Purification and characterization of an early glycoprotein from adenovirus type 2-infected cells

Transcription mapping and characterization of 284R and 121R proteins produced from early region 3 of bovine adenovirus type 3

We established the transcription map of early region (E) 3 of bovine adenovirus 3 (BAV-3) by Northern blot, S1 nuclease protection assays, cDNA sequencing, and RT-PCR analysis. Five major classes of mRNAs were identified, which shared the 3' ends. Four classes of mRNAs transcribed from the E3 promoter also shared the 5' end, while one major class of mRNA transcribed from the major late promoter contained a tripartite leader sequence at the 5' end. These five transcripts have the potential to encode four proteins, namely 284R, 121R, 86R, and 82R. To identify the proteins, rabbit antiserum was prepared using a bacterial fusion protein encoding 284R or 121R protein. Serum against 284R immunoprecipitated protein of 26-32 kDa in in vitro translated and transcribed mRNA and three proteins of 48, 67, and 125 kDa from BAV-3-infected cells. Western blots and enzymatic digestions confirmed that the 284R protein is a glycoprotein, which contains only N-linked oligosaccharides, both high mannose (48 kDa) and complex types (67 kDa). Serum against 121R immunoprecipitated a protein of 14.5 kDa from in vitro translated and transcribed mRNA and BAV-3-infected cells. Although 121R protein shows limited sequence similarity to a 14.7-kDa protein of human adenovirus 5, the 284R protein appears to be unique to BAV-3. Since proteins encoded by the E3 region appear to influence adenovirus pathogenesis, the 284R protein may contribute to the unique pathogenic properties of BAV-3.

Molecular basis of immune evasion strategies by adenoviruses

Human adenoviruses have provided valuable insights into virus-host interactions at the clinical and experimental levels. In addition to the medical importance of adenoviruses in acute infections and the ability of the virus to persist in the host, adenovirus-based recombinants are being developed as potential vaccine vectors. It is now clear that adenoviruses employ various strategies to modulate the innate and the adaptive host immune defences. Adenovirus genome-coded products that interact with the immune response of the host have been identified, and to a large extent the molecular mechanisms of their functions have been revealed. Such knowledge will no doubt influence our approach to the areas of viral pathogenesis, vaccine development and immune modulation for disease management.

Analysis of early region 3 mutants of mouse adenovirus type 1

Early region 3 (E3) of mouse adenovirus type 1 has the potential to produce three proteins which have identical amino termini but unique carboxy-terminal sequences. Three recombinant deletion viruses were constructed so that each could produce only one of the three E3 proteins. A fourth mutant that should produce no E3 proteins was also constructed. These recombinants were able to grow in mouse 3T6 cells and produced wild-type levels of viral mRNAs and proteins except for those specifically deleted by the mutations. Early mRNA production from the mutant viruses was analyzed by reverse transcriptase PCR and confirmed that each deletion mutant would be able to produce only one of the three E3 proteins. Late mRNA production was analyzed by Northern (RNA) blotting and found to be similar in wild-type and mutant viruses. Capsid morphology was unaltered in the mutant viruses as seen by electron microscopy. Immunoprecipitation of E3 proteins from infections of mouse 3T6 cells using an antiserum specific for all three E3 proteins was used to examine the effect of the introduced mutations on protein expression. Two mutants produced only one class of E3 protein as predicted from their specific mutations and mRNA expression profiles. One mutant virus failed to produce any detectable E3 proteins. The predicted E3-null mutant was found to be leaky and could produce low levels of E3 proteins. Outbred Swiss mice were infected with the E3 mutant viruses to determine if the E3 proteins have an effect on the pathogenicity of the virus in mice. All of the mutants showed decreased pathogenicity as determined by increased 50% lethal doses, indicating that the proteins of the E3 region are important determinants of the pathogenesis of mouse adenovirus in its natural host.

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.

A 6700 MW membrane protein is encoded by region E3 of adenovirus type 2

There is an open reading frame between ATG1022 and TGA1205 in the E3 transcription unit of adenovirus 2 that could encode a protein of MW 6700 (6.7K) (61 amino acids). To address whether this protein is expressed, we prepared an antiserum against a synthetic peptide corresponding to residues 47-61 in the 6.7K protein. This antiserum immunoprecipitated two series of protein bands, a 7K-8K doublet and a 15K-16K doublet or triplet, as observed by electrophoresis on 10-18% gradient SDS-polyacrylamide gels. These bands were not obtained from cells infected with mutants that lack the 6.7K gene. Most, if not all, of the 7K-8K and 15K-16K bands were detected by immunoblot, indicating that they are modified versions of the 6.7K protein. Only an 8K band was observed after cell-free translation of hybridization-purified mRNA, suggesting that this may be the primary translation product. As judged by DNA sequence, the 6.7K protein has a hydrophobic domain of at least 22 residues (residues 16-37), suggesting that 6.7K may be a membrane protein. Consistent with this, the 7K-8K and 15K-16K bands were observed in the crude membrane but not the cytosol or nuclear fractions of biochemically fractionated cells. The 6.7K protein was underproduced by mutants which underproduce E3 mRNAs a and c, indicating that 6.7K is translated from these mRNAs. Since the E3-gp 19K protein is also translated from mRNAs a and c, these mRNAs are bicistronic. The 6.7K protein is well-conserved in Ad5 (Ad2 and Ad5 are group C adenoviruses), and appears to be marginally conserved in Ad3 (group B).

