Polylactosaminoglycan modification of a small integral membrane glycoprotein, influenza B virus NB

MERS-CoV and SARS-CoV-2 membrane proteins are modified with polylactosamine chains

Coronaviruses are positive-stranded RNA enveloped viruses. The helical nucleocapsid is surrounded by a lipid bilayer in which are anchored three viral proteins: the spike (S), membrane (M) and envelope (E) proteins. The M protein is the major component of the viral envelope and is believed to be its building block. The M protein of Middle East respiratory syndrome coronavirus (MERS-CoV) and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) contains a short N-terminal domain with an N-glycosylation site. We investigated their N-glycosylation and show that polylactosamine chains are conjugated to SARS-CoV-2 and MERS-CoV M proteins in transfected and infected cells. Acidic residues present in the first transmembrane segments of the proteins are required for their glycosylation. No specific signal to specify polylactosamine conjugation could be identified and high mannose-conjugated protein was incorporated into virus-like particles.

NB protein does not affect influenza B virus replication in vitro and is not required for replication in or transmission between ferrets

The influenza B virus encodes a unique protein, NB, a membrane protein whose function in the replication cycle is not, as yet, understood. We engineered a recombinant influenza B virus lacking NB expression, with no concomitant difference in expression or activity of viral neuraminidase (NA) protein, an important caveat since NA is encoded on the same segment and initiated from a start codon just 4 nt downstream of NB. Replication of the virus lacking NB was not different to wild-type virus with full-length NB in clonal immortalized or complex primary cell cultures. In the mouse model, virus lacking NB induced slightly lower IFN-α levels in infected lungs, but this did not affect virus titres or weight loss. In ferrets infected with a mixture of viruses that did or did not express NB, there was no fitness advantage for the virus that retained NB. Moreover, virus lacking NB protein was transmitted following respiratory droplet exposure of sentinel animals. These data suggest no role for NB in supporting replication or transmission in vivo in this animal model. The role of NB and the nature of selection to retain it in all natural influenza B viruses remain unclear.

Uukuniemi Phlebovirus assembly and secretion leave a functional imprint on the virion glycome

Uukuniemi virus (UUKV) is a model system for investigating the genus Phlebovirus of the Bunyaviridae. We report the UUKV glycome, revealing differential processing of the Gn and Gc virion glycoproteins. Both glycoproteins display poly-N-acetyllactosamines, consistent with virion assembly in the medial Golgi apparatus, whereas oligomannose-type glycans required for DC-SIGN-dependent cellular attachment are predominant on Gc. Local virion structure and the route of viral egress from the cell leave a functional imprint on the phleboviral glycome.

Palmitoylation is required for intracellular trafficking of influenza B virus NB protein and efficient influenza B virus growth in vitro

All influenza viruses bud and egress from lipid rafts within the apical plasma membrane of infected epithelial cells. As a result, all components of progeny virions must be transported to these lipid rafts for assembly and budding. Although the mechanism of transport for other influenza proteins has been elucidated, influenza B virus (IBV) glycoprotein NB subcellular localization and transport are not understood completely. To address the aforementioned properties of NB, a series of trafficking experiments were conducted. Here, we showed that NB co-localized with markers specific for the endoplasmic reticulum (ER) and Golgi region. The data from chemical treatment of NB-expressing cells by Brefeldin A, a fungal antibiotic and a known chemical inhibitor of the protein secretory pathway, further confirmed that NB is transported through the ER-Golgi pathway as it restricted NB localization to the perinuclear region. Using NB deletion mutants, the hydrophobic transmembrane domain was identified as being required for NB transport to the plasma membrane. Furthermore, palmitoylation was also required for transport of NB to the plasma membrane. Systematic mutation of cysteines to serines in NB demonstrated that cysteine 49, likely in a palmitoylated form, is also required for transport to the plasma membrane. Surprisingly, further analysis demonstrated that in vitro replication of NBC49S mutant virus was delayed relative to the parental IBV. The results demonstrated that NB is the third influenza virus protein to have been shown to be palmitoylated and together these findings may aid in future studies aimed at elucidating the function of NB.

