombinant vaccinia virus. The polypeptides were studied with respect to the nature of glycosylation, localization, and proteolytic processing. The polypeptides derived from the cDNAs that contained the N-terminal signal peptide were modified with N -linked high mannose type structures and low levels of O -linked oligosaccharides, whereas the polypeptides from the cDNAs that lacked the signal peptide were not glycosylated. The GPI anchor moiety is either absent or present at a very low level in the polypeptide expressed from the cDNA that contained both the signal peptide and GPI signal sequences. Together, these data establish that whereas the signal peptide of MSP-1 is functional, the GPI anchor signal is either nonfunctional or poorly functional in mammalian cells. The polypeptides expressed from the cDNAs that contained the signal peptide were proteolytically cleaved at their C-termini, whereas the polypeptides expressed from the cDNAs that lacked the signal peptide were uncleaved. While the polypeptide expressed from the cDNA containing both the signal peptide and GPI anchor signal was truncated by approximately 14 kDa at the C-terminus, the polypeptide derived from the cDNA with only the signal peptide was processed to remove approximately 6 kDa, also from the C-terminus. Furthermore, the polypeptides derived from cDNAs that lacked the signal peptide were exclusively localized intra-cellularly, the polypeptides from cDNAs that contained the signal peptide were predominantly intracellular, with low levels on the cell surface; none of the polypeptides was secreted into the culture medium to a detectable level. These results suggest that N -glycosylation alone is not sufficient for the efficient extracellular transport of the recombinant MSP-1 polypeptides through the secretory pathway in mammalian cells.

Heterologous expression of a Trypanosoma cruzi surface glycoprotein (gp82) in mammalian cells indicates the existence of different signal sequence requirements and processing

Metacyclic trypomastigotes of Trypanosoma cruzi express a developmentally regulated 82 kDa surface glycoprotein (gp82) that has been implicated in the mammalian cell invasion. When the non-infective epimastigote stage of the parasite was transfected with a vector containing the gp82 gene, an 82 kDa surface glycoprotein, which was indistinguishable from the metacyclic stage protein, was expressed. In contrast, when the same gene was expressed in transfected mammalian cells, although a large amount of protein was produced, it was not imported into the endoplasmic reticulum and glycosylated. This blockage in targeting and processing could be partially compensated for by the addition of a virus haemagglutinin signal peptide to the amino terminus of gp82. Thus, the requirements for membrane protein processing are distinct in mammals and T. cruzi, and an intrinsic feature of the gp82 prevents subsequent sorting to the mammalian cell surface. These results could be useful in the development of new DNA vaccines against T. cruzi employing parasite genes encoding immunodominant surface glycoproteins.

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.

The polymorphism of acetylcholinesterase: post-translational processing, quaternary associations and localization

The molecular forms of acetylcholinesterase (AChE) correspond to various quaternary structures and modes of anchoring of the enzyme. In vertebrates, these molecules are generated from a single gene: the catalytic domain may be associated with several types of C-terminal peptides, that define distinct types of catalytic subunits (AChE(S), AChE(H), AChE(T)) and determine their post-translational maturation. AChE(S) generates soluble monomers, in the venom of Elapid snakes. AChE(H) generates GPI-anchored dimers, in Torpedo muscles and on mammalian blood cells. AChE(T) is the only type of catalytic subunit that exists in all vertebrate cholinesterases; it produces the major forms in adult brain and muscle. AChE(T) generates multiple structures, ranging from monomers and dimers to collagen-tailed and hydrophobic-tailed forms, in which catalytic tetramers are associated with anchoring proteins that attach them to the basal lamina or to cell membranes. In the collagen-tailed forms, AChE(T) subunits are associated with a specific collagen, ColQ, which is encoded by a single gene in mammals. ColQ contains a short peptidic motif, the proline-rich attachment domain (PRAD), that triggers the formation of AChE(T) tetramers, from monomers and dimers. The critical feature of this motif is the presence of a string of prolines, and in fact synthetic polyproline shows a similar capacity to organize AChE(T) tetramers. Although the COLQ gene produces multiple transcripts, it does not generate the hydrophobic tail. P, which anchors AChE in mammalian brain membranes. The coordinated expression of AChE(T) subunits and anchoring proteins determines the pattern of molecular forms and therefore the localization and functionality of the enzyme.

