The amino-terminal signal peptide on the porcine transmissible gastroenteritis coronavirus matrix protein is not an absolute requirement for membrane translocation and glycosylation

Coronaviruses: An Updated Overview of Their Replication and Pathogenesis

Coronaviruses (CoVs), enveloped positive-sense RNA viruses, are characterized by club-like spikes that project from their surface, an unusually large RNA genome, and a unique replication strategy. CoVs cause a variety of diseases in mammals and birds ranging from enteritis in cows and pigs, and upper respiratory tract and kidney disease in chickens to lethal human respiratory infections. Most recently, the novel coronavirus, SARS-CoV-2, which was first identified in Wuhan, China in December 2019, is the cause of a catastrophic pandemic, COVID-19, with more than 8 million infections diagnosed worldwide by mid-June 2020. Here we provide a brief introduction to CoVs discussing their replication, pathogenicity, and current prevention and treatment strategies. We will also discuss the outbreaks of the highly pathogenic Severe Acute Respiratory Syndrome Coronavirus (SARS-CoV) and Middle Eastern Respiratory Syndrome Coronavirus (MERS-CoV), which are relevant for understanding COVID-19.

Characterization of an Immunodominant Epitope in the Endodomain of the Coronavirus Membrane Protein

The coronavirus membrane (M) protein acts as a dominant immunogen and is a major player in virus assembly. In this study, we prepared two monoclonal antibodies (mAbs; 1C3 and 4C7) directed against the transmissible gastroenteritis virus (TGEV) M protein. The 1C3 and 4C7 mAbs both reacted with the native TGEV M protein in western blotting and immunofluorescence (IFA) assays. Two linear epitopes, 243YSTEART249 (1C3) and 243YSTEARTDNLSEQEKLLHMV262 (4C7), were identified in the endodomain of the TGEV M protein. The 1C3 mAb can be used for the detection of the TGEV M protein in different assays. An IFA method for the detection of TGEV M protein was optimized using mAb 1C3. Furthermore, the ability of the epitope identified in this study to stimulate antibody production was also evaluated. An immunodominant epitope in the TGEV membrane protein endodomain was identified. The results of this study have implications for further research on TGEV replication.

Mutation in the cytoplasmic retrieval signal of porcine epidemic diarrhea virus spike (S) protein is responsible for enhanced fusion activity

Murine-adapted porcine epidemic diarrhea virus (PEDV), MK-p10, shows high neurovirulence and increased fusion activity compared with a non-adapted MK strain. MK-p10 S protein had four mutations relative to the original virus S, and one of these (H→R at position 1381, H1381R) in the cytoplasmic tail (CT) was suggested to be responsible for the increased fusion activity. To explore this, we examined fusion activity using recombinant S proteins. We expressed and compared the fusion activity of MK-p10 S, S with the H1381R mutation, S with the three other mutations that were not thought to be involved in high fusion activity, and the original S protein. The MK-p10 and MK-H1381R S proteins induced larger cell fusions than others. We also examined the distribution of these S proteins; the MK-p10 and MK-H1381R S proteins were transported onto the cell surface more efficiently than others. These findings suggest that the H1381R mutation is responsible for enhanced fusion activity, which may be attributed to the efficient transfer of S onto the cell surface. H1381 is a component of the KxHxx motif in the CT region, which is a retrieval signal of the S protein for the endoplasmic reticulum-Golgi intermediate compartment (ERGIC). Loss of this motif could allow for the efficient transfer of S proteins from ERGIC onto the cell surface and subsequent increased fusion activity.

A conserved domain in the coronavirus membrane protein tail is important for virus assembly

