Primary sequence domains required for the retention of rotavirus VP7 in the endoplasmic reticulum

The endoplasmic reticulum of plant cells and its role in protein maturation and biogenesis of oil bodies

The endoplasmic reticulum (ER) is the port of entry of proteins into the endomembrane system, and it is also involved in lipid biosynthesis and storage. This organelle contains a number of soluble and membrane-associated enzymes and molecular chaperones, which assist the folding and maturation of proteins and the deposition of lipid storage compounds. The regulation of translocation of proteins into the ER and their subsequent maturation within the organelle have been studied in detail in mammalian and yeast cells, and more recently also in plants. These studies showed that in general the functions of the ER in protein synthesis and maturation have been highly conserved between the different organisms. Yet, the ER of plants possesses some additional functions not found in mammalian and yeast cells. This compartment is involved in cell to cell communication via the plasmodesmata, and, in specialized cells, it serves as a storage site for proteins. The plant ER is also equipped with enzymes and structural proteins which are involved in the process of oil body biogenesis and lipid storage. In this review we discuss the components of the plant ER and their function in protein maturation and biogenesis of oil bodies. Due to the large number of cited papers, we were not able to cite all individual references and in many cases we refer the readers to reviews and references therein. We apologize to the authors whose references are not cited.

A sorting motif localizes the foamy virus glycoprotein to the endoplasmic reticulum

We recently identified an endoplasmic reticulum (ER) retrieval signal-the dilysine motif-in the glycoproteins of all five foamy viruses (FVs) for which sequences were available (P. A. Goepfert, G. Wang, and M. J. Mulligan, Cell 82:543-544, 1995). In the present study, expression of recombinant human FV (HFV) glycoprotein and analyses of oligosaccharide modifications and precursor cleavage indicated that the protein was localized to the ER. HFV glycoproteins encoding seven different dilysine motif mutations were then expressed. The results indicated that disruptions of the dilysine motif resulted in higher levels of forward transport of the HFV glycoprotein from the ER through the Golgi apparatus to the plasma membrane. We conclude that the dilysine motif is responsible for ER sorting of the FV glycoprotein. Signal-mediated ER localization has not previously been described for a retroviral glycoprotein.

Membrane binding and endoplasmic reticulum retention sequences of rotavirus VP7 are distinct: role of carboxy-terminal and other residues in membrane binding

The sequences responsible for binding rotavirus glycoprotein VP7 to the membrane of the endoplasmic reticulum (ER) have not been identified. Here we show that the sequences which promote membrane binding in vitro are distinct from the N-terminal sequences which promote retention of VP7 in the ER in vivo. The role of the C-terminal region in membrane binding was also examined by using truncation mutants. Membrane binding in vitro was reduced but not abolished by removing up to 102 residues from the C terminus. The data suggest that the last 36 residues of VP7 may be present in the membrane or translocation pore, possibly with the C terminus protruding into the cytoplasm, since these residues contribute to, but do not account for, membrane binding. Surprisingly, modified forms of VP7 which are secreted from transfected cells showed the same membrane-binding properties in vitro as the protein retained in the ER membrane. Thus, secreted VP7 may not be present as a soluble polypeptide in the ER. A model to explain these results is presented. Previously published data are consistent with the idea that the highly conserved C terminus of nascent VP7 could have a cytoplasmic orientation which is important for assembly of mature virus particles.

Expression of the rotavirus SA11 protein VP7 in the simple eukaryote Dictyostelium discoideum

The outer capsid protein of rotavirus, VP7, is a major neutralization antigen and is considered a necessary component of any subunit vaccine developed against rotavirus infection. For this reason, significant effort has been directed towards producing recombinant VP7 that maintains the antigenic characteristics of the native molecule. Using a relatively new expression system, the simple eukaryote Dictyostelium discoideum, we have cloned the portion of simian rotavirus SA11 genome segment 9, encoding the mature VP7 protein, downstream of a native D. discoideum secretion signal sequence in a high-copy-number extrachromosomal vector. The majority of the recombinant VP7 expressed by transformants was intracellular and was detected by Western immunoblot following gel electrophoresis as two or three bands with an apparent molecular mass of 35.5 to 37.5 kDa. A small amount of VP7 having an apparent molecular mass of 37.5 kDa was secreted. Both the intracellular VP7 and the secreted VP7 were N glycosylated and sensitive to endoglycosidase H digestion. Under nonreducing electrophoresis conditions, over half the intracellular VP7 migrated as a monomer while the remainder migrated with an apparent molecular mass approximately twice that of the monomeric form. In an enzyme-linked immunosorbent assay, intracellular VP7 reacted with both nonneutralizing and neutralizing antibodies. The monoclonal antibody recognition pattern paralleled that found with VP7 expressed in either vaccinia virus or herpes simplex virus type 1 and confirms that D. discoideum-expressed VP7 is able to form the major neutralization domains present on viral VP7. Because D. discoideum cells are easy and cheap to grow, this expression system provides a valuable alternative for the large-scale production of recombinant VP7 protein.