Maturation of the axonal plasma membrane requires upregulation of sphingomyelin synthesis and formation of protein-lipid complexes

Neuronal maturation is a gradual process; first axons and dendrites are established as distinct morphological entities; next the different intracellular organization of these processes occurs; and finally the specialized plasma membrane domains of these two compartments are formed. Only when this has been accomplished does proper neuronal function take place. In this work we present evidence that the correct distribution of a class of axonal membrane proteins requires a mechanism which involves formation of protein-lipid (sphingomyelin/cholesterol) detergent-insoluble complexes (DIGs). Using biochemistry and immunofluorescence microscopy we now show that in developing neurons the randomly distributed Thy-1 does not interact with lipids into DIGs (in fully developed neurons the formation of such complexes is essential for the correct axonal targeting of this protein). Using lipid mass spectrometry and thin layer chromatography we show that the DIG lipid missing in the developing neurons is sphingomyelin, but not cholesterol or glucosylceramide. Finally, by increasing the intracellular levels of sphingomyelin in the young neurons the formation of Thy-1/DIGs was induced and, consistent with a role in sorting, proper axonal distribution was facilitated. These results emphasize the role of sphingomyelin in axonal, and therefore, neuronal maturation.

Glycosylphosphatidylinositol-dependent secretory transport in Trypanosoma brucei

We have investigated the role of glycosylphosphatidylinositol (GPI) anchors in forward secretory trafficking using African trypanosomes as a model system. Soluble GPI-minus forms of variant surface glycoprotein (VSG), in which the C-terminal GPI-addition peptide signal is deleted, are secreted from transformed procyclic trypanosomes with 5-fold reduced kinetics, relative to matched GPI-anchored constructs. Cell fractionation and immunofluorescence localization studies indicate that the GPI-minus VSG reporters accumulate in the endoplasmic reticulum (ER). This transport defect is specific, since overexpression of GPI-minus VSG has no effect on the rate of transport of a second soluble secretory reporter (BiPN) when co-expressed in the same cells. Two results suggest that delayed forward transport cannot be accounted for by failure to fold/assemble in the absence of a GPI anchor, thereby leading to prolonged association with ER quality-control machinery. First, no evidence was found for elevated association of GPI-minus VSG with the ER molecular chaperone, BiP. Secondly, newly synthesized GPI-minus VSG is dimerized efficiently, as judged by velocity-sedimentation analysis. GPI-dependent transport is not confined to the VSG reporters, because a similar dependence is found with another trypanosomal GPI-anchored protein, trans-sialidase. These findings suggest that GPI structures act in a positive manner to mediate efficient forward transport of some, and perhaps all, GPI-anchored proteins in the early secretory pathway of trypanosomes. Possible mechanisms for GPI-dependent transport are discussed with respect to current models of vesicular trafficking.

The hydrophobic domains in the carboxyl-terminal signal for GPI modification and in the amino-terminal leader peptide have similar structural requirements

Proteins having a glycosyl-phosphatidylinositol (GPI) membrane anchor are synthesized with a carboxyl-terminal signal that is cleaved in the endoplasmic reticulum prior to GPI modification. The signal is characterized by a moderately hydrophobic domain downstream from the cleavage/modification site. The essential features of this domain were characterized using a truncated version of folate receptor (FR) type beta (FR-beta delta 5) in which its five carboxyl-terminal amino acid residues were deleted without affecting the efficiency of GPI modification. The amino acids at various positions in the hydrophobic domain were systematically altered and the extent of GPI modification of the recombinant proteins was determined by measuring [3H]folic acid binding at the cell surface, by Western blot analysis and from the sensitivity of the proteins to phosphatidylinositol-specific phospholipase C (PI-PLC). The results indicate that a threshold level of hydrophobicity exists at a single position below which the efficiency of GPI modification decreases with increasing hydrophilicity. Further, the hydrophobic domain is characterized by a hydrophobicity profile and not merely a minimum overall hydrophobicity. Thus, a leucine-rich core hydrophobic segment of six to eight amino acid residues is more sensitive to relatively small hydrophilic substitutions compared to its flanking regions and such mutations could be compensated by a hydrophobic substitution elsewhere within this core segment. Such a hydrophobicity profile is characteristic of the amino-terminal leader peptide. When the entire hydrophobic domain of the leader peptide of FR-beta (12 amino acid residues) was substituted with the hydrophobic domain of the GPI signal (13 amino acids), it was possible to obtain expression of FR-beta on the cell surface. In this construct, point mutations in the core hydrophobic segment and in the flanking regions within the substituting peptide produced a similar pattern of effects on the cell surface receptor expression compared to the corresponding mutations in the GPI signal of FR-beta. The results suggest that common principles may govern interactions of the hydrophobic domains of the GPI signal and the leader peptide with the endoplasmic reticulum.