Suppression of acute anti-friend virus CD8+ T-cell responses by coinfection with lactate dehydrogenase-elevating virus

Friend virus (FV) and lactate dehydrogenase-elevating virus (LDV) are endemic mouse viruses that can cause long-term chronic infections in mice. We found that numerous mouse-passaged FV isolates also contained LDV and that coinfection with LDV delayed FV-specific CD8(+) T-cell responses during acute infection. While LDV did not alter the type of acute pathology induced by FV, which was severe splenomegaly caused by erythroproliferation, the immunosuppression mediated by LDV increased both the severity and the duration of FV infection. Compared to mice infected with FV alone, those coinfected with both FV and LDV had delayed CD8(+) T-cell responses, as measured by FV-specific tetramers. This delayed response accounted for the prolonged and exacerbated acute phase of FV infection. Suppression of FV-specific CD8(+) T-cell responses occurred not only in mice infected concomitantly with LDV but also in mice chronically infected with LDV 8 weeks prior to infection with FV. The LDV-induced suppression was not mediated by T regulatory cells, and no inhibition of the CD4(+) T-cell or antibody responses was observed. Considering that most human adults are carriers of chronically infectious viruses at the time of new virus insults and that coinfections with viruses such as human immunodeficiency virus and hepatitis C virus are currently epidemic, it is of great interest to determine how infection with one virus may impact host responses to a second infection. Coinfection of mice with LDV and FV provides a well-defined, natural host model for such studies.

The NB protein of influenza B virus is not necessary for virus replication in vitro

The NB protein of influenza B virus is thought to function as an ion channel and therefore would be expected to have an essential function in viral replication. Because direct evidence for its absolute requirement in the viral life cycle is lacking, we generated NB knockout viruses by reverse genetics and tested their growth properties both in vitro and in vivo. Mutants not expressing NB replicated as efficiently as the wild-type virus in cell culture, whereas in mice they showed restricted growth compared with findings for the wild-type virus. Thus, the NB protein is not essential for influenza B virus replication in cell culture but promotes efficient growth in mice.

Influenza C virus CM2 integral membrane glycoprotein is produced from a polypeptide precursor by cleavage of an internal signal sequence

The influenza C virus CM2 protein is a small glycosylated integral membrane protein (115 residues) that spans the membrane once and contains a cleavable signal sequence at its N terminus. The coding region for CM2 (CM2 ORF) is located at the C terminus of the 342-amino acid (aa) ORF of a colinear mRNA transcript derived from influenza C virus RNA segment 6. Splicing of the colinear transcript introduces a translational stop codon into the ORF and the spliced mRNA encodes the viral matrix protein (CM1) (242 aa). The mechanism of CM2 translation was investigated by using in vitro and in vivo translation of RNA transcripts. It was found that the colinear mRNA derived from influenza C virus RNA segment 6 serves as the mRNA for CM2. Furthermore, CM2 translation does not depend on any of the three in-frame methionine residues located at the beginning of CM2 ORF. Rather, CM2 is a proteolytic cleavage product of the p42 protein product encoded by the colinear mRNA: a cleavage event that involves the recognition and cleavage of an internal signal peptide presumably by signal peptidase resident in the endoplasmic reticulum. Alteration of the predicted signal peptidase cleavage site by mutagenesis blocked generation of CM2. The other polypeptide species resulting from the cleavage of p42, designated p31, contains the CM1 coding region and an additional C-terminal 17 aa (formerly the CM2 signal peptide). Protein p31, in comparison to CM1, displays characteristics of an integral membrane protein.

Characterization of a second protein (CM2) encoded by RNA segment 6 of influenza C virus

The biochemical properties of a second protein (CM2) encoded by RNA segment 6 of influenza C virus were investigated. Three forms of CM2 with different electrophoretic mobilities (CM2(0), CM2a, and CM2b) were detected in infected cells by immunoprecipitation with antiserum to the glutathione S-transferase (GST)-CM2 fusion protein. Treatment of infected cells with tunicamycin and digestion of immunoprecipitated proteins with endoglycosidase H or peptide-N-glycosidase F suggested that a mannose-rich oligosaccharide core is added to unglycosylated CM2(0) (Mr, approximately 16,000) to form CM2a (Mr, approximately 18,000) and that the processing of the carbohydrate chain from the high-mannose type to the complex type converts CM2a into CM2b, which is heterogeneous in electrophoretic mobility (Mr, approximately 22,000 to 30,000). Labeling of infected cells with [3H]palmitic acid showed that CM2 is fatty acylated. The fatty acid bond was sensitive to treatment with hydroxylamine and mercaptoethanol, which indicates a labile thioester-type linkage. The CM2 protein was also found to form disulfide-linked dimers and tetramers on sodium dodecyl sulfate-polyacrylamide gels under nonreducing conditions. Trypsin treatment of infected cell surfaces as well as of microsome vesicles from infected cells followed by immunoprecipitation with antiserum to the GST fusion protein containing the 56 C-terminal amino acid residues of CM2 suggested that this C-terminal domain is intracellular and exposed to the cytoplasms of microsomes. Furthermore, evidence that a small amount of CM2 is incorporated into progeny virus particles was obtained by Western blot analysis. These results, altogether, suggest that CM2 is an integral membrane protein with biochemical properties similar to those of influenza A virus M2 and influenza B virus NB proteins.