Requirements for the association of adenovirus type 2 E3/19K wild-type and mutant proteins with HLA antigens

The E3/19K protein of human adenovirus type 2 is a resident of the endoplasmic reticulum (ER). Immediately after synthesis, it associates with major histocompatibility complex class I antigens and prevents their intracellular transport and cell surface expression. We have generated several C-terminal deletion mutants of the E3/19K protein that are preterminated at various positions on both sides of the membrane-spanning segment of the protein. One of these mutants is terminated at the luminal side of the membrane (M310), and two are terminated in the hydrophobic segment (M374 and M392), whereas mutant M621 is terminated on the cytoplasmic side of the ER membrane. The M310, M374, and M392 mutants are soluble proteins. They do not associate with HLA antigens in transfected 293 cells, and they are, to some extent, secreted into the medium. The M621 mutant protein is integrated in the ER membrane, associates immediately after its synthesis with HLA antigens, and exits from the ER. By using either an in vitro translation system supplemented with microsomes or overexpression in insect cells, we showed that M374 and E3/19K are able to associate with HLA antigens. These results indicate that the conformation of the luminal part of the E3/19K protein is not grossly altered by the mutations. Rapid transport of the M374 mutant out of the ER and partial degradation of this protein may prevent the interaction with HLA class I antigens in transfected 293 cells.

Transcription mapping of mouse adenovirus type 1 early region 3

Early region 3 (E3) of mouse adenovirus type 1 was analyzed using S1 nuclease protection and primer extension assays, cDNA sequencing, and genomic sequencing. We present the genomic sequence from 79 to 83 map units of the viral genome, the precise ends and splice sites of the E3 mRNAs, and the predicted protein sequence encoded by the mRNAs. Three major classes of early mRNAs were identified; all were approximately 1 kb long, consisted of three exons, and shared 5' and 3' ends. The three classes had alternative splicing at the junction between the second and third exon. The three proteins predicted by the three mRNAs were slightly similar to the E3 19K glycoprotein of human adenovirus type 3; the longest of the three was the most similar. Open reading frames corresponding to late proteins were also identified in the translated mouse adenovirus type 1 DNA sequence. In mouse adenovirus, as in the human adenoviruses, L4 overlaps E3, and L5 starts just downstream of the E3 region.

A 10,400-molecular-weight membrane protein is coded by region E3 of adenovirus

Previous studies with adenovirus mutants have indicated that a 10,400-molecular-weight (10.4K) protein predicted to be coded by an open reading frame in region E3 of adenovirus functions to down regulate the epidermal growth factor receptor (C. R. Carlin, A. E. Tollefson, H. A. Brady, B. L. Hoffman, and W. S. M. Wold, Cell 57:135-144, 1989). We now demonstrate that the 10.4K protein is in fact synthesized in cells infected by group C adenoviruses. This was done by immunoprecipitation of 10.4K from cells infected by a variety of E3 mutants, using antisera against three different synthetic peptides corresponding to the predicted 10.4K sequence. The 10.4K protein was translated primarily from E3 mRNA f, as indicated by cell-free translation of mRNA purified by hybridization from cells infected with an RNA processing mutant that synthesizes predominantly mRNA f. The 10.4K protein was overproduced or underproduced in vivo, respectively, by mutants that overproduce or underproduce E3 mRNA f, also indicating that the 10.4K protein is translated primarily from mRNA f. The 10.4K protein migrated as two bands with apparent molecular weights of 16,000 and 11,000 (10 to 18% gradient gels); both bands contained 10.4K epitopes, as shown by Western blot (immunoblot). Only the 16K band was obtained by cell-free translation, suggesting that the 16K protein is the precursor to the 11K protein. The 10.4K protein is a membrane protein, as shown by cell fractionation experiments and as predicted from its sequence. The predicted 10.4K sequence as well as a putative N-terminal signal sequence and 30-residue transmembrane domain are conserved in adenovirus types 2 and 5 (group C) and in types 3, 7, and 35 (group B).

An adenovirus mRNA which encodes a 14,700-Mr protein that maps to the last open reading frame of region E3 is expressed during infection

The E3 regions of adenovirus types 2 and 5, respectively, are known to synthesize proteins of 19,000 Mr (19K) and 11.6K, but information regarding the identity and characterization of other potential E3 proteins encoded by the six remaining open reading frames (ORFs) is lacking. In this study, we show that the last ORF of region E3, which encodes a 14.7K protein, is expressed in adenovirus-infected cells. This information was largely derived from analysis of an E3 deletion mutant (H2dl801) in which an extensive deletion (1,939 base pairs) was found to eliminate all ORFs except for two proteins of 12.5K and 14.7K. The 14.7K protein was translated from RNA isolated from H2dl801-infected cells that had been hybridization selected to E3 DNA; hybridization-selected RNA from wild-type adenovirus type 5-infected cells translated both the 19K and the 14.7K proteins. Moreover, an antiserum directed against a bacterial 14.7K fusion protein (A. E. Tollefson and W. S. M. Wold, J. Virol. 62:33-39, 1988) immunoprecipitated the 14.7K translation product synthesized by wild-type and mutant H2dl801 adenovirus mRNAs.