Proteolysis of the carboxyl-terminal GPI signal independent of GPI modification as a mechanism for selective protein secretion

Variable amounts of soluble forms of a variety of glycosyl-phosphatidylinositol (GPI)-anchored proteins occur extracellularly, but the molecular mechanisms governing their release are not entirely clear. When the GPI-anchored folate receptor (FR) type beta was expressed transiently in human 293 fibroblasts, there was a roughly equal distribution of [3H]folic acid binding protein between the cell surface and the medium after 24 h over a wide range of expression levels of FR-beta. The difference in apparent molecular masses between the soluble FR-beta and the PI-PLC-treated membrane protein indicated that the former was not released from the membrane by the action of phospholipase. Brefeldin A inhibited the release of soluble FR-beta from both the transfected 293 cells and stable recombinant CHO (CHO-FR-beta) cells while pre-existing levels of cell surface FR were unaltered suggesting the absence of a precursor-product relationship between the membrane-associated FR-beta and the soluble protein in the medium. [35S]Cysteine pulse-chase analysis was consistent with this finding. Interchanging of carboxyl-terminal peptides between FR-beta and FR-alpha revealed that the nature of the processed signal for GPI modification was responsible for the quantitative membrane anchoring of FR-alpha and the production of soluble FR-beta. When total cell lysates were analyzed by Western blot, a diffuse band of apparent 41 kDa and three additional sharp bands of apparent 35, 33, and 29.3 kDa were seen. The 41 kDa band was identified as the PI-PLC sensitive cell surface receptor. Several mutant constructs of FR-beta, in which the carboxyl-terminal signal for GPI modification was either disrupted or deleted only gave the three lower bands. The three sharp bands from the wild-type and the mutant forms of FR-beta were identified as nonglycosylated (29.3 kDa) or glycosylated polypeptides in which the carboxyl-terminal peptide was at least partially proteolyzed without GPI modification. All of the mutations in the GPI signal resulted in the recovery of [3H]folic acid binding protein in the media which, similar to the wild-type FR recovered from the media, were converted to the 29.3 kDa band by N-glycanase. The results from this study indicate that a carboxyl-terminal peptide in FR-beta is efficiently proteolyzed intracellularly by a pathway that is independent of GPI signal recognition resulting in proper protein folding and secretion. Such carboxyl-terminal sequences could represent a simple adaptation for proteins whose physiologic functions reside both at the cell surface and in extracellular fluids, allowing their selective and tissue-specific release.

Prion protein aggregation reverted by low temperature in transfected cells carrying a prion protein gene mutation

Prion diseases are characterized by the conversion of the normal cellular prion protein (PrPC), a glycoprotein that is anchored to the cell membrane by a glycosylphosphatidylinositol moiety, into an isoform that is protease-resistant (PrPres) and pathogenic. In inherited prion diseases, mutations in the prion protein (PrPM) engender the conversion of PrPM into PrPres. We developed a cell model of Gerstmann-Sträussler-Scheinker disease, a neurodegenerative condition characterized by PrPM-containing amyloid deposits and neuronal loss, by expressing the Gerstmann-Sträussler-Scheinker haplotype Q217R-129V in human neuroblastoma cells. By comparison to PrPC, this genotype results in the following alterations of PrPM: 1) expression of an aberrant form lacking the glycosylphosphatidylinositol anchor, 2) increased aggregation and protease resistance, and 3) impaired transport to the cell surface. Most of these alterations are temperature-sensitive, indicating that they are due to misfolding of PrPM.

Intracellular transport, cell-surface exposure and release of recombinant Tamm-Horsfall glycoprotein