Differential glycosylation, virion incorporation, and sensitivity to neutralizing antibodies of human immunodeficiency virus type 1 envelope produced from infected primary T-lymphocyte and macrophage cultures

Two primary cell targets for human immunodeficiency virus type 1 (HIV-1) infection in vivo are CD4+ T lymphocytes and monocyte-derived macrophages (MDM). HIV-1 encodes envelope glycoproteins which mediate virus entry into these cells. We have utilized infected and radiolabelled primary peripheral blood mononuclear cell (PBMC) and MDM cultures to examine the biochemical and antigenic properties of the HIV-1 envelope produced in these two cell types. The gp120 produced in MDM migrates as a broad, diffuse band in sodium dodecyl sulfate-polyacrylamide gel electrophoresis gels compared with that of the more homogeneous gp120 released from PBMCs. Glycosidase analyses indicated that the diffuse appearance of the MDM gp120 is due to the presence of asparagine-linked carbohydrates containing lactosaminoglycans, a modification not observed with the gp120 produced in PBMCs. Neutralization experiments, using isogeneic PBMC and MDM-derived macrophage-tropic HIV-1 isolates, indicate that 8- to 10-fold more neutralizing antibody, directed against the viral envelope, is required to block virus produced from MDM. These results demonstrate that HIV-1 released from infected PBMC and MDM cultures differs in its biochemical and antigenic properties.

The ion channel activity of the influenza virus M2 protein affects transport through the Golgi apparatus

High level expression of the M2 ion channel protein of influenza virus inhibits the rate of intracellular transport of the influenza virus hemagglutinin (HA) and that of other integral membrane glycoproteins. HA coexpressed with M2 is properly folded, is not associated with GRP78-BiP, and trimerizes with the same kinetics as when HA is expressed alone. Analysis of the rate of transport of HA from the ER to the cis and medial golgi compartments and the TGN indicated that transport through the Golgi apparatus is delayed. Uncleaved HA0 was not expressed at the cell surface, and accumulation HA at the plasma membrane was reduced to 75-80% of control cells. The delay in intracellular transport of HA on coexpression of M2 was not observed in the presence of the M2-specific ion channel blocker, amantadine, indicating that the Golgi transport delay is due to the M2 protein ion channel activity equilibrating pH between the Golgi lumen and the cytoplasm, and not due to saturation of the intracellular transport machinery. The Na+/H+ ionophore, monensin, which also equilibrates pH between the Golgi lumen and the cytoplasm, caused a similar inhibition of intracellular transport as M2 protein expression did for HA and other integral membrane glycoproteins. EM data showed a dilation of Golgi cisternae in cells expressing the M2 ion channel protein. Taken together, the data suggest a similarity of effects of M2 ion channel activity and monensin on intracellular transport through the Golgi apparatus.

Lactate dehydrogenase-elevating virus: an ideal persistent virus?

LDV contradicts all commonly held views about mechanisms of virus persistence, namely that persistence is primarily associated with noncytopathic viruses, or the selection of immune escape variants or other mutants, or a decrease in expression of certain viral proteins by infected cells, or replication in “immune-privileged sites”, or a general suppression of the host immune system, etc. [1, 2, 5, 54, 77, 78]. LDV is a highly cytocidal virus that invariably establishes a life-long, viremic, persistence in mice, in spite of normal anti-viral immune responses.