Coronavirus membrane (M) proteins play key roles in virus assembly, through M-M, M-spike (S), and M-nucleocapsid (N) protein interactions. The M carboxy-terminal endodomain contains a conserved domain (CD) following the third transmembrane (TM) domain. The importance of the CD (SWWSFNPETNNL) in mouse hepatitis virus was investigated with a panel of mutant proteins, using genetic analysis and transient-expression assays. A charge reversal for negatively charged E(121) was not tolerated. Lysine (K) and arginine (R) substitutions were replaced in recovered viruses by neutrally charged glutamine (Q) and leucine (L), respectively, after only one passage. E121Q and E121L M proteins were capable of forming virus-like particles (VLPs) when coexpressed with E, whereas E121R and E121K proteins were not. Alanine substitutions for the first four or the last four residues resulted in viruses with significantly crippled phenotypes and proteins that failed to assemble VLPs or to be rescued into the envelope. All recovered viruses with alanine substitutions in place of SWWS residues had second-site, partially compensating, changes in the first TM of M. Alanine substitution for proline had little impact on the virus. N protein coexpression with some M mutants increased VLP production. The results overall suggest that the CD is important for formation of the viral envelope by helping mediate fundamental M-M interactions and that the presence of the N protein may help stabilize M complexes during virus assembly.

The molecular biology of coronaviruses

Coronaviruses are large, enveloped RNA viruses of both medical and veterinary importance. Interest in this viral family has intensified in the past few years as a result of the identification of a newly emerged coronavirus as the causative agent of severe acute respiratory syndrome (SARS). At the molecular level, coronaviruses employ a variety of unusual strategies to accomplish a complex program of gene expression. Coronavirus replication entails ribosome frameshifting during genome translation, the synthesis of both genomic and multiple subgenomic RNA species, and the assembly of progeny virions by a pathway that is unique among enveloped RNA viruses. Progress in the investigation of these processes has been enhanced by the development of reverse genetic systems, an advance that was heretofore obstructed by the enormous size of the coronavirus genome. This review summarizes both classical and contemporary discoveries in the study of the molecular biology of these infectious agents, with particular emphasis on the nature and recognition of viral receptors, viral RNA synthesis, and the molecular interactions governing virion assembly.

Downstream ribosomal entry for translation of coronavirus TGEV gene 3b

Gene 3b (ORF 3b) in porcine transmissible gastroenteritis coronavirus (TGEV) encodes a putative nonstructural polypeptide of 27.7 kDa with unknown function that during translation in vitro is capable of becoming a glycosylated integral membrane protein of 31 kDa. In the virulent Miller strain of TGEV, ORF 3b is 5'-terminal on mRNA 3-1 and is presumably translated following 5' cap-dependent ribosomal entry. For three other strains of TGEV, the virulent British FS772/70 and Taiwanese TFI and avirulent Purdue-116, mRNA species 3-1 is not made and ORF 3b is present as a non-overlapping second ORF on mRNA 3. ORF 3b begins at base 432 on mRNA 3 in Purdue strain. In vitro expression of ORF 3b from Purdue mRNA 3-like transcripts did not fully conform to a predicted leaky scanning pattern, suggesting ribosomes might also be entering internally. With mRNA 3-like transcripts modified to carry large ORFs upstream of ORF 3a, it was demonstrated that ribosomes can reach ORF 3b by entering at a distant downstream site in a manner resembling ribosomal shunting. Deletion analysis failed to identify a postulated internal ribosomal entry structure (IRES) within ORF 3a. The results indicate that an internal entry mechanism, possibly in conjunction with leaky scanning, is used for the expression of ORF 3b from TGEV mRNA 3. One possible consequence of this feature is that ORF 3b might also be expressed from mRNAs 1 and 2.

Unique N-linked glycosylation of murine coronavirus MHV-2 membrane protein at the conserved O-linked glycosylation site

The membrane (M) proteins of murine coronavirus (MHV) strains have been reported to contain only O-linked oligosaccharides. The predicted O-glycosylation site consisting of four amino acid residues of Ser-Ser-Thr-Thr is located immediately adjacent to the initiator Met and is well conserved among MHV strains investigated so far. We analyzed the nucleotide sequence of a highly virulent strain MHV-2 M-coding region and demonstrated that MHV-2 had a unique amino acid, Asn, at position 2 at the conserved O-glycosylation site. We also demonstrated that this substitution added N-linked glycans to MHV-2 M protein resulting in increment of molecular mass of MHV-2 M protein compared with JHM strain having only O-linked glycans.