Human Tamm-Horsfall glycoprotein (T-H), first described as the major urinary glycoprotein, is a glycosylphosphatidyl-inositol (GPI)-anchored membrane protein which mainly resides at the luminal face of cells of the thick ascending limb of Henle's loop (TAL) and early distal convoluted tubules of nephron. Since no human renal cell-line producing T-H is available, T-H cDNA was transfected in HeLa cells and a cell line was selected in which 95% of the cells stably expressed T-H, in order to elucidate the biosynthesis, mechanisms regulating the transport of T-H along the exocytic pathway, exposure at the cell surface and release in soluble form. Treatment of cells with an exogenous reducing agent results in a drastic delay in the conversion from precursor to mature T-H. Since the accumulating T-H-precursor carries glycans not yet processed by Golgi-mannosidases, we propose that the formation of a correct set of intrachain disulphide bonds is required for T-H exit out the endoplasmic reticulum. Even the treatment of cells with an inhibitor of GPI-anchor biosynthesis results in an intracellular accumulation of T-H precursor, loss of T-H localization into Golgi apparatus and reduced surface exposure. These results indicate that the GPI-anchor addition is necessary for T-H delivery to the cell-surface. The release rate of new synthesized T-H shows an initial lag time very likely depending on the time required for T-H surface exposure. A portion of released T-H appears to contain ethanolamine, a component of GPI anchor, indicating that, at least in HeLa cells, a GPI-specific phospholipase contributes to the T-H release. Exposure of cells to monensin and brefeldin A results in a loss of accumulation of T-H in the Golgi perinuclear region and a reduced delivery to the cell surface. Under monensin treatment an intermediate T-H form non-exposed at the cell surface is released in the medium, indicating that a soluble T-H may be produced inside the cell under conditions that alter the Golgi apparatus. If such an event occurs in polarized kidney cells, a T-H release from the basolateral face may be postulated, inasmuch as the GPI-anchor is an apical sorting signal. Since T-H is a powerful autoantigen, the accumulation of soluble T-H in the interstitium of TAL may cause the formation of immunocomplexes.

Mutational analysis of the C-terminal signal peptide of bovine liver 5'-nucleotidase for GPI anchoring: a study on the significance of the hydrophilic spacer region

Bovine liver 5'-nucleotidase is a GPI-anchored protein whose Ser523 attaches to GPI as the omega-site. For GPI-modification, pro-protein of the enzyme possesses a signal peptide at the C-terminus, comprising a hydrophilic spacer sequence of 8 amino acid residues and the following hydrophobic region of 17 amino acid residues. The C-terminal signal peptide is replaced by GPI on a luminal leaflet of endoplasmic reticulum. To characterize the C-terminal signal peptide for GPI modification, we constructed a series of deletion and elongation mutant genes, altering length of the hydrophilic spacer sequence by site-directed mutagenesis. Systematic deletion and Ala insertion of the sequence showed that the sequence of 6-14 residues were compatible for GPI modification. For GPI transfer to the pro-protein, the optimum length of spacer sequence would be 8, being consistent with natural selection. The spacer sequence may play a role for leading the omega-residue correctly to the active site of putative GPI transamidase. The elongation of the spacer is more permissible than deletion. Nevertheless, the length of the spacer sequence may influence efficiency of GPI modification by its positive or negative control.

Quality control of glycosylphosphatidylinositol anchor attachment in mammalian cells: a biochemical study

hGHDAF28 is a chimaeric protein consisting of human growth hormone fused to a crippled signal sequence for glycosylphosphatidylinositol (GPI)-anchor addition from decay-accelerating factor, and serves as a model for quality control of GPI-anchor addition. hGHDAF28 is retained in a pre-Golgi compartment and degraded intracellularly by a mechanism with similarity to that for other endoplasmic reticulum (ER)-retained proteins (Field, Moran, Lee, Keller and Caras (1994) J. Biol. Chem. 269, 10830-10837). We have studied the specific pathway of degradation for hGHDAF28 using a number of compounds which affect protein folding and trafficking pathways in eukaryotic cells. We found that high concentrations of dithiothreitol (DTT) accelerated loss of hGHDAF28 by degradation from cell lysates, without promoting secretion or alteration of disulphide-bond distribution, in contrast to a number of other examples of ER-retained proteins where DTT alters disulphide-bond formation. Additionally, degradation of hGHDAF28 was sensitive to pH, being promoted at pH 6.0 and inhibited at pH 8.0; however, the latter effect was transient, indicating incomplete blockade. Degradation was also partially enhanced by depletion of ER calcium with thapsigargin, but this was again a partial and transient effect. Furthermore, degradation was temperature sensitive, with a gradual decrease in rate observed at lower temperatures. However, a sharp decrease in turnover between 15 degrees C and 20 degrees C, indicative of a requirement for transport to a post-ER compartment, was not observed. Degradation of hGHDAF28 was insensitive to treatment with nocodozole or compounds preventing cytoplasmic autophagy, suggesting that ER degradation is independent of classical autophagy and microtubule-dependent processes. In addition, disruption of N-glycosylation with tunicamycin, or inhibition of processing of immature N-glycan chains with castanospermine or deoxynojirimycin, had little effect on the stability of hGHDAF28, suggesting that disruption of the BiP/calnexin quality-control system by bulk cellular secretory proteins does not influence the ER-degradation pathway of hGHDAF28. Intermolecular hGHDAF28 cysteine bonds result in the formation of aggregates which are probably important in the retention of the molecule. The insensitivity of this structure to reduction in vivo, together with the enhanced degradation rate, indicates that DTT mediates its effect on stability via a molecule involved in degradation of hGHDAF28, possibly a thiol-sensitive protease.