Detection of oligomeric and monomeric forms of P-glycoprotein in multidrug resistant cells

P-glycoprotein (P-gp) is thought to function as a drug efflux pump in multidrug resistant (MDR) cells. The functional form of P-gp in its native state is not known. Previous results from radiation target size analysis have suggested that P-gp occurs as dimers in MDR cell plasma membranes [Boscoboinik et al. (1990) Biochim. Biophys. Acta 1027, 225-228]. In this study, we used sucrose gradient velocity sedimentation to determine if P-gp oligomers could be retrieved from detergent extracts of hamster and human MDR cell lines. The proportion of P-gp recovered as higher order oligomers was dependent on the detergents used for solubilization of the cells. When a detergent such as CHAPS was used, 50% or more of the P-gp sedimented as higher order oligomers. In contrast, in the presence of SDS, only monomers were retrieved, but naturally occurring oligomers could be preserved if the cells were treated with a cross-linker prior to detergent solubilization. The oligomers and monomers were both able to bind the photoactive analog of ATP (8-azido[alpha-32P]ATP) or the drug [3H]azidopine in membrane preparations. P-gp is a phosphoprotein, and its phosphorylated state is thought to be important for function. When MDR cells were labeled with [32P]orthophosphate in vivo, we observed that the monomer and dimer were more highly phosphorylated than the larger oligomers, suggesting that these different forms of P-gp may be functionally distinct. The assembly of oligomers appears to occur in an early bisynthetic compartment, and asparagine-linked glycosylation is not required for their formation. Our findings indicate that oligomers of P-gp exist in MDR cells and raise the possibility that the dynamics of oligomer formation and dissociation may be important in the mechanism of action of P-gp.

Retention by the endoplasmic reticulum of rotavirus VP7 is controlled by three adjacent amino-terminal residues

The rotavirus outer capsid glycoprotein, VP7, is an endoplasmic reticulum (ER) membrane-associated glycoprotein in both infected and transfected cells. It was previously demonstrated in this laboratory and by others that both the cleaved signal sequence (H2) and the first NH2-terminal 61 amino acids of VP7 are sufficient and necessary for ER retention of this molecule. Using site-specific mutagenesis and transfection techniques, we show that residues Ile-9, Thr-10, and Gly-11 were specifically necessary for ER retention. These results further define the ER retention sequence of VP7 and demonstrate that conservative changes, apparently innocuous in only three adjacent amino acids, can lead to major solubility and compartmentalization changes. It was found that placement of the first 31 mature NH2-terminal residues of VP7, in addition to the cleaved ER translocation signal sequence, was sufficient to retain the enzymatically active chimeric alpha-amylase in the ER; this enzyme is normally secreted. Deletions of the residues Ile-9, Thr-10, and Gly-11 within the amylase chimera containing 31 VP7 amino acids resulted in secretion of enzymatically active protein. It was also observed that the residues of VP7 presented in certain chimeras were able to abolish alpha-amylase enzymatic activity. These chimeras are presumably misfolded since it was demonstrated by pulse-chase experiments that these molecules are degraded in the ER. We surmise that a favorable conformation is necessary for retention since ER retention and activity of the chimeras depend on the primary sequence context.