The major product of porcine transmissible gastroenteritis coronavirus gene 3b is an integral membrane glycoprotein of 31 kDa

The open reading frame potentially encoding a polypeptide of 27.7 kDa and located as the second of three ORFs (gene 3b) between the S and M genes in the genome of the Purdue strain of porcine transmissible gastroenteritis coronavirus (TGEV) was cloned and expressed in vitro to examine properties of the protein. Gene 3b has a postulated role in pathogenesis, but its truncated form in some laboratory-passaged strains of TGEV has led to the suggestion that it is not essential for virus replication. During synthesis in vitro in the presence of microsomes, the 27.7-kDa polypeptide became an integral membrane protein, retained its postulated hydrophobic N-terminal signal sequence, and underwent glycosylation on apparently two asparagine linkage sites to attain a final molecular mass of 31 kDa. A 20-kDa N-terminally truncated, nonglycosylated, nonanchored form of the protein was also made via an unknown mechanism. The existence of both transmembrane and soluble forms of the gene 3 product in the cell is suggested by immunofluorescence patterns showing both a punctuated perinuclear and diffuse intracytoplasmic distribution. No gene 3b product was found on gradient-purified Purdue TGEV by a Western blotting procedure that would have detected as few as 4 molecules/virion, indicating the protein probably is not a structural component of the virion.

Coronavirus genomic and subgenomic minus-strand RNAs copartition in membrane-protected replication complexes

The majority of porcine transmissible gastroenteritis coronavirus plus-strand RNAs (genome and subgenomic mRNAs), at the time of peak RNA synthesis (5 h postinfection), were not found in membrane-protected complexes in lysates of cells prepared by Dounce homogenization but were found to be susceptible to micrococcal nuclease (85%) or to sediment to a pellet in a cesium chloride gradient (61%). They therefore are probably free molecules in solution or components of easily dissociable complexes. By contrast, the majority of minus-strand RNAs (genome length and subgenomic mRNA length) were found to be resistant to micrococcal nuclease (69%) or to remain suspended in association with membrane-protected complexes following isopycnic sedimentation in a cesium chloride gradient (85%). Furthermore, 35% of the suspended minus strands were in a dense complex (1.20 to 1.24 g/ml) that contained an RNA plus-to-minus-strand molar ratio of approximately 8:1 and viral structural proteins S, M, and N, and 65% were in a light complex (1.15 to 1.17 g/ml) that contained nearly equimolar amounts of plus- and minus-strand RNAs and only trace amounts of proteins M and N. In no instance during fractionation were genome-length minus strands found segregated from sub-genome-length minus strands. These results indicate that all minus-strand species are components of similarly structured membrane-associated replication complexes and support the concept that all are active in the synthesis of plus-strand RNAs.

The molecular biology of coronaviruses

This chapter discusses the manipulation of clones of coronavirus and of complementary DNAs (cDNAs) of defective-interfering (DI) RNAs to study coronavirus RNA replication, transcription, recombination, processing and transport of proteins, virion assembly, identification of cell receptors for coronaviruses, and processing of the polymerase. The nature of the coronavirus genome is nonsegmented, single-stranded, and positive-sense RNA. Its size ranges from 27 to 32 kb, which is significantly larger when compared with other RNA viruses. The gene encoding the large surface glycoprotein is up to 4.4 kb, encoding an imposing trimeric, highly glycosylated protein. This soars some 20 nm above the virion envelope, giving the virus the appearance-with a little imagination-of a crown or coronet. Coronavirus research has contributed to the understanding of many aspects of molecular biology in general, such as the mechanism of RNA synthesis, translational control, and protein transport and processing. It remains a treasure capable of generating unexpected insights.

The molecular biology of coronaviruses

This chapter discusses the manipulation of clones of coronavirus and of complementary DNAs (cDNAs) of defective-interfering (DI) RNAs to study coronavirus RNA replication, transcription, recombination, processing and transport of proteins, virion assembly, identification of cell receptors for coronaviruses, and processing of the polymerase. The nature of the coronavirus genome is nonsegmented, single-stranded, and positive-sense RNA. Its size ranges from 27 to 32 kb, which is significantly larger when compared with other RNA viruses. The gene encoding the large surface glycoprotein is up to 4.4 kb, encoding an imposing trimeric, highly glycosylated protein. This soars some 20 nm above the virion envelope, giving the virus the appearance-with a little imagination-of a crown or coronet. Coronavirus research has contributed to the understanding of many aspects of molecular biology in general, such as the mechanism of RNA synthesis, translational control, and protein transport and processing. It remains a treasure capable of generating unexpected insights.