One secret of LDV's success in persistence is its specificity for a renewable, nonessential population of cells that is continuously regenerated, namely a subpopulation of macrophages. Since the continuous destruction of these cells is not associated with any obvious health effects, this macrophage population seems nonessential to the well-being of its host. The only function identified for this subpopulation of macrophages is clearance of the muscle type of LDH and some other enzymes [59, 67, 68]. Furthermore, the effects of LDV infection on the host immune system, namely the polyclonal activation of B cells and its associated production of autoantibodies, and the slight impairment of primary and secondary antibody responses also do not seem to be severe enough to cause any clinical consequences.

But how does LDV replication in macrophages escape all host defenses? Persistence is not dependent on the seletion of immune escape variants or other mutants ([58] and Palmer, Even and Plagemann, unpublished results). Also, LDV replication is not restricted to immune-privileged sites [5]. LDV replication persists in the liver, lymphoidal tissues and testis [66]. Only the latter could be considered a site not readily accessible to immune surveillance.

Most likely, resistance of LDV replication to antiviral immune responses is related to the unique structure of its envelope proteins and the production of large quantities of viral antigens. High titers of anti-LDV antibodies are generated in infected mice but they neutralize LDV infectivity only very inefficiently and, even though the antiviral antibodies are mainly of the IgG2a and IgG2b isotypes, they do not mediate complement lyses of virions [31]. Interaction of the antibodies and complement with the VP-3/VP-2 heterodimers in the viral envelope may be impeded by the exposure of only very short peptide segments of these proteins at the envelope surface and the presence of large oligosaccharide side chains. Furthermore, since LDV maturation is restricted to intracytoplasmic cisternae [59, 71], the question arises of whether any of the viral proteins are available on the surface of infected cells for ADCC.

CTLs also fail to control LDV replication. Altough CTLs specific for N/VP-1 are rapidly generated, these have disappeared by 30 days p.i. [26]. The reasons for this loss are unknown, but high-dose clonal exhaustion [41, 51, 77, 78] is a reasonable possibility since, regardless of the infectious dose, large amounts of LDV proteins are present in all the lymphoidal tissues at the time of the induction of the CTL response. Furthermore, after exhaustion of CTLs in the periphery, continuous replication of LDV in the thymus [65] assures that the mice become permanently immunologically tolerant with respect to LDV antigen-specific CTLs as a result of negative selection in the thymus. LDV might be a primary example for the effectiveness of a permanent clonal CTL deletion in adult animals under natural conditions of infection.

The presumed modes of transmission of LDV in nature and the events associated with its infection of mice are strikingly similar to those observed during the acute and asymptomatic phases of infection with human immunodeficiency virus (HIV) [24, 29, 74, 78]. These include: (1) primary inefficient transmission via sexual and transplacental routes but effective transmission via blood; (2) primary replication in renewable populations of lymphoidal cells with production of large amounts of virus after the initial infection of the host followed by persistent low level of viremia in spite of antiviral immune responses; (3) persistence, reflecting continuous rounds of productive, cytocidal infection of permissive cells [59, 74] and the rate of generation of permissive cells which may be the main factor in determining the level of virus production (in the case of HIV, the rate of activation of CD4⁺ T cells to support a productive HIV replication might be the factor determining the rate of virus production and the progression of the disease); (4) rapid antibody formation but delayed production of neutralizing antibodies with limited neutralizing capacity; (5) rapid but transient generation of virus-specific CTLs; and (6) accumulation of large amounts of virus in newly formed germinal centers in the spleen and lymph nodes concomitant with an initiation of a permanent polyclonal activation of B cells resulting in an elevation of plasma IgG2a.

The events described under points 2–6 might be generally associated with natural viremic persistent virus infections. Such persistent viruses, by necessity, have evolved properties that allow them to escape all host defenses and control of their infection by immunological processes is, therefore, difficult, if not impossible. Prevention of infection and chemotherapy may be the only approaches available to combat such virus infections.