Bisecting GlcNAc structures act as negative sorting signals for cell surface glycoproteins in forskolin-treated rat hepatoma cells

The bisecting N-acetylglucosamine residue is formed by UDP-N-acetylglucosamine:beta-D-mannoside-beta-1, 4-N-acetylglucosaminyltransferase III (GnT-III), a key branching enzyme for N-glycans. We found that forskolin, an adenylyl cyclase activator, markedly enhanced GnT-III at the transcriptional level in various hepatoma cells and hepatocytes, resulting in an increase of bisecting GlcNAc residues in various glycoproteins, as judged from the lectin binding to erythroagglutinating phytohemagglutinin (E-PHA). In whole cell lysates, the E-PHA binding was increased, and leukoagglutinating phytohemagglutinin (L-PHA) binding was decreased at 12 h after forskolin treatment, by time, both GnT-III activity and mRNA had reached the maximum levels. In contrast, the binding capacity as to E-PHA, determined by fluorescence-activated cell sorting on the cell surface, was decreased, suggesting that bisecting GlcNAc structures in certain glycoproteins changed the expression levels of glycoproteins and decreased their sorting on the cell surface. Fractionated organelles of M31 cells showed that the binding capacity as to E-PHA was mainly localized in Golgi membranes and lysosomes. This was also supported by a fluorescence microscopy. In order to determine whether or not the bisecting GlcNAc residue acts as a sorting signal for glycoproteins, N-oligosaccharide structures of lysosomal-associated membrane glycoprotein 1 and beta-glucuronidase, gamma-glutamyltranspeptidase, and secretory glycoproteins such as ceruloplasmin and alpha-fetoprotein were measured by E-PHA and L-PHA blotting after immunoprecipitation. The expression levels of lysosomal membrane glycoprotein 1 and gamma-glutamyltranspeptidase on the cell surface were decreased at 12 h after forskolin treatment, indicating that the bisecting GlcNAc structure may act as a negative sorting signal for the cell surface glycoproteins and may alter the characteristics of hepatoma cells. This is the first report on glycoprotein sorting related to a specific structure of oligosaccharides, bisecting GlcNAc.

An uncleaved glycosylphosphatidylinositol signal mediates Ca(2+)-sensitive protein degradation

Inv-gp80 is a chimeric protein which contains a signal for the attachment of a glycosylphosphatidylinositol (GPI) anchor. When expressed in Dictyostelium discoideum, this protein fails to become GPI anchored and is retained within the cell as an integral membrane protein. We have compared the subcellular localization and degradation of Inv-gp80 with that of its intracellular but soluble counterpart, Inv-gp80sc. Inv-gp80sc lacks the hydrophobic C-terminal 22 amino acids of Inv-gp80. The N-linked oligosaccharides of both Inv-gp80 and Inv-gp80sc remained sensitive to endoglycosidase H, and both proteins co-fractionated with endoplasmic reticulum marker enzymes on Percoll gradients. Under normal conditions, Inv-gp80 displayed a half-life (t 1/2) of 90 min, while Inv-gp80sc displayed a t 1/2 of 120 min. The degradation of both proteins required ATP, was inhibited by tosyl phenylalanylchloromethane (Tos-Phe-CH2Cl) and was insensitive to inhibitors of lysosomal function. While depletion of Ca2+ from the endoplasmic reticulum had no effect on the degradation of Inv-gp80sc, it stimulated the degradation of Inv-gp80. When the GPI anchor signal sequence of Inv-gp80 was replaced with the transmembrane domain of the interleukin-2 receptor, the degradation of the protein was no longer influenced by Ca2+ fluxes. The data suggest that while the GPI anchor sequence of Inv-gp80 does not contain determinants regulating the degradation of the protein under basal conditions, it targets Inv-gp80 for rapid degradation under conditions where Ca2+ is depleted from the endoplasmic reticulum.