Membrane cycling between the ER and Golgi apparatus and its role in biosynthetic transport

Membrane traffic between the ER and Golgi is now recognized as a carefully regulated process controlled by distinct anterograde (to the Golgi) and retrograde (to the ER) pathways. These pathways link two organelles with different morphologies, structures, and localizations within the cell. The ER, which is involved in multiple cellular functions including protein biosynthesis and folding, extends to the cell periphery and forms a dynamic tubule reticulum. By contrast, the Golgi apparatus, which functions in membrane sorting and recycling events, is localized at the center of the cell near the MTOC and is comprised of compact cisternal units. The required transport into the Golgi apparatus of newly synthesized proteins exported from the ER offers a twofold advantage to the cell. First, the rate of movement of membrane and protein through the biosynthetic pathway can be controlled by the selective use of a recycling pathway. Second, membrane moving through the biosynthetic pathway enters a structure specialized for sorting of membrane to different final destinations in the cell Control of biosynthetic transport within the ER/Golgi system involves the utilization of two alternative transport pathways: anterograde (ER to Golgi) and retrograde (Golgi to ER). These two pathways share a common regulatory system involving membrane assembly/disassembly of cytosolic coatomer proteins. Thus, conditions that favor irreversible coatomer binding (i.e., GTP gamma S) inhibit retrograde transport while producing anterograde transport intermediates. Conditions that prevent coatomer binding (i.e., BFA) inhibit anterograde transport and enhance retrograde transport. The underlying biochemical machinery that normally balances anterograde and retrograde membrane traffic between the ER and Golgi is only just beginning to be understood. Any model to explain this system, however, must account for the morphologic characteristics of the membranes involved. Whereas anterograde traffic involves discontinuous "coated" structures moving from peripheral sites in the ER toward the central Golgi, retrograde traffic utilizes continuous "noncoated" tubule structures that move from a central site (i.e., the CGN) to the peripheral ER (see Figure 3). Such a system maximizes volume transport (utilizing vacuolar structures) in the anterograde direction and membrane transport (utilizing tubules) in the retrograde direction. It is therefore ideal for sorting of bulk flow lumenal contents from recycling membrane early in the biosynthetic pathway.

SHR3: a novel component of the secretory pathway specifically required for localization of amino acid permeases in yeast

Mutations in SHR3 block amino acid uptake into yeast by reducing the levels of multiple amino acid permeases within the plasma membrane. SHR3 is a novel integral membrane protein component of the endoplasmic reticulum (ER). shr3 null mutants specifically accumulate amino acid permeases in the ER; other plasma membrane proteins, secretory proteins, and vacuolar proteins are processed and targeted correctly. Our findings suggest that SHR3 interacts with a structural domain shared by amino acid permeases, an interaction required for permease-specific processing and transport from the ER. Even in the presence of excess amino acids, shr3 mutants exhibit starvation responses. shr3 mutants constitutively express elevated levels of GCN4, and mutant shr3/shr3 diploids undergo dimorphic transitions that result in filamentous growth at enhanced frequencies.

Cell biology of viruses that assemble along the biosynthetic pathway

In this review we discuss five groups of viruses that bud into, or assemble from, different compartments along the biosynthetic pathway. These are herpes-, rota-, corona-, bunya- and pox-viruses. Our main emphasis will be on the virally-encoded membrane glycoproteins that are responsible for determining the site of virus assembly. In a number of cases these proteins have been well characterized and appear to serve as resident markers of the budding compartments. The assembly and dissemination of these viruses raises many questions of cell biological interest.

Monoclonal antipeptide antibodies recognize epitopes upon VP4 and VP7 of simian rotavirus SA11 in infected MA104 cells

To study morphogenetic events of rotavirus SA11-infected MA104 cells with strictly defined reagents we produced monoclonal antibodies against synthetic peptides from both outer capsid proteins VP4 (aa residues 228-241: QNTRNIVPVSIVSR) and VP7 (aa residues 319-326: SAAFYYRV) of simian rotavirus SA11. Two of the selected monoclonal antibodies proved to be reactive with determinants of SA11-infected MA104 rhesus monkey kidney cells, with purified SA11 as well as with the particular peptides used for immunization. The anti-VP4 antibody had a demonstrable neutralizing titer of 200 (50% focus reduction) whereas the anti-VP7 MuMAb revealed no detectable neutralizing activity. In peptide-inhibition experiments, the corresponding peptide inhibited its MuMAb whereas the noncorresponding peptide had no effect on antibody binding to intracellular viral antigen. Localization of VP7 was preceded by VP4 as shown by immunofluorescence microscopy.

Golgi retention signals: do membranes hold the key?