Influenza A virus M2 ion channel protein: a structure-function analysis

A structure-function analysis of the influenza A virus M2 ion channel protein was performed. The M2 protein of human influenza virus A/Udorn/72 and mutants containing changes on one face of the putative alpha helix of the M2 transmembrane (TM) domain, several of which lead to amantadine resistance when found in virus, were expressed in oocytes of Xenopus laevis. The membrane currents of oocytes expressing mutant M2 ion channels were measured at both normal and low pH, and the amantadine-resistant mutant containing the change of alanine at residue 30 to threonine was found to have a significantly attenuated low pH activation response. The specific activity of the channel current of the amantadine-resistant mutants was investigated by measuring the membrane current of individual oocytes followed by quantification of the amount of M2 protein expressed in these single oocytes by immunoblotting analysis. The data indicate that changing residues on this face of the putative alpha helix of the M2 TM domain alters properties of the M2 ion channel. Some of the M2 proteins containing changes in the TM domain were found to be modified by addition of an N-linked carbohydrate chain at an asparagine residue that is membrane proximal and which is not modified in the wild-type M2 protein. These N-linked carbohydrate chains were further modified by addition of polylactosaminoglycan. A glycosylated M2 mutant protein (M2 + V, A30T) exhibited an ion channel activity with a voltage-activated, time-dependent kinetic component. Prevention of carbohydrate addition did not affect the altered channel activity. The ability of the M2 protein to tolerate deletions in the TM domain was examined by expressing three mutants (del29-31, del28-31, and del27-31) containing deletions of three, four, and five residues in the TM domain. No ion channel activity was detected from expression of M2 del29-31 and del27-31, whereas expression of M2 del28-31 resulted in an ion channel activity that was activated by hyperpolarization (and not low pH) and was resistant to amantadine block. Examination of the oligomeric form of M2 del28-31 indicated that the oligomer is different from wild-type M2, and the data were consistent with M2 del28-31 forming a pentamer.

Glycosylation requirements for intracellular transport and function of the hemagglutinin of influenza virus

The contribution of each of the seven asparagine-linked oligosaccharide side chains on the hemagglutinin of the A/Aichi/68 (X31) strain of influenza virus was assessed with respect to its effect on the folding, intracellular transport, and biological activities of the molecule. Twenty mutant influenza virus hemagglutinins were constructed and expressed, each of which had one or more of the seven glycosylation sites removed. Investigations of these mutant hemagglutinins indicated that (i) no individual oligosaccharide side chain is necessary or sufficient for the folding, intracellular transport, or function of the molecule, (ii) at least five oligosaccharide side chains are required for the X31 hemagglutinin molecule to move along the exocytic pathway to the plasma membrane, and (iii) mutant hemagglutinins having less than five oligosaccharide side chains form intracellular aggregates and are retained in the endoplasmic reticulum.

Multiple glycoproteins synthesized by the smallest RNA segment (S10) of bluetongue virus

The genome of bluetongue virus, an orbivirus, consists of 10 double-stranded RNAs, each encoding at least one polypeptide. The smallest RNA segment (S10) encodes two minor nonstructural proteins, NS3 and NS3A, the structures and functions of which are not understood. We have expressed these two proteins in mammalian cells by using the T7 cytoplasmic transient expression system. Using a deletion mutant (lacking the first AUG initiation codon), we have demonstrated that the second initiation codon is used to initiate the synthesis of NS3A protein and that the two initiation codons are responsible for the synthesis not only of NS3 and NS3A but also of high-molecular-weight forms of both proteins. These higher-molecular-weight forms (GNS3 and GNS3A) are glycosylated. We have also demonstrated that the carbohydrate chains of GNS3 and GNS3A could be further modified by heterogeneous extension to polylactosaminoglycan forms. The glycosylated and nonglycosylated forms are found in similar intracellular locations in the Golgi complex. In the presence of cycloheximide, NS3 and NS3A immunofluorescence staining was pronounced in the Golgi complex, confirming that NS3 and NS3A are competent for transport to the Golgi apparatus after synthesis. We conclude that S10 gene products are integral membrane glycoproteins.

Structural proteins of equine arteritis virus

We have recently shown that the genome of equine arteritis virus (EAV) contains seven open reading frames (ORFs). We now present data on the structural proteins of EAV and the assignment of their respective genes. Virions are composed of a 14-kDa nucleocapsid protein (N) and three membrane proteins designated M, GS, and GL. M is an unglycosylated protein of 16 kDa, and GS and GL are N-glycosylated proteins of 25 and 30 to 42 kDa, respectively. The broad size distribution of GL results from heterogeneous N-acetyllactosamine addition since it is susceptible to digestion by endo-beta-galactosidase. Using monospecific antisera as well as an antivirion serum, and by expression of individual ORFs, the genes for the structural proteins were identified: ORF 7 codes for N, ORF 6 for M, ORF 5 for GL, and ORF 2 for GS. With the exception of GS, the proteins are about equally abundant in EAV virions, being present at a molar ratio of 3 (N):2 (M):3 (GL). The GS protein, which is expressed at a level similar to that of M in infected cells, is strikingly underrepresented in virus particles (1 to 2%). Our data justify a distinct taxonomic position for EAV, together with lactate dehydrogenase-elevating virus and simian hemorrhagic fever virus; although coronavirus- and toroviruslike in features of transcription and translation, the virion architecture of EAV is fundamentally different.