Bip/GRP78 but not calnexin associates with a precursor of glycosylphosphatidylinositol-anchored protein

When fused in-frame with a C-terminal propeptide of placental alkaline phosphatase (PLAP), rat alpha 2u-globulin (alpha GL), a nonglycosylated secretory protein, was expressed on the cell surface as a glycosylphosphatidylinositol (GPI)-linked chimaeric protein (alpha GL-PLAP). In contrast with the wild-type alpha GL-PLAP, a mutant, in which Asp at the cleavage/attachment site of GPI was replaced by Trp, failed to become a GPI-linked mature form and was retained as a precursor form within the cell [Oda, Cheng, Saku, Takami, Sohda, Misumi, Ikehara and Millán (1994) Biochem. J. 301, 577-583]. To elucidate the molecular interactions involved in the retention of the proform within the cell, we examined the association of the proform with molecular chaperones in the endoplasmic reticulum (ER). Antibody against the ER retrieval motif KDEL coimmunoprecipitated a 25 kDa proform, but not a 22 kDa GPI-linked mature form. Pulse-chase experiments showed that the wild-type alpha GL-PLAP with a cleavable propeptide was converted into the mature form, whereas the mutant alpha GL-PLAP with an uncleavable propeptide remained associated with ER-resident proteins with a KDEL motif and underwent rapid degradation in a pre-Golgi compartment. Chemical cross-linking studies showed that, of the several ER-resident proteins immunoreactive with the anti-KDEL antibody, a 78 kDa protein was the only protein associated with the proform. Furthermore this 78 kDa protein was dissociated from the precursor molecule on incubation with ATP, allowing us tentatively to assign it as Bip/GRP78. Anticalnexin antibody, however, failed to coprecipitate any form of the chimaeric protein. Immunoelectron microscopy showed that the proform with the uncleavable propeptide was localized in the ER, but not detected in the Golgi apparatus or plasma membranes. Taken together, these results suggest that Bip/GRP78 is associated with pro alpha GL-PLAP and retains it within the ER until pro alpha GL-PLAP is either modified by GPI or degraded, thereby participating in the quality control of this GPI-linked chimaeric protein.

Cell surface expression of a functional rubella virus E1 glycoprotein by addition of a GPI anchor

Rubella virus (RV) envelope glycoproteins E1 and E2 are targeted to the Golgi as heterodimers. While E2 contains a transmembrane Golgi retention signal, E1 is arrested in a pre-Golgi compartment in the absence of E2, and appears to require heterodimerization in order to reach the Golgi. Various forms of E1 with deletions in the ectodomain or lacking the cytoplasmic (CT) and transmembrane (TM) domains, as well as the 29 C-terminal amino acid residues of the ectodomain were also retained intracellularly. We therefore investigated the possibility of targetting E1 to the plasma membrane by addition of a glycosylphosphatidylinositol (GPI) anchor. We found that E1GPI was transported to the cell surface where it retained the hemadsorption activity characteristic of the wild-type E1/E2 heterodimer. Furthermore, coexpression of a mammalian GPI-specific phospholipase D (GPI-PLD) resulted in the release of E1GPI and in constitutive expression of a soluble form of E1. This study thus demonstrates that the GPI anchor has a dominant effect over the E1 pre-Golgi retention signal and that E1 is sufficient for hemadsorption.

Properties and localization of a tyrosine phosphorylated form of hexokinase in mouse sperm

Mouse sperm possess a phosphotyrosine-containing hexokinase type 1 (HK1) that is associated with the plasma membrane fraction of these cells (Kalab et al., 1994; J. Biol Chem 269:3810-3817). This apparent plasma membrane association appears unique, since somatic HK1 is normally cytoplasmic or bound to the outer mitochondrial membrane via contact sites with a voltage-dependent anion channel (porin) through a porin-binding domain. In male germ cells, three cDNA clones have been described that encode unique HK1 isoforms (HK1-sa, HK1-sb, HK1-sc) that do not contain porin binding domains (Mori et al., 1993: Biol Reprod 49:191-203). This suggests that these proteins might not be localized to the outer mitochondrial membrane and could have alternative functions in germ cells and/or sperm. We demonstrate in the mouse that male germ cells and sperm could potentially express four HK1 isoforms (HK1-sa, HK1-sb, HK1-sc, and the somatic HK1). At the protein level, at least one of the HK1 isoforms becomes phosphorylated on tyrosine residues during spermatogenesis. Treatment of sperm membrane fractions to dissociate the phosphotyrosine-containing HK1 (pY-mHK1) yields results demonstrating that pY-mHK1 has properties of an integral membrane protein. Indirect immunofluorescence using a monoclonal antibody to HK1 demonstrates specific staining both in the head and tail regions of sperm. Surface biotinylation of intact sperm followed by precipitation with either polyclonal HK1 antiserum or with avidin-Sepharose suggests that pY-mHK1 possesses an extracellular domain. These results suggest that mouse sperm contain at least one HK1 isoform that is present on the sperm head, has an extracellular domain, and behaves as an integral membrane protein.