Membrane protein molecules of transmissible gastroenteritis coronavirus also expose the carboxy-terminal region on the external surface of the virion

The binding domains of four monoclonal antibodies (MAbs) specific for the M protein of the PUR46-MAD strain of transmissible gastroenteritis coronavirus (TGEV) have been located in the 46 carboxy-terminal amino acids of the protein by studying the binding of MAbs to recombinant M protein fragments. Immunoelectron microscopy using these MAbs demonstrated that in a significant proportion of the M protein molecules, the carboxy terminus is exposed on the external surface both in purified viruses and in nascent TGEV virions that recently exited infected swine testis cells. The same MAbs specifically neutralized the infectivity of the PUR46-MAD strain, indicating that the C-terminal domain of M protein is exposed on infectious viruses. This topology of TGEV M protein probably coexists with the structure currently described for the M protein of coronaviruses, which consists of an exposed amino terminus and an intravirion carboxy-terminal domain. The presence of a detectable number of M protein molecules with their carboxy termini exposed on the surface of the virion has relevance for viral function, since it has been shown that the carboxy terminus of M protein is immunodominant and that antibodies specific for this domain both neutralize TGEV and mediate the complement-dependent lysis of TGEV-infected cells.

Membrane assembly of the triple-spanning coronavirus M protein. Individual transmembrane domains show preferred orientation

The M protein of mouse hepatitis virus strain A59 is a triple-spanning membrane protein which assembles with an uncleaved internal signal sequence, adopting an NexoCcyt orientation. To study the insertion mechanism of this protein, domains potentially involved in topogenesis were deleted and the effects analyzed in topogenesis were deleted and the effects analyzed in several ways. Mutant proteins were synthesized in a cell-free translation system in the presence of microsomal membranes, and their integration and topology were determined by alkaline extraction and by protease-protection experiments. By expression in COS-1 and Madin-Darby canine kidney-II cells, the topology of the mutant proteins was also analyzed in vivo. Glycosylation was used as a biochemical marker to assess the disposition of the NH2 terminus. An indirect immunofluorescence assay on semi-intact Madin-Darby canine kidney-II cells using domain-specific antibodies served to identify the cytoplasmically exposed domains. The results show that each membrane-spanning domain acts independently as an insertion and anchor signal and adopts an intrinsic preferred orientation in the lipid bilayer which corresponds to the disposition of the transmembrane domain in the wild-type assembled protein. These observations provide further insight into the mechanism of membrane integration of multispanning proteins. A model for the insertion of the coronavirus M protein is proposed.

Membrane assembly of the triple-spanning coronavirus M protein. Individual transmembrane domains show preferred orientation

The M protein of mouse hepatitis virus strain A59 is a triple-spanning membrane protein which assembles with an uncleaved internal signal sequence, adopting an NexoCcyt orientation. To study the insertion mechanism of this protein, domains potentially involved in topogenesis were deleted and the effects analyzed in topogenesis were deleted and the effects analyzed in several ways. Mutant proteins were synthesized in a cell-free translation system in the presence of microsomal membranes, and their integration and topology were determined by alkaline extraction and by protease-protection experiments. By expression in COS-1 and Madin-Darby canine kidney-II cells, the topology of the mutant proteins was also analyzed in vivo. Glycosylation was used as a biochemical marker to assess the disposition of the NH2 terminus. An indirect immunofluorescence assay on semi-intact Madin-Darby canine kidney-II cells using domain-specific antibodies served to identify the cytoplasmically exposed domains. The results show that each membrane-spanning domain acts independently as an insertion and anchor signal and adopts an intrinsic preferred orientation in the lipid bilayer which corresponds to the disposition of the transmembrane domain in the wild-type assembled protein. These observations provide further insight into the mechanism of membrane integration of multispanning proteins. A model for the insertion of the coronavirus M protein is proposed.