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The DNA sequence of the early E3 transcription unit of adenovirus 2 (Ad2) (J. Hérissé et al., Nucleic Acids Res. 8:2173-2192, 1980), indicates that an open reading frame exists between nucleotides 1860 and 2163 that could encode a protein of Mr 11,600 (11.6K). We have determined the DNA sequence of the corresponding region in Ad5 (closely related to Ad2) and have established that this putative gene is conserved in Ad5 (a 10.5K protein). To determine whether this protein is expressed, we prepared an antiserum in rabbits against a synthetic peptide corresponding to amino acids 66 to 74 in the 11.6K protein of Ad2. The peptide antiserum immunoprecipitated a ca. 13K-14K protein doublet, as estimated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, from [35S]methionine-labeled Ad2- or Ad5-early-infected KB cells. The antiserum also immunoprecipitated a 13K-14K protein doublet translated in vitro from Ad2 or Ad5 early E3-specific mRNA purified by hybridization to Ad2 EcoRI-D (nucleotides -236 to 2437). The synthetic peptide successfully competed with the 13K-14K protein doublet in immunoprecipitation experiments, thereby confirming the specificity of the antiserum. As deduced from the DNA sequence, the 11.6K protein (and the corresponding 10.5K Ad5 protein) has a conserved 22-amino-acid hydrophobic domain, suggesting that the protein may be associated with membranes. We conclude that a gene located at nucleotides 1860 to 2143 in the Ad2 E3 transcription unit (nucleotides 1924 to 2203) in the Ad5 E3 transcription unit) encodes an 11.6K protein (10.5K in Ad5).

Identification and gene mapping of a 14,700-molecular-weight protein encoded by region E3 of group C adenoviruses

Early region E3 of adenovirus type 5 should encode at least nine proteins as judged by the DNA sequence and the spliced structures of the known mRNAs. Only two E3 proteins have been proved to exist, a glycoprotein (gp19K) and an 11,600-molecular-weight protein (11.6K protein). Here we describe an abundant 14.7K protein coded by a gene in the extreme 3' portion of E3. To identify this 14.7K protein, we constructed a bacterial vector which synthesized a TrpE-14.7K fusion protein, then we prepared antiserum against the fusion protein. This antiserum immunoprecipitated the 14.7K protein from cells infected with adenovirus types 5 and 2, as well as with a variety of E3 deletion mutants. Synthesis of the 14.7K protein correlated precisely with the presence or absence of the 14.7K gene and with the synthesis of the mRNA (mRNA h) which encodes the 14.7K protein. The 14.7K protein appeared as a triplet on immunoprecipitation gels and Western blots (immunoblots).

Characterization of a major histocompatibility complex class I antigen-binding glycoprotein from adenovirus type 35, a type associated with immunocompromised hosts

Adenovirus type 35 (Ad35) is a group B adenovirus that has been isolated primarily from patients with acquired immunodeficiency syndrome and other immunodeficiency disorders. We have studied the interaction of this unique adenovirus with the immune system by analyzing Ad35 early viral proteins in infected HeLa cells. We have identified a 29,000-Mr Ad35 early glycoprotein, E29, which associates with class I antigens of the major histocompatibility complex (MHC) in the endoplasmic reticulum. Ad35 E29 is analogous to the group C Ad2 early glycoprotein E3-19K (E19), which has been shown to interfere with the expression of class I antigens on the cell surface (H. Burgert and S. Kvist, Cell 41:987-997, 1985). In contrast to the Ad2 glycoprotein, Ad35 E29 was synthesized in much smaller amounts, was more extensively glycosylated, and did not cross-react with polyclonal antibody against the Ad2 protein. As a control, a class I antigen-binding glycoprotein from another group B adenovirus, Ad7, was also characterized and was found to have properties similar to those of Ad35 E29. Therefore, the differences in the glycosylation and quantity of class I antigen-binding glycoproteins between Ad35 and Ad2 are group related. Inhibition of the expression of MHC class I antigens, which are needed for cytotoxic-T-lymphocyte recognition of virus-infected cells, appears to play a vital role in the adenovirus life cycle in vivo. Our data indicate that this function has been conserved despite significant differences in the MHC class I antigen-binding glycoprotein and in the pathogenicity between serotypes.

A short sequence in the COOH-terminus makes an adenovirus membrane glycoprotein a resident of the endoplasmic reticulum

The E19 protein of adenoviruses is a transmembrane protein that abrogates the intracellular transport of class I antigens by forming complexes with them in the ER. We show here that the E19 protein is retained in the ER even in the absence of class I antigens. To define the region conferring residency in the ER, we examined two mutant forms of the viral protein. A 5 amino acid extension of the 15-membered cytoplasmic tail of the protein reduces its interaction with class I antigens but does not change its intracellular distribution. Shortening the tail to 7 amino acids also diminishes the affinity for class I antigens; however, this mutant E19 protein becomes transported to the cell surface. Thus, we concluded that a small stretch of amino acids exposed on the cytoplasmic side of the ER membrane is responsible for the retention of the E19 protein in the ER.

Adenoviruses of subgenera B, C, D, and E modulate cell-surface expression of major histocompatibility complex class I antigens

The immune defense against viral infections involves cytotoxic T lymphocytes that recognize viral products in the context of class I major histocompatibility complex (MHC) antigens. To evade such immune surveillance viruses may have evolved various strategies to manipulate the expression of class I antigens. Adenovirus 2 manufactures an early glycoprotein, E19, that binds to nascent class I antigens in the endoplasmic reticulum and impedes their transport to the cell surface. We now show that adenoviruses typical of all viral subgenera except the highly oncogenic subgenus A dramatically reduce the cell-surface expression of class I antigens. It has been shown that subgenus A viruses abolish class I antigen expression in transformed cells by reducing mRNA levels. Thus, all adenoviruses can modulate the cell-surface expression of class I antigens.