Quality control in the secretory pathway

Whereas newly synthesized proteins that have acquired a properly folded and assembled structure are transported from the endoplasmic reticulum to their final destinations, incompletely folded and assembled proteins are, as a rule, retained and eventually degraded. The molecular mechanisms of this unique molecular sorting phenomenon, called 'quality control', have been illuminated by recent studies.

trans-Golgi retention of a plasma membrane protein: mutations in the cytoplasmic domain of the asialoglycoprotein receptor subunit H1 result in trans-Golgi retention

Unlike the wild-type asialoglycoprotein receptor subunit H1 which is transported to the cell surface, endocytosed and recycled, a mutant lacking residues 4-33 of the 40-amino acid cytoplasmic domain was found to be retained intracellularly upon expression in different cell lines. The mutant protein accumulated in the trans-Golgi, as judged from the acquisition of trans-Golgi-specific modifications of the protein and from the immunofluorescence staining pattern. It was localized to juxtanuclear, tubular structures that were also stained by antibodies against galactosyltransferase and gamma-adaptin. The results of further mutagenesis in the cytoplasmic domain indicated that the size rather than the specific sequence of the cytoplasmic domain determines whether H1 is retained in the trans-Golgi or transported to the cell surface. Truncation to less than 17 residues resulted in retention, and extension of a truncated tail by an unrelated sequence restored surface transport. The transmembrane segment of H1 was not sufficient for retention of a reporter molecule and it could be replaced by an artificial apolar sequence without affecting Golgi localization. The cytoplasmic domain thus appears to inhibit interaction(s) of the exoplasmic portion of H1 with trans-Golgi component(s) for example by steric hindrance or by changing the positioning of the protein in the membrane. This mechanism may also be functional in other proteins.

Anchoring of an immunogenic Plasmodium falciparum circumsporozoite protein on the surface of Dictyostelium discoideum

The circumsporozoite protein (CSP), a major antigen of Plasmodium falciparum, was expressed in the slime mold Dictyostelium discoideum. Fusion of the parasite protein to a leader peptide derived from Dictyostelium contact site A was essential for expression. The natural parasite surface antigen, however, was not detected at the slime mold cell surface as expected but retained intracellularly. Removal of the last 23 amino acids resulted in secretion of CSP, suggesting that the C-terminal segment of the CSP, rather than an ectoplasmic domain, was responsible for retention. Cell surface expression was obtained when the CSP C-terminal segment was replaced by the D. discoideum contact site A glycosyl phosphatidylinositol anchor signal sequence. Mice were immunized with Dictyostelium cells harboring CSP at their surface. The raised antibodies recognized two different regions of the CSP. Anti-sporozoite titers of these sera were equivalent to anti-peptide titers detected by enzyme-linked immunosorbent assay. Thus, cell surface targeting of antigens can be obtained in Dictyostelium, generating sporozoite-like cells having potentials for vaccination, diagnostic tests, or basic studies involving parasite cell surface proteins.

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.

Lack of glycosyl-phosphatidylinositol anchoring leads to precursor retention by a unique mechanism in Dictyostelium discoideum

Gp80, a cell-adhesion molecule in Dictyostelium discoideum, is modified by N- and O-linked oligosaccharides, and a glycosylphosphatidylinositol (GPI) anchor. To identify sequences important for the addition of these modifications to gp80, we created a hybrid protein in which the C-terminal 136 amino acids of yeast invertase were replaced by the C-terminal 110 amino acids of gp80. When expressed in D. discoideum, this protein (Inv-gp80) was not GPI-anchored and was retained in a pre-Golgi compartment. Inv-gp80 did, however, display characteristics of a transmembrane protein, suggesting a novel mechanism for its retention. We also expressed a truncated version of the hybrid protein in which the C-terminal 22 amino acids of the Inv-gp80 were deleted. The truncated protein (Inv-gp80stop) was O-glycosylated and secreted. These observations indicate that the hybrid protein is not abnormally folded and demonstrate the importance of the C-terminal 22 amino acids in the retention of Inv-gp80. Together, the data suggest that oligomerization of the protein blocks its GPI anchoring.