The diverse forms and functions of cellular organelles are, presumably, a consequence of their particular molecular compositions. The generation and maintenance of this diversity is achieved by the targeting of newly synthesized proteins to specific locations and their subsequent retention there. Sequences that retain proteins in the endoplasmic reticulum (ER) have been identified at the C-termini of resident ER proteins, where they are readily accessible to potential receptors. By contrast, recent results have demonstrated that retention of proteins in the Golgi complex involves sequences located within transmembrane domains. This suggests the novel possibility that the membrane composition of the Golgi complex plays a role in retention of resident Golgi proteins.

The endoplasmic reticulum retention signal of the E3/19K protein of adenovirus type 2 consists of three separate amino acid segments at the carboxy terminus

The E3/19K protein of adenovirus type 2 is a resident of the ER. Immediately after synthesis it binds to human major histocompatibility complex class I antigens and prevents their departure from the ER compartment. The ER retention signal of the E3/19K protein is contained within the 15 amino acids that protrude on the cytoplasmic side at the carboxy terminus of the protein. To define the ER retention sequence in more detail, we have generated 10 mutants of the E3/19K protein that differ only within this segment. Analysis of the rate of intracellular transport and cell surface expression of HLA antigens associated to these mutants, show that the sequences Ser-Phe-Ile, located in the middle of the 15-residue segment and Met-Pro, at the extreme carboxy terminus, are crucial for retention. Four charged residues, Asp-Glu-Lys-Lys, are located between these two retention elements but are of little or no importance. The basic cluster of amino acids close to the membrane also has some effect on retention. Thus, the retention signal of the E3/19K protein is not a contiguous sequence of amino acids but has a complex spatial arrangement.

The rate of bulk flow from the Golgi to the plasma membrane

A truncated analog of the backbone of sphingomyelin and glycolipids was synthesized. This truncated C8C8 ceramide was soluble in water (but was still able to cross cell membranes) and was utilized by the Golgi apparatus of living cells to produce water-soluble truncated phospholipids and glycolipids that were then secreted into the medium. Sphingomyelin is synthesized in a proximal (likely the cis) Golgi compartment. At 37 degrees C in CHO cells, the sphingomyelin analog is secreted with a half time of about 10 min. With this rate of bulk flow, no special signal is needed to pass through the Golgi to the plasma membrane. At 30 degrees C the half time of secretion of a lumenal ER marker is about 18 min, and that of the truncated sphingomyelin is about 14 min. Comparison of these rates sets an upper limit of about 4 min for half of the ER to be drained into the proximal Golgi at 30 degrees C.

Sequences in rotavirus glycoprotein VP7 that mediate delayed translocation and retention of the protein in the endoplasmic reticulum

Glycosylation and translocation of the simian rotavirus protein VP7, a resident ER protein, does not occur co-translationally in vivo. In pulse-chase experiments in COS cells, nonglycosylated VP7 was still detectable after a 25-min chase period, although the single glycosylation site was only 18 residues beyond the signal peptide cleavage site. After labeling, glycosylated and nonglycosylated VP7 was recovered in microsomes but the latter was sensitive to trypsin (i.e., the nascent protein became membrane associated) but most of it entered the ER posttranslationally because of a rate-limiting step early in translocation. In contrast with the simian protein, bovine VP7 was glycosylated and translocated rapidly. Thus, delayed translocation per se was not required for retention of VP7 in the ER. By constructing hybrid proteins, it was further shown that the signal peptide together with residues 64-111 of the simian protein caused delayed translocation. The same sequences were also necessary and sufficient for retention of simian VP7 in the ER. The data are consistent with the idea that certain proteins are inserted into the ER membrane in a loop configuration.

Identification of the gene encoding the group B rotavirus VP7 equivalent: primary characterization of the ADRV segment 9 RNA

Gene segment 9 of the adult diarrheal rotavirus, ADRV, has been cloned, and the full-length gene 9 cDNA has been sequenced. Sequences at the 5' and 3' termini of the gene 9 double-stranded RNA were confirmed by direct RNA sequencing. The ninth RNA segment is 814 bases in length with a single open reading frame beginning at base 10 and terminating at base 757. The encoded polypeptide contains 249 amino acids with a calculated molecular weight of 28.5 kDa. The protein contains three potential N-linked glycosylation sites and a hydrophobic signal-like sequence at its amino terminus. A search of the NBRF protein data base with the encoded polypeptide revealed extensive similarities with VP7 proteins from a number of group A rotaviruses. Direct comparisons of the ADRV gene 9 polypeptide and the group A rotavirus VP7 demonstrate that the two proteins share 78% amino acid similarity and 28% identity as well as predicted secondary structure similarities. These findings suggest that the ADRV gene segment 9 encodes the VP7 polypeptide equivalent of group A rotaviruses.