Subtypes of bovine adenovirus type 2 exhibit major differences in region E3

The genomes of two adenovirus type 2 strains which were isolated from different hosts have been investigated. One of these strains designated ORT-111 was originally isolated from a lamb in Hungary during an outbreak of pneumoenteritis. This isolate was typed as bovine adenovirus type 2 (Ad bos 2) in a neutralization assay. The genome of ORT-111 was compared to that of the prototype strain of Ad bos 2, a virus which exclusively has been isolated from cattle. Electron microscopic heteroduplex analysis showed that 95% of the genomes were well matched, forming stable duplexes at Tm -6 degrees. Two distinct substitution loops were, however, seen which were approximately 0.5 and 1.0 kbp long. The centers of the two loops were located 5.3 and 7.7 kbp from one end of the Ad bos 2 genome. In order to map these regions relative to the gene map of human adenovirus type 2 (Ad2), restriction enzyme cleavage fragments of the two bovine viruses were cloned and hybridized to different sets of restriction fragments of human Ad2. From these results it was apparent that the centers of the two substitution loops were located at coordinates 76 and 83, respectively; thus at positions which fall within region E3 and the adjacent gene for polypeptide VIII of human Ad2. The observed differences between the genomes of the two Ad bos 2 strains are in sharp contrast to those previously observed when the genomes of different human adenovirus serotypes were compared. In the latter case the hexon and the fiber genes showed the most pronounced variation.

Structural and functional dissection of an MHC class I antigen-binding adenovirus glycoprotein

The early transmembrane glycoprotein E19 of adenovirus-2 binds to class I antigens of the major histocompatibility complex (MHC). The association is initiated in the endoplasmic reticulum of infected cells and abrogates the intracellular transport of the class I molecules. To examine which parts of the E19 molecule are responsible for the association with the class I antigens and which parts confine the protein to the endoplasmic reticulum we have constructed a series of mutated E19 genes, which have been expressed in an improved mammalian expression vector. By various manipulations the membrane anchoring and the cytoplasmic domains were removed from the protein. The biosynthesis of the mutant protein was examined. All mutant proteins were secreted from the cells suggesting that the transmembrane and/or cytoplasmic portions of the E19 molecule are responsible for its confinement to the endoplasmic reticulum. The ability to associate with class I antigens was retained by the lumenal domain of the E19 protein.

Biosynthesis and properties of the adenovirus 2 L1-encoded 52,000- and 55,000-Mr proteins

The adenovirus type 2 L1 region, which is located at 30.7 to 39.2 map units on the viral genome, is transcribed from the major late promoter during both early and late stages of virus replication, and a 52,000-Mr (52K) protein-55K protein doublet has been translated in vitro on L1-specific RNA. To investigate the biosynthesis and properties of the L1 52K and 55K proteins, we prepared antibody against a synthetic peptide encoded near the predicted N terminus. As determined by immunoprecipitation and immunoblot analysis, the antipeptide antibody recognized major 52K and 55K proteins synthesized in adenovirus type 2-infected cells that appeared to be identical to the 52K-55K doublet translated in vitro. The immunoprecipitated 52K and 55K proteins were very closely related, as shown by a peptide map analysis. Both L1 proteins were phosphorylated, and they were phosphorylated at similar sites. No precursor-product relationship was detected between the 52K and 55K proteins by a pulse-chase analysis. Biosynthesis of the L1 52K and 55K proteins began about 6 to 7 h postinfection, after biosynthesis of the early region 1A and early region 1B 19K (175R) T antigens, and reached a maximum rate at about 15 h; the maximum rate was maintained until at least 25 h postinfection. At all times, the 55K protein appeared to be synthesized at a severalfold-higher level than the 52K protein. Both proteins were quite stable and accumulated until late times after infection. Viral DNA replication was not essential for formation of the L1 proteins. Thus, the L1 52K-55K gene appears to be regulated in a manner different from the classical early and late viral genes but similar to the protein encoded by the i-leader (Symington et al., J. Virol. 57:849-856, 1986). The L1 proteins were detected in the cell nucleus by immunofluorescence microscopy with antipeptide antibody and were found to be primarily associated with the nuclear membrane by an immunoblot analysis of subcellular fractions.

Evidence that AGUAUAUGA and CCAAGAUGA initiate translation in the same mRNA region E3 of adenovirus