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.

Requirements for glycosylphosphatidylinositol attachment are similar but not identical in mammalian cells and parasitic protozoa

The general features of the glycosylphosphatidylinositol (GPI) signal have been conserved in evolution. To test whether the requirements for GPI attachment are indeed the same in mammalian cells and parasitic protozoa, we expressed the prototype GPI-linked protein of Trypanosoma brucei, the variant surface glycoprotein (VSG), in COS cells. Although large amounts of VSG were produced, only a small fraction became GPI linked. This impaired processing is not caused by the VSG ectodomain, since replacement of the VSG GPI signal with that of decay accelerating factor (DAF) produced GPI-linked VSG. Furthermore, whereas fusion of the DAF GPI signal to the COOH terminus of human growth hormone (hGH) produces GPI-linked hGH, an analogous hGH fusion using the VSG GPI signal does not, indicating that the VSG GPI signal functions poorly in mammalian cells. By constructing chimeric VSG-DAF GPI signals and fusing them to the COOH terminus of hGH, we show that of the two critical elements that comprise the GPI-signal--the cleavage/attachment site and the COOH terminal hydrophobic domain--the former is responsible for the impaired activity of the VSG GPI signal in COS cells. To confirm this, we show that the VSG GPI signal can be converted to a viable signal for mammalian cells by altering the amino acid configuration at the cleavage/attachment site. We also show that when fused to the COOH terminus of hGH, the putative GPI signal from the malaria circumsporozoite (CS) protein produces low levels of GPI-anchored hGH, suggesting that the CS protein is indeed GPI linked, but that the CS protein GPI signal, like the VSG-signal, functions poorly in COS cells. The finding that the requirements for GPI attachment are similar but not identical in parasitic protozoa and mammalian cells may allow for the development of selective inhibitors of GPI-anchoring that might prove useful as antiparasite therapeutics.

Mutational analysis of the COOH-terminal hydrophobic domain of bovine liver 5'-nucleotidase as a signal for glycosylphosphatidylinositol (GPI) anchor attachment

In order to address the minimum domain of the COOH-terminal hydrophobic region responsible for GPI modification of bovine liver 5'-nucleotidase, we constructed a series of the deletion mutants of the COOH-terminus and expressed them in COS cells. Cells transfected by the deletion mutant of 6 amino acids (-IIILYQ) from the hydrophobic domain (-FSLIFLSVLAVIII-LYQ) did not show any elevation of cell surface-associated 5'-nucleotidase activity, whereas the 2 (-YQ) or 4 (-ILYQ) amino acid deletion mutant retained the bovine liver-derived activity on the cell surface as a GPI-anchored protein. Loss of half the hydrophobic domain (6 or 8 amino acids) resulted in accumulation of the activity in the cell. On the other hand, deletion of the whole hydrophobic domain (17 amino acids) or the entire cleaved-off domain (25 amino acids) made the product secreted into the medium. In conclusion, the hydrophobicity of 13 amino acids in length was enough for the GPI modification of the bovine liver 5'-nucleotidase.

Addition of lipid substituents of mammalian protein glycosylphosphoinositol anchors

A single metabolic path leading to synthesis of ether lipids is known in animal cells, the major products of which are plasmalogens. To learn whether this peroxisomal path is also responsible for the synthesis of base-resistant lipid components of glycosylphosphoinositol (GPI)-anchored membrane proteins, we have investigated the structure of anchor precursor mannolipids both in wild-type cells (CHO-K1 and a macrophage-like line, RAW 264.7) and in two corresponding mutant cells in which ether lipid biosynthesis is severely impaired. We observe that the precursor mannolipids of both the wild-type and mutant cells do not include alkylglycerol. Nevertheless, both wild-type and mutant cells express cell surface GPI-anchored placental alkaline phosphatase (AP) which includes alkali-resistant hydrophobic chains in its anchor moiety. Thus, (i) in normal AP GPI anchor synthesis, any ether-linked substituents must be added either immediately before, during, or after anchor addition to AP, and (ii) the classical peroxisomal path for ether lipid synthesis appears not to contribute to the synthesis of GPI anchors.