Rotavirus proteins VP7, NS28, and VP4 form oligomeric structures

Sucrose gradient sedimentation analysis of rotavirus SA11-infected Ma104 cells revealed the presence of oligomers of VP7, the structural glycoprotein, and NS28, the nonstructural glycoprotein. Cross-linking the proteins, either before or after sucrose gradient centrifugation, stabilizes oligomers, which can be analyzed by nonreducing sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) after immunoprecipitation. The major NS28 oligomer was tetrameric, though dimers and higher-order structures were observed as well. VP7 formed predominantly dimers, and again tetramers and higher oligomeric forms were present. Each oligomer of VP7 and NS28 sedimented at the same characteristic rate through the sucrose gradient either in the presence or absence of cross-linking. Monomers could not be cross-linked to form oligomers, demonstrating that cross-linked oligomers were not artifactually derived from monomers. Reversing the cross-linking of immunoprecipitated VP7 on reducing SDS-PAGE resulted in the appearance of only the monomeric form of VP7. Dissociation of the NS28 oligomers resulted in stable dimers as well an monomers. In the faster-sedimenting fractions, a 16S to 20S complex, which contained the rotavirus outer shell proteins VP7 and VP4 cross-linked to NS28, was observed. These complexes were shown not to be associated with any known subviral particle. The association of VP4, NS28, and VP7 may represent sites on the endoplasmic reticulum membrane that participate in the budding of the single-shelled particles into the lumen of the endoplasmic reticulum, where maturation to double-shelled particles occurs.

Cell type-specific post-Golgi apparatus localization of a "resident" endoplasmic reticulum glycoprotein, glucosidase II

Glucosidase II, an asparagine-linked oligosaccharide processing enzyme, is a resident glycoprotein of the endoplasmic reticulum. In kidney tubular cells, in contrast to previous findings on hepatocytes, we found by light and electron microscopy immunoreactivity for glucosidase II predominantly in post-Golgi apparatus structures. The majority of immunolabel was in endocytotic structures beneath the plasma membrane. Immunoprecipitation confirmed presence of the glucosidase II subunit in purified brush border preparations. Kidney glucosidase II contained species carrying endo H-sensitive, high mannose as well as endo H-resistant oligosaccharide chains. Some species of glucosidase II contained sialic acid. The sialylated species were enzymatically active. This study demonstrates than an enzyme presumed to be a resident of the endoplasmic reticulum may show alternative localizations in some cell types.

Preferred apical distribution of glycosyl-phosphatidylinositol (GPI) anchored proteins: a highly conserved feature of the polarized epithelial cell phenotype

We use a sensitive biotin polarity assay to survey the surface distribution of glycosyl-phosphatidylinositol (GPI) anchored proteins in five model epithelial cell lines derived from different species (dog, pig, man) and tissues, i.e., kidney (MDCK I, MDCK II, LLC-PK1) and intestine (Caco-2 and SK-CO15). After biotinylation of apical or basolateral surfaces of confluent monolayers grown on polycarbonate filters, GPI-anchored proteins are identified by their shift from a Triton X-114 detergent-rich phase to a detergent-poor phase in the presence of phosphatidylinositol-specific phospholipase C. All GPI-anchored proteins detected (3-9 per cell type, at least 13 different proteins) are found to be apically polarized; no GPI-anchored protein is observed preferentially localized to the basal surface. One of the GPI-anchored proteins is identified as carcinoembryonic antigen (CEA). Survey of MDCK II-RCAr, a mutant cell line with a pleiotropic defect in galactosylation of glycoproteins and glycolipids (that presumably affects GPI anchors) also reveals an apical polarization of all GPI-anchored proteins. In contrast, analysis of MDCK II-ConAr (a mutant cell line with an unknown defect in glycosylation) revealed five GPI-anchored proteins, two of which appeared relatively unpolarized. Our results indicate that the polarized apical distribution of GPI-anchored proteins is highly conserved across species and tissue-type and may depend on glycosylation.