We described a simple method to introduce site-specific mutations into region E3 of adenovirus (Ad). Mutations are made in cloned Ad2 EcoRI-D (map position 76-83), then ligated between Ad5 EcoRI-A (map position 0-76) and EcoRI-B (map position 83-100) to complete the viral genome. We have used this method to isolate a viable virus mutant (dl702) that is relevant to the problems of translation initiation and gene organization in the E3 complex transcription unit. mRNA a in region E3 encodes an abundant glycoprotein termed gp19K. There are two AUGs in mRNA a that are 5' to AUG1204 which initiates gp19K. One of these, AUG1022, could initiate a 6.7K protein, although this protein has not been identified in infected cells. Mutant dl702 has a deletion such that the 6.7K gene is fused in-frame to the gp19K gene. We report that the 6.7K-gp19K fusion protein is synthesized both in dl702-infected cells and after cell free translation of infected cell RNA. The quantity of fusion protein made is much less than that of wild type gp19K. The sequence context of AUG1022 for 6.7K is AGUAUAUGA, and that of AUG1204 for gp19K is CCAAGAUGA. The consensus sequence of eukaryotic initiation codons is CCPuCCAUGG, with the Pu at -3 being important (M. Kozak, Nucleic Acids Res. 12, 857-872, 1984). Our results suggest that (i) AUG1022 can initiate translation in vivo and therefore the 6.7K protein probably is made in infected cells, (ii) that mRNA a is a dicistronic mRNA encoding the 6.7K and gp19K proteins, and (iii) that the initiation codon for 6.7K may be much less efficient than that for gp19K. Thus, the E3 genes may be organized such that the relative abundance of the 6.7K and gp19K proteins is controlled by the efficiency of their initiation codons in the same mRNA.

Region E3 of human adenoviruses; differences between the oncogenic adenovirus-3 and the non-oncogenic adenovirus-2

The nucleotide sequence of a 4379-bp-long DNA segment, located between map coordinates 76.5 and 89.2 in the Ad3 genome, was established. The segment includes the entire early transcription unit 3 (E3) and the 3'-part of the gene for the late polypeptide pVIII. The established sequence was compared to the sequence of the corresponding region in the Ad2 genome [Hérissé et al., Nucl. Acids Res. 8, (1980) 2173-2192; Hérissé and Galibert, Nucl. Acids Res. 9, (1981) 1229-1249]. Although Ad2 and Ad3 belong to different serological and oncogenic subgroups, their E3 regions are well conserved (55-60% homology). In total, the E3 region of Ad3 appears to encode eight to nine different polypeptides many of which are likely to be membrane-associated. The most conspicuous difference between the E3 regions of Ad2 and Ad3 is the presence of a 950-bp-long A + T-rich insert in the Ad3 sequence. This insert contains two open reading frames, each encoding a polypeptide with a predicted Mr of about 20,000. The mRNAs encoding these novel polypeptides were identified by S1 nuclease analysis.

An adenovirus type 2 glycoprotein blocks cell surface expression of human histocompatibility class I antigens

The adenovirus type 2 encoded protein E3/19K binds to human histocompatibility class I antigens (HLA). This association occurs both in adenovirus-infected cells and in cells that have been transfected with the gene encoding the E3/19K protein. The formation of the HLA-E3/19K complex prevents the HLA antigens from being correctly processed by inhibiting their terminal glycosylation. This effect is specific for HLA antigens and does not generally involve the glycosyltransferases. Furthermore, the HLA-E3/19K association dramatically reduces the cell surface expression of the HLA antigens. This reduced level of antigens might influence the cytotoxic T cell response. Therefore, our results show a possible molecular mechanism whereby adenoviruses, and perhaps other viruses, delay or escape the cellular immune system of the host.

Mapping the 5' ends, 3' ends, and splice sites of mRNAs from the early E3 transcription unit of adenovirus 5

Using nuclease gel analyses, the sites in the approximately 4000 nucleotide (nt) E3 transcription unit of adenovirus 5 (Ad5) that encode the 5' ends, 3' ends, and 5' and 3' splice sites of the approximately 10 E3 mRNAs were determined. Transcription initiation of all mRNAs occurs at two major (nt 1 and 8) and approximately two minor sites, situated 20-30 nt 3' to a TATA box. There are two major 3' end sites (nt 2227 and 3308), located approximately 20 nt downstream from ATTAAA and an AATAAA sequences, respectively. Thus, ATTAAA, as well as the usual AATAAA, apparently can function as a 3' end signal. There are two 5' splice sites (nt 372 and 923), both with GT at the intron boundary. There are four 3' splice sites (nt 766, 1817, 2201, and 2880), all with AG at the intron boundary. The nt 1817, 2201, and 2880 3' splice sites are located immediately upstream from open reading frames, such that splicing at the different sites allows synthesis of completely different proteins.

DNA sequence of the early E3 transcription unit of adenovirus 5

The DNA sequence of the early E3 transcription unit of adenovirus 5 (Ad5) has been determined and it has been compared to Ad2, as published previously [J. Hérissé, G. Courtois, and F. Galibert (1980), Nucl. Acids Res. 8, 2173-2192; J. Hérissé and F. Galibert (1981), Nucl. Acids Res. 9, 1229-1240]. The E3 regions of Ad5 and Ad2 are quite homologous despite being nonessential for Ad growth in cultured cells. The major differences are "gaps" that exist either in Ad5 or Ad2 in intergenic regions. The conservation of sequences suggests that E3 plays a beneficial role in natural infection of humans. E3 appears to encode about seven to nine proteins; based on sequence, seven of these may be membrane proteins. Thus, E3 may be a transcription unit devoted to the synthesis of membrane proteins. The E3 genes lie essentially one after the other along the genome, and which gene is expressed from a given primary transcript is determined by the choice of the 3' end site and the 5' and 3' splice sites. Almost all E3 mRNAs contain nonfunctional AUGs that are 5' to the initiation codon. Codon usage is nonrandom. Although the CG dinucleotide frequency is low, CG clusters exist in the promoter and other regions.