New aspects of influenza viruses

Influenza virus infections continue to cause substantial morbidity and mortality with a worldwide social and economic impact. The past five years have seen dramatic advances in our understanding of viral replication, evolution, and antigenic variation. Genetic analyses have clarified relationships between human and animal influenza virus strains, demonstrating the potential for the appearance of new pandemic reassortants as hemagglutinin and neuraminidase genes are exchanged in an intermediate host. Clinical trials of candidate live attenuated influenza virus vaccines have shown the cold-adapted reassortants to be a promising alternative to the currently available inactivated virus preparations. Modern molecular techniques have allowed serious consideration of new approaches to the development of antiviral agents and vaccines as the functions of the viral genes and proteins are further elucidated. The development of techniques whereby the genes of influenza viruses can be specifically altered to investigate those functions will undoubtedly accelerate the pace at which our knowledge expands.

The 1A protein of respiratory syncytial virus is an integral membrane protein present as multiple, structurally distinct species

The respiratory syncytial virus (RSV) 1A protein was previously identified as a 7.5-kilodalton (kDa) nonglycosylated species that, on the basis of its predicted sequence determined from the sequence of its mRNA, contains a hydrophobic central domain that was suggestive of membrane interaction. Here, four major, structurally distinct intracellular species of the 1A protein were identified in cells infected by RSV or by a recombinant vaccinia virus expressing the 1A gene. The four species of 1A were: (i) the previously described, nonglycosylated 7.5-kDa species that appeared to be the full-length, unmodified 1A protein; (ii) a nonglycosylated 4.8-kDa species that was carboxy-coterminal with the 7.5-kDa species and might be generated by translational initiation at the second AUG in the sequence; (iii) a 13- to 15-kDa species that contained one or two N-linked carbohydrate side chains of the high-mannose type; and (iv) a 21- to 30-kDa glycosylated species that appeared to be generated from the 13- to 15-kDa species by further modification of the N-linked carbohydrate. All four forms of the 1A protein were synthesized and processed on intracellular membranes, and several lines of biochemical evidence showed that all four species were integral membrane proteins. Thus, the 1A protein is a third RSV integral membrane protein and is present as such in both glycosylated and nonglycosylated forms. With the use of antiserum raised against a synthetic peptide representing the C terminus of the 1A protein, indirect immunofluorescence showed that the 1A protein was expressed at the cell surface. Antibody-antigen complexes formed at the surface of intact infected cells were immunoprecipitated, showing that the 7.5-kDa, 13- to 15-kDa, and 21- to 30-kDa, but not the 4.8-kDa, species, were accessible to extracellular antibodies. Thus, the 1A protein is a candidate to be a viral surface antigen. The small size, gene map location integral membrane association, and cell surface expression of the 1A protein strongly suggested that it is a counterpart to the SH protein that has been described for simian virus type 5. We suggest that, in the future, the RSV 1A protein be given the same designation, namely, SH.

Cell surface expression and orientation in membranes of the 44-amino-acid SH protein of simian virus 5

Antiserum was raised against a synthetic peptide containing the N-terminal hydrophilic domain of the small hydrophobic protein (SH) of simian virus 5 (SV5) and used to characterize properties of the SH protein. SH demonstrated properties of an integral membrane protein. Indirect immunofluorescence experiments showed that the protein is involved in the exocytotic pathway, and isolation of plasma membranes from SV5-infected cells showed an enrichment of SH, indicating that SH is transported to the infected-cell surface. Biochemical analysis of the orientation of SH in membranes by proteolysis of intact SV5-infected cell surfaces and intracellular microsomal vesicles indicated that SH is oriented in membranes with its N-terminal hydrophilic domain exposed on the cytoplasmic face of the plasma membrane and the C terminus of approximately five amino acid residues exposed at the cell surface. These data are discussed with respect to positive-acting signals being necessary in the ectodomain of SH for cell surface expression.