The 9-kDa hydrophobic protein encoded at the 3' end of the porcine transmissible gastroenteritis coronavirus genome is membrane-associated

The open reading frame potentially encoding a 78 amino acid, 9101 Da hydrophobic protein (HP) and, mapping at the 3' end of the porcine transmissible gastroenteritis coronavirus (TGEV) genome, was shown to be expressed during virus replication. The cloned HP gene was placed in a plasmid under control of the T7 RNA polymerase promoter and in vitro translation of transcripts generated in vitro yielded a 9.1-kDa protein that was immunoprecipitable with porcine hyperimmune anti-TGEV serum. Antiserum raised in rabbits against a 31 amino acid synthetic polypeptide that represented the central hydrophilic region of HP specifically immunoprecipitated HP from TGEV-infected cells. HP was further shown to become associated with microsomal membranes during synthesis in vitro and was found to be closely associated with the endoplasmic reticulum and cell surface membranes in infected cells. The intracellular location of HP suggests that it may play a role in the membrane association of replication complexes or in virion assembly.

Enhancement of the vaccinia virus/phage T7 RNA polymerase expression system using encephalomyocarditis virus 5'-untranslated region sequences

A recombinant vaccinia virus producing the bacteriophage T7 RNA polymerase was used to express foreign genes in eukaryotic cells. Translation efficiency in this expression system was enhanced significantly by employing the encephalomyocarditis virus (EMCV) 5'-untranslated region (UTR) which confers cap-independent translation by directing internal initiation of translation. The enhancement was accomplished by fusing open reading frames (ORFs) to the N terminus of the EMCV polyprotein coding region, thus utilizing its highly efficient translation initiation site. Expression vectors were constructed to allow cloning in all three reading frames. As reporter genes, we used the lacZ gene and a number of genes encoding coronavirus structural proteins: among others the genes encoding glycoproteins with N-terminal signal sequences. The signal sequences of these glycoproteins are located internally in the primary translation product. We demonstrated that this did not interfere with translocation and glycosylation and yields biologically active proteins. The usefulness of sequences that direct internal initiation was extended by using EMCV UTRs to express two and three ORFs from polycistronic mRNAs.

Fixed-cell immunoperoxidase technique for the study of surface antigens induced by the coronavirus of transmissible gastroenteritis (TGEV)

An immunoperoxidase technique performed on the TGEV-infected cells was developed for detection of virus-induced antigens. The presence of M antigen of TGEV on the surface of infected cells was demonstrated by this technique. This finding is in contrast to the M protein of murine hepatitis coronavirus which migrates to the Golgi apparatus but is not transported to the plasma membrane. The time course of appearance M and S antigens on the surface of TGEV-infected cell can be studied by this technique.

A Golgi retention signal in a membrane-spanning domain of coronavirus E1 protein

The E1 glycoprotein from an avian coronavirus is a model protein for studying retention in the Golgi complex. In animal cells expressing the protein from cDNA, the E1 protein is targeted to cis Golgi cisternae (Machamer, C. E., S. A. Mentone, J. K. Rose, and M. G. Farquhar. 1990. Proc. Natl. Acad. Sci. USA. 87:6944-6948). We show that the first of the three membrane-spanning domains of the E1 protein can retain two different plasma membrane proteins in the Golgi region of transfected cells. Both the vesicular stomatitis virus G protein and the alpha-subunit of human chorionic gonadotropin (anchored to the membrane by fusion with the G protein membrane-spanning domain and cytoplasmic tail) were retained in the Golgi region of transfected cells when their single membrane-spanning domains were replaced with the first membrane-spanning domain from E1. Single amino acid substitutions in this sequence released retention of the chimeric G protein, as well as a mutant E1 protein which lacks the second and third membrane-spanning domains. The important feature of the retention sequence appears to be the uncharged polar residues which line one face of a predicted alpha helix. This is the first retention signal to be defined for a resident Golgi protein. The fact that it is present in a membrane-spanning domain suggests a novel mechanism of retention in which the membrane composition of the Golgi complex plays an instrumental role in retaining its resident proteins.