Mapping a new gene that encodes an 11,600-molecular-weight protein in the E3 transcription unit of adenovirus 2

The DNA sequence of the early E3 transcription unit of adenovirus 2 (Ad2) (J. Hérissé et al., Nucleic Acids Res. 8:2173-2192, 1980), indicates that an open reading frame exists between nucleotides 1860 and 2163 that could encode a protein of Mr 11,600 (11.6K). We have determined the DNA sequence of the corresponding region in Ad5 (closely related to Ad2) and have established that this putative gene is conserved in Ad5 (a 10.5K protein). To determine whether this protein is expressed, we prepared an antiserum in rabbits against a synthetic peptide corresponding to amino acids 66 to 74 in the 11.6K protein of Ad2. The peptide antiserum immunoprecipitated a ca. 13K-14K protein doublet, as estimated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, from [35S]methionine-labeled Ad2- or Ad5-early-infected KB cells. The antiserum also immunoprecipitated a 13K-14K protein doublet translated in vitro from Ad2 or Ad5 early E3-specific mRNA purified by hybridization to Ad2 EcoRI-D (nucleotides -236 to 2437). The synthetic peptide successfully competed with the 13K-14K protein doublet in immunoprecipitation experiments, thereby confirming the specificity of the antiserum. As deduced from the DNA sequence, the 11.6K protein (and the corresponding 10.5K Ad5 protein) has a conserved 22-amino-acid hydrophobic domain, suggesting that the protein may be associated with membranes. We conclude that a gene located at nucleotides 1860 to 2143 in the Ad2 E3 transcription unit (nucleotides 1924 to 2203) in the Ad5 E3 transcription unit) encodes an 11.6K protein (10.5K in Ad5).

Adenovirus VA RNAI: a positive regulator of mRNA translation

We have developed a sensitive transient expression assay in 293 cells to study the effect of VA RNAs on the translation of adenovirus mRNAs. Monolayers of 293 cells were transfected with mixtures of recombinant plasmids encoding adenovirus-specific transcription units and plasmids encoding VA RNAs. Transfected cells were labeled with [35S]methionine for ca. 15 h, and labeled cell extracts were prepared. Changes in the protein expression caused by VA RNA cotransfection were measured by immunoprecipitation, using monospecific antisera prepared against adenovirus-specific polypeptides. Using this experimental design, we demonstrate that VA RNAI stimulates the translation of both early and late adenovirus mRNAs. Synthesis of the E3 19,000-dalton glycoprotein and the E2A 72,000-dalton DNA binding protein was stimulated between 10 and 20 times by VA RNAI cotransfection. Synthesis of the late hexon polypeptide was also stimulated, although translation of hexon was from an aberrant mRNA lacking the second and third segments of the common tripartite leader attached to late adenovirus mRNAs. VA RNAII, although very homologous to VA RNAI, does not function as a translational stimulator.

Adenovirus VA RNAI: a positive regulator of mRNA translation

We have developed a sensitive transient expression assay in 293 cells to study the effect of VA RNAs on the translation of adenovirus mRNAs. Monolayers of 293 cells were transfected with mixtures of recombinant plasmids encoding adenovirus-specific transcription units and plasmids encoding VA RNAs. Transfected cells were labeled with [35S]methionine for ca. 15 h, and labeled cell extracts were prepared. Changes in the protein expression caused by VA RNA cotransfection were measured by immunoprecipitation, using monospecific antisera prepared against adenovirus-specific polypeptides. Using this experimental design, we demonstrate that VA RNAI stimulates the translation of both early and late adenovirus mRNAs. Synthesis of the E3 19,000-dalton glycoprotein and the E2A 72,000-dalton DNA binding protein was stimulated between 10 and 20 times by VA RNAI cotransfection. Synthesis of the late hexon polypeptide was also stimulated, although translation of hexon was from an aberrant mRNA lacking the second and third segments of the common tripartite leader attached to late adenovirus mRNAs. VA RNAII, although very homologous to VA RNAI, does not function as a translational stimulator.

Isolation of immunogenic and suppressogenic glycoproteins from adenovirus type 12 hamster tumor cells

Two types of glycoproteins were isolated from the membrane fraction of adenovirus type 12 (Ad12) hamster tumor cells by recovering detergent-solubilized glycoproteins using concanavalin A-affinity chromatography and gel filtration. One of the glycoproteins consisted of a polypeptide of 130,000 daltons (130K) with a pI value of 4.7-5.1, and the other consisted of a polypeptide of 18,500 daltons (18.5K) with a pI value of 6.3-6.6. The glycoproteins were immunologically different. The 18.5K glycoprotein induced in vivo resistance to tumor growth and anti-tumor cytotoxic T cells, while the 130K glycoprotein induced in vivo suppressor T cells which inhibited the activity of anti-tumor cytotoxic T cells.

Gene fusion vectors based on the gene for staphylococcal protein A

Two plasmid vectors, containing the gene coding for staphylococcal protein A and adapted for gene fusion, have been constructed. These vectors will allow fusion of any gene to the protein A gene, thus giving hybrid proteins which can be purified, in a one-step procedure, by IgG affinity chromatography. As an example of the practical use of such vectors, the protein A gene has been fused to the lacZ gene of Escherichia coli. E. coli strains containing such plasmids produce hybrid proteins with both IgG binding and beta-galactosidase activities. The hybrid protein(s) can be immobilized on IgG-Sepharose by its protein A moiety with high efficiency without losing its enzymatic activity and they can be eluted from the column by competitive elution with pure protein A. The fused protein(s) also binds to IgG-coated microtiter wells which means that the in vivo product can be used as an enzyme conjugate in ELISA tests.