Novel N-terminal amino acid sequence required for retention of a hepatitis B virus glycoprotein in the endoplasmic reticulum

The preS1 surface glycoprotein of hepatitis B virus is targeted to the endoplasmic reticulum (ER) and is retained in this organelle when expressed in the absence of other viral gene products. The protein is also acylated at its N terminus with myristic acid. Sequences responsible for its ER retention have been identified through examination of mutants bearing lesions in the preS1 coding region. These studies reveal that such sequences map to the N terminus of the molecule, between residues 6 and 19. Molecules in which this region was present remained in the ER; those in which it had been deleted were secreted from the cell. Although all deletions which allowed efficient secretion also impaired acylation of the polypeptide, myristylation alone was not sufficient for ER retention: point mutations which eliminated myristylation did not lead to secretion. These data indicate that an essential element for ER retention resides in a 14-amino-acid sequence that is unrelated to previously described ER retention signals.

Rotavirus gene structure and function

Knowledge of the structure and function of the genes and proteins of the rotaviruses has expanded rapidly. Information obtained in the last 5 years has revealed unexpected and unique molecular properties of rotavirus proteins of general interest to virologists, biochemists, and cell biologists. Rotaviruses share some features of replication with reoviruses, yet antigenic and molecular properties of the outer capsid proteins, VP4 (a protein whose cleavage is required for infectivity, possibly by mediating fusion with the cell membrane) and VP7 (a glycoprotein), show more similarities with those of other viruses such as the orthomyxoviruses, paramyxoviruses, and alphaviruses. Rotavirus morphogenesis is a unique process, during which immature subviral particles bud through the membrane of the endoplasmic reticulum (ER). During this process, transiently enveloped particles form, the outer capsid proteins are assembled onto particles, and mature particles accumulate in the lumen of the ER. Two ER-specific viral glycoproteins are involved in virus maturation, and these glycoproteins have been shown to be useful models for studying protein targeting and retention in the ER and for studying mechanisms of virus budding. New ideas and approaches to understanding how each gene functions to replicate and assemble the segmented viral genome have emerged from knowledge of the primary structure of rotavirus genes and their proteins and from knowledge of the properties of domains on individual proteins. Localization of type-specific and cross-reactive neutralizing epitopes on the outer capsid proteins is becoming increasingly useful in dissecting the protective immune response, including evaluation of vaccine trials, with the practical possibility of enhancing the production of new, more effective vaccines. Finally, future analyses with recently characterized immunologic and gene probes and new animal models can be expected to provide a basic understanding of what regulates the primary interactions of these viruses with the gastrointestinal tract and the subsequent responses of infected hosts.

Novel N-terminal amino acid sequence required for retention of a hepatitis B virus glycoprotein in the endoplasmic reticulum

The preS1 surface glycoprotein of hepatitis B virus is targeted to the endoplasmic reticulum (ER) and is retained in this organelle when expressed in the absence of other viral gene products. The protein is also acylated at its N terminus with myristic acid. Sequences responsible for its ER retention have been identified through examination of mutants bearing lesions in the preS1 coding region. These studies reveal that such sequences map to the N terminus of the molecule, between residues 6 and 19. Molecules in which this region was present remained in the ER; those in which it had been deleted were secreted from the cell. Although all deletions which allowed efficient secretion also impaired acylation of the polypeptide, myristylation alone was not sufficient for ER retention: point mutations which eliminated myristylation did not lead to secretion. These data indicate that an essential element for ER retention resides in a 14-amino-acid sequence that is unrelated to previously described ER retention signals.

The signal peptide of the rotavirus glycoprotein VP7 is essential for its retention in the ER as an integral membrane protein

The rotavirus glycoprotein VP7 has a cleavable signal peptide and is normally resident as an integral membrane protein in the ER of infected cells. A gene was constructed in which the VP7 H2 signal peptide was replaced by one from influenza hemagglutinin. COS cells transfected with this gene produced VP7 with the correct amino terminus, but the protein was rapidly secreted. Uncleaved VP7 from either precursor was not detected in cells after brief pulse-labeling, suggesting that the signal peptide was not acting as a temporary anchor; rather, it exerted its effect despite rapid cleavage. By splicing the H2 signal peptide onto another reporter protein, the malaria S-antigen, we demonstrated that H2 was necessary, but not itself sufficient, for targeting and retention. We propose that an interaction between the cleaved signal peptide and other downstream sequences in VP7 is required for retention of this protein in the ER as an integral membrane polypeptide.