Another triple-spanning envelope protein among intracellularly budding RNA viruses: the torovirus E protein

The nucleotide sequence of the Berne virus envelope (E) protein gene was determined and its 26.5K translation product was identified by in vitro transcription and translation. Computer analysis of the protein sequence revealed the characteristics of a class III membrane protein lacking a cleaved signal sequence but containing three successive transmembrane alpha-helices in the N-terminal half, much the same as the coronavirus membrane (M) protein. The disposition of the E protein in the membrane was studied by in vitro translation in the presence of microsomes and by subsequent proteinase K digestion. Only small portions of either end of the polypeptide were found to be exposed on opposite sides of the vesicle membranes. Experiments with a hybrid E protein (EM) containing the C-terminal tail of a coronavirus M protein, to which an anti-peptide serum was available, showed that this C-terminus was present at the cytoplasmic side of the membrane, which is another similarity to the coronavirus M protein. Immunofluorescence experiments indicated that the EM protein, expressed by a recombinant vaccinia virus, accumulated in intracellular membranes, predominantly those of the endoplasmic reticulum. The common features of the torovirus E and the coronavirus M protein support our hypothesis that an evolutionary relationship exists between these groups of intracellularly budding viruses.

Primary structure of the membrane and nucleocapsid protein genes of feline infectious peritonitis virus and immunogenicity of recombinant vaccinia viruses in kittens

Feline infectious peritonitis virus (FIPV) causes a mostly fatal, immunologically mediated disease in cats. Previously, we demonstrated that immunization with a recombinant vaccinia virus expressing the FIPV spike protein (S) induced early death after challenge with FIPV (Vennema et al., 1990, J. Virol. 64, 1407-1409). In this paper we describe similar immunizations with the FIPV membrane (M) and nucleocapsid (N) proteins. The genes encoding these proteins were cloned and sequenced. Comparison of the amino acid sequences with the corresponding sequences of porcine transmissible gastroenteritis virus revealed 84.7 and 77% identity for M and N, respectively. Vaccinia virus recombinants expressing the cloned genes induced antibodies in immunized kittens. Immunization with neither recombinant induced early death after challenge with FIPV, strongly suggesting that antibody-dependent enhancement is mediated by antibodies against S only. Immunization with the N protein recombinant had no apparent effect on the outcome of challenge. However, three of eight kittens immunized with the M protein recombinant survived the challenge, as compared to one of eight kittens of the control group.

Minus-strand copies of replicating coronavirus mRNAs contain antileaders

The 5' leader sequence on mRNAs of the porcine transmissible gastroenteritis coronavirus was determined and found to be 90 nucleotides in length. An oligodeoxynucleotide with a sequence from within the leader was used as a probe in Northern analysis on RNA from infected cells, and an antileader (a minus-strand copy of the leader sequence) was shown to be present on all mRNA minus-strand species. RNase protection analysis showed the antileader to be approximately the same length as the leader. The kinetics of antileader appearance was the same as that for the appearance of minus-strand RNA species. This, along with a demonstration that viral mRNAs become packaged, gives further support to the idea that coronavirus mRNAs can undergo replication via subgenomic mRNA-length replicative intermediates, and that input mRNAs from infecting virions may serve as initial templates for their own replication. In this sense, then, coronaviruses behave in part like RNA viruses with segmented genomes.

Expression of swine transmissible gastroenteritis virus envelope antigens on the surface of infected cells: epitopes externally exposed

The peplomer protein (S) and the transmembrane protein (M) of transmissible gastroenteritis virus (TGEV) of swine were identified by iodination and serologically on the surface of infected cells. Of a total of 4 monoclonal antibodies (mAb) directed against four antigenic sites of S protein (Correa et al., 1988), 3 specific for sites A, B and D attached to the plasma membrane of infected cells, as disclosed by indirect immunofluorescence and by complement-mediated cytolysis. Four of the mAbs assayed were specific for the viral protein M and two of them gave plasma membrane immunofluorescence and mediated cytolysis in the presence of complement. The viral nucleoprotein N could not be demonstrated on the surface of infected cells either by iodination or employing 3 mAbs against this protein. Finally, a time course infection experiment demonstrated that S and M proteins were expressed on the surface of infected cells at 4 h after infection, before infective virus was released from infected cells.