Structure of three spliced mRNAs from region E3 of adenovirus type 2

A cDNA library representing early adenovirus type 2 (Ad2) mRNA was constructed. The cDNA copies were inserted into the PstI cleavage site of the pBR322 plasmid, and clones containing sequences from region E3 of the Ad2 genome were identified by colony hybridization. Selected clones were characterized by restriction enzyme cleavage, hybridization, and partial DNA sequence analysis. The precise structure of three spliced mRNAs was established by comparing the results with the DNA sequence of region E3 from Ad2 (Herissé et al., Nucl. Acids Res. 8 (1980) 2173--2191; Herissé and Galibert, Nucl. Acids Res. 9 (1981) 1229--1249). One of the characterized mRNA species encodes the E3/19K glycoprotein, whereas the other two most likely encode the E3/14K protein. The results demonstrate, moreover, that certain splice points which are used to generate the major E3 mRNAs are also used to splice the supplementary leader segments to the fibre mRNA at late times after infection. Two separate poly(A)-addition sites were identified in region E3 by analysis of the cDNA clones; one is preceded by the hexanucleotide sequence AAUAAA, whereas the other is preceded by an altered hexanucleotide, having the sequence AUUAAA.

Control of adenovirus gene expression: cellular gene products restrict expression of adenovirus host range mutants in nonpermissive cells

Adenovirus type 5 (Ad5) host range mutants dl312 and hr-1, with lesions in region E1A (0 to 4.5 map units) of the viral genome, fail to accumulate virus-specific early RNA during infection in HeLa cells. In a recent report, we showed that the addition of anisomycin, a stringent inhibitor of protein synthesis, at 1 h after infection of HeLa cells with hr-1 virus resulted in the accumulation of properly spliced and translatable mRNA from all early regions (M. G. Katze, H. Persson, and L. Philipson, Mol. Cell. Biol. 1:807-813, 1981). Based on these results we proposed a model in which expression of early mutant RNA was achieved through inactivation of a cellular protein normally causing a reduction in the amount of viral RNA. These studies have been extended in the present report, which shows that early viral proteins can be detected in Ad5 dl312- and Ad5 hr-1-infected HeLa cells which have been treated for several hours with anisomycin either shortly after infection or before infection. A pulse of drug treatment also resulted in expression of substantial amounts of adenovirus structural proteins after infection with both Ad5 hr-1 and Ad5 dl312, whereas in drug-free controls no late proteins were detected. The Ad5 hr-1 virus previously reported to be DNA replication negative in nonpermissive HeLa cells was found to replicate its DNA, albeit at low levels, when anisomycin was present either from 1 to 5 h postinfection or for 5 h before infection. When infectious virus production was examined in mutant-infected cells the titer of Ad5 dl312 virus was found to increase at least 500-fold in anisomycin-treated HeLa cells. Taken together, these and our previous results suggest that the block in gene expression characteristic for complementation group I Ad5 host range mutants in HeLa cells can be overcome by inactivating cellular gene products serving as negative regulators of viral gene expression.

Gene for staphylococcal protein A

The gene for protein A from Staphylococcus aureus was cloned into pBR322 in Escherichia coli. An immunoassay was used to detect production of the protein. Protein A produced in E. coli was found in the periplasmic space and was purified and concentrated by IgG-Sepharose affinity chromatography. DNA sequence assay of the gene revealed a region with the general features of a prokaryotic signal peptide and a fifth structural region homologous to the four repetitive regions found earlier by amino acid sequence determination of the mature protein. Upstream from the structural gene there is a possible promoter region and a ribosomal binding sequence typical of gram-positive bacteria. The initiation codon is TTG.

Purification of a native membrane-associated adenovirus tumor antigen

A 15,000-dalton protein was purified from HeLa cells infected with adenovirus type 2. Proteins solubilized from a membrane fraction of lytically infected cells was used as the starting material for purification. Subsequent purification steps involved lentil-lectin, phosphocellulose, hydroxyapatite, DEAE-cellulose, and aminohexyl-Sepharose chromatographies. A monospecific antiserum, raised against the purified protein, immunoprecipitated a 15,000-dalton protein encoded in early-region E1B (E1B/15K protein) of the adenovirus type 2 DNA. Tryptic finger print analysis revealed that the purified protein was identical to the E1B/15K protein encoded in the transforming part of the viral genome. The antiserum immunoprecipitated the E1B/15K protein from a variety of viral transformed cell lines isolated from humans, rats, or hamsters. The E1B/15K protein was associated with the membrane fraction of both lytically and virus-transformed cell lines and could only be released by detergent treatment. Furthermore, a 11,000- to 12,000-dalton protein that could be precipitated with the anti-E1B/15K serum was recovered from membranes treated with trypsin or proteinase K, suggesting that a major part of the E1B/15K protein is protected in membrane vesicles. Translation of early viral mRNA in a cell-free system, supplemented with rough microsomes, showed that this protein was associated with the membrane fraction also in vitro.

Viral gene products in adenovirus type-2 transformed hamster cells

I have analyzed viral gene products expressed in five adenovirus type 2 (Ad2)- cytoplasmic, viral RNA which was selected by hybridization to cloned restriction endonuclease fragments of Ad2 DNA. Proteins synthesized in vitro were analyzed by electrophoresis in sodium dodecyl sulfate-polyacrylamide gels and compared with those directed by RNAs prepared from productively infected cells. The early regions E1 and E4 of adenovirus type 2 (Ad2) were found to be expressed in all of five Ad2-transformed hamster embryo cells lines. RNA transcribed from early region E2, which codes for the 72,000-molecular-weight (72K) DNA-binding protein was detected in cell line HE1 only, and early region E3 was expressed exclusively in cell line HE4. RNA transcribed from the region between approximately 12 and 35 map units, coding for immediate early (13.5K, 52/53K) and immediate early proteins (13.6K, 16K, 17K, 87K), as well as RNA from late genes, was not found in any of the cell lines HE1 to HE5 had electrophoretic mobilities similar to those programmed by RNA from productively infected cells.

Structures of the oligosaccharides of the glycoprotein coded by early region E3 of adenovirus 2

Early region E3 of adenovirus 2 encodes a glycoprotein, E3-gp25K, that is a good model with which to study structure-function relationships in transmembrane glycoproteins. We have determined the structures of the oligosaccharides linked to E3-gp25K. The oligosaccharides were labeled with [2-(3)H]mannose in adenovirus 2-early infected KB cells for 5.5h (pulse) or for 5.5 h followed by a 3-h chase (pulse-chase). E3-gp25K was extracted and purified by chromatography on DEAE-Sephacel in 7 M urea, followed by gel filtration on a column of Bio-Gel A-1.5m in 6 M guanidine hydrochloride. An analysis of the purified protein by sodium dodecyl sulfate-polyacrylamide gel electrophoresis indicated that it was >95% pure. The oligosaccharides were isolated by pronase digestion followed by gel filtration on a column of Bio-Gel P-6, then by digestion with endo-beta-N-acetylglucosaminidase H, followed by gel filtration on Bio-Gel P-6, and finally by paper chromatography. The pulse sample contained equal amounts of Man(9)GlcNAc and Man(8)GlcNAc and small amounts of Man(7)GlcNAc and Man(6)GlcNAc. The pulse-chase sample had predominantly Man(8)GlcNAc and much less Man(9)GlcNAc, indicating that processing of the Man(9)GlcNAc to Man(8)GlcNAc had occurred during the chase period. Thus, Man(8)GlcNAc is the major oligosaccharide on mature E3-gp25K. The structures of these oligosaccharides were established by digestion with alpha-mannosidase, methylation analysis, and acetolysis. The oligosaccharides found had typical high-mannose structures that have been observed in other membrane and soluble glycoproteins, and the branching patterns and linkages of the mannose residues of Man(9)GlcNAc were identical to those of the lipid-linked Glc(3)Man(9)GlcNAc(2) donor. Thus, adenovirus 2 infection (early stages) apparently does not affect the usual cellular high-mannose glycosylation pathways, and despite being virus coded, E3-gp25K is glycosylated in the same manner as a typical mammalian cell-coded glycoprotein.

Adenovirus early gene products may control viral mRNA accumulation and translation in vivo

The mechanisms controlling early adenovirus gene expression in vivo have been studied using inhibitors of protein synthesis. When inhibitors were added shortly before or at the onset of infection, viral mRNA from all early regions was transcribed, spliced and accumulated over a 7 hr period. After longer pretreatment, accumulation of several early mRNAs were suppressed. Addition of inhibitors 1 hr after infection enhanced the accumulation of viral mRNA in the cytoplasm. Translation of early mRNA selected on adenovirus DNA in a cell-free system reflected the amount of viral mRNA present. A viral coded product may therefore control accumulation of viral mRNA. A different pattern emerged when inhibitors of protein synthesis were removed at 5 hr postinfection and cells were removed at 5 hr postinfection and cells were pulse-labeled in vivo. If inhibitors were introduced at or before infection, early viral proteins were synthesized only after a lag of 1-3 hr. However, if treatment was introduced 1 hr postinfection, reversion of the protein synthesis block was instantaneous. It appears that protein synthesis inhibitors reveal an in vivo translational block for viral mRNA. This block could be overcome by preinfection with a related virus. Furthermore, no block was observed in a virus-transformed human embryonic kidney cell line (293) which expresses early region 1 of the viral genome. Viral gene product(s) encoded in early region 1 may control translation of early adenovirus messenger RNA in vivo.

Multiple mRNA species for the precursor to an adenovirus-encoded glycoprotein: identification and structure of the signal sequence

Early region 3 of the adenovirus type 2 genome encodes three proteins with molecular weights of 16,000, 14,500, and 14,000 (E2/16, E3/14.5, and E3/14). The E3/16 protein is the precursor to the E3/19 glycoprotein and is around 1500 daltons larger than the unglycosylated E3/19O protein. The E3/14.5 and E3/14 proteins are structurally related to each other but different from E3/16. Three mRNA species were identified for E3/16; all have common 5' ends with the same spliced region but with different 3' ends. E3/14 was translated from a 13S mRNA with the same 5' structure as the E3/16 mRNA but followed by a second spliced region with a different 3' end. A partial amino acid sequence was determined for E3/16 after radioactive labeling in vitro and this sequence can be aligned with a known DNA sequence. It contains a hydrophobic signal sequence, two presumptive glycosylation sites, and a hydrophobic region close to the COOH terminus.