Sequence analysis of the membrane protein gene of human coronavirus 229E

Human coronaviruses (HCV) are ubiquitous pathogens which cause respiratory, gastrointestinal, and possibly neurological disorders. To better understand the molecular biology of the prototype HCV-229E strain, the complete nucleotide sequence of the membrane protein (M) gene was determined from cloned cDNA. The open reading frame is preceded by a consensus transcriptional initiation sequence UCUAAACU, identical to the one found upstream of the N gene. The M gene encodes a 225-amino acid polypeptide with a molecular weight (MW) of 25,822, slightly higher than the apparent MW of 19,000-22,000 observed for the unprocessed M protein obtained after in vitro translation and immunoprecipitation. The M amino acid sequence presents a significant degree of homology (38%) with its counterpart of transmissible gastroenteritis coronavirus (TGEV). The M protein of HCV-229E is highly hydrophobic and its hydropathicity profile shows a transmembranous region composed of three major hydrophobic domains characteristic of a typical coronavirus M protein. About 10% (20 amino acids) of the HCV-229E M protein constitutes a hydrophilic and probably external portion. One N-glycosylation and three potential O-glycosylation sites are found in this exposed domain.

Expression of the porcine transmissible gastroenteritis coronavirus M protein

Cloned cDNA encoding the M protein of the porcine transmissible gastroenteritis coronavirus (TGEV) was introduced into a vaccinia virus to examine the function of the amino-terminal signal peptide. The M protein expressed by the recombinant virus was targeted to the Golgi region of infected cells, as is the M protein in cells infected with TGEV. The protein appeared not to undergo processing other than glycosylation. However, the vaccinia-expressed M protein was slightly larger than the protein found in TGEV-infected cells, suggesting that a difference in modification exists between the proteins.

Sequence and analysis of BECV F15 matrix protein

Clones from the bovine enteric coronavirus (F15) cDNA library were cloned in pBR322 and sequenced by the method of Sanger and Coulson. This led to the identification of a sequence of 1,300 bases which contained a single open reading frame of 690 bases yielding a protein having properties of the matrix protein (M). It was comprised of 230 amino acids with a molecular weight of 26,376 Da. It was hydrophobic and had a net charge of +8 at neutral pH. Analysis of its secondary structure could not establish a simple transmembrane arrangement of the amino acids. Comparison of its nucleotide sequence with that of BECV Mebus strain showed only a two-base change resulting in a 100% homology between the two amino acid sequences. Furthermore, a very conserved structure of M appeared on comparison with the Dayoff optimal alignment of MHV-A59, MHV-JHM, TGEV, IBV Beaudette and IBV 6/82M amino acid sequences. As the two strains of BECV, F15 and Mebus present some antigenic differences, this led us to reconsider the role of M in viral antigen specificity. A hypothesis is that, as it seems to possess the necessary information on its transmembrane region, it is an ideal candidate for the viral budding process.

Coronavirus subgenomic minus-strand RNAs and the potential for mRNA replicons

The genome of the porcine transmissible gastroenteritis coronavirus is a plus-strand, polyadenylylated, infectious RNA molecule of approximately 20 kilobases. During virus replication, seven subgenomic mRNAs are generated by what is thought to be a leader-priming mechanism to form a 3'-coterminal nested set. By using radiolabeled, strand-specific, synthetic oligodeoxynucleotide probes in RNA blot hybridization analyses, we have found a minus-strand counterpart for the genome and for each subgenomic mRNA species in the cytoplasm of infected cells. Subgenomic minus strands were found to be components of double-stranded replicative forms and in numbers that surpass full-length antigenome. We propose that subgenomic mRNA replication, in addition to leader-primed transcription, is a significant mechanism of mRNA synthesis and that it functions to amplify mRNAs. It is a mechanism of amplification that has not been described for any other group of RNA viruses. Subgenomic replicons may also function in a manner similar to genomes of defective interfering viruses to lead to the establishment of persistent infections, a universal property of coronaviruses.

Nucleotide sequence of the gene encoding the membrane protein of human coronavirus 229 E

The sequence of the gene encoding the membrane protein of human coronavirus 229 E (HCV 229 E) has been determined. The primary translation product, deduced from the DNA sequence, is a polypeptide of 225 amino acids with a predicted molecular weight of 26,000. The polypeptide has 3 potential N-glycosylation sites. Many structural similarities with the membrane proteins of other coronaviruses can be recognized.