Complex formation between the spike protein and the membrane protein during mouse hepatitis virus assembly

Studies on membrane topology, N-glycosylation and functionality of SARS-CoV membrane protein

The glycosylated membrane protein M of the severe acute respiratory syndrome associated coronavirus (SARS-CoV) is the main structural component of the virion and mediates assembly and budding of viral particles. The membrane topology of SARS-CoV M and the functional significance of its N-glycosylation are not completely understood as is its interaction with the surface glycoprotein S. Using biochemical and immunofluorescence analyses we found that M consists of a short glycosylated N-terminal ectodomain, three transmembrane segments and a long, immunogenic C-terminal endodomain. Although the N-glycosylation site of M seems to be highly conserved between group 1 and 3 coronaviruses, studies using a recombinant SARS-CoV expressing a glycosylation-deficient M revealed that N-glycosylation of M neither influence the shape of the virions nor their infectivity in cell culture. Further functional analysis of truncated M proteins showed that the N-terminal 134 amino acids comprising the three transmembrane domains are sufficient to mediate accumulation of M in the Golgi complex and to enforce recruitment of the viral spike protein S to the sites of virus assembly and budding in the ERGIC.

SARS coronavirus replicase proteins in pathogenesis

Much progress has been made in understanding the role of structural and accessory proteins in the pathogenesis of severe acute respiratory syndrome coronavirus (SARS-CoV) infections. The SARS epidemic also brought new attention to the proteins translated from ORF1a and ORF1b of the input genome RNA, also known as the replicase/transcriptase gene. Evidence for change within the ORF1ab coding sequence during the SARS epidemic, as well as evidence from studies with other coronaviruses, indicates that it is likely that the ORF1ab proteins play roles in virus pathogenesis distinct from or in addition to functions directly involved in viral replication. Recent reverse genetic studies have confirmed that proteins of ORF1ab may be involved in cellular signaling and modification of cellular gene expression, as well as virulence by mechanisms yet to be determined. Thus, the evolution of the ORF1ab proteins may be determined as much by issues of host range and virulence as they are by specific requirements for intracellular replication.

Characterization of severe acute respiratory syndrome coronavirus membrane protein

The coronavirus membrane protein (M) is the key player in the assembly of virions at intracellular membranes between endoplasmic‐reticulum and Golgi‐complex. Using a newly established human monoclonal anti‐M antibody we detected glycosylated and nonglycosylated membrane‐associated M in severe acute respiratory syndrome‐associated coronavirus (SARS‐CoV) infected cells and in purified virions. Further analyses revealed that M contained a single N‐glycosylation site at asparagine 4. Recombinant M was transported to the plasma membrane and gained complex‐type N‐glycosylation. In SARS‐CoV infected cells and in purified virions, however, N‐glycosylation of M remained endoglycosidase H‐sensitive suggesting that trimming of the N‐linked sugar side chain is inhibited.

Molecular interactions in the assembly of coronaviruses

This chapter describes the interactions between the different structural components of the viruses and discusses their relevance for the process of virion formation. Two key factors determine the efficiency of the assembly process: intracellular transport and molecular interactions. Many viruses have evolved elaborate strategies to ensure the swift and accurate delivery of the virion components to the cellular compartment(s) where they must meet and form (sub) structures. Assembly of viruses starts in the nucleus by the encapsidation of viral DNA, using cytoplasmically synthesized capsid proteins; nucleocapsids then migrate to the cytosol, by budding at the inner nuclear membrane followed by deenvelopment, to pick up the tegument proteins.

The structural characterization and antigenicity of the S protein of SARS-CoV

The corona-like spikes or peplomers on the surface of the virion under electronic microscope are the most striking features of coronaviruses. The S (spike) protein is the largest structural protein, with 1,255 amino acids, in the viral genome. Its structure can be divided into three regions: a long N-terminal region in the exterior, a characteristic transmembrane (TM) region, and a short C-terminus in the interior of a virion. We detected fifteen substitutions of nucleotides by comparisons with the seventeen published SARS-CoV genome sequences, eight (53.3%) of which are non-synonymous mutations leading to amino acid alternations with predicted physiochemical changes. The possible antigenic determinants of the S protein are predicted, and the result is confirmed by ELISA (enzyme-linked immunosorbent assay) with synthesized peptides. Another profound finding is that three disulfide bonds are defined at the C-terminus with the N-terminus of the E (envelope) protein, based on the typical sequence and positions, thus establishing the structural connection with these two important structural proteins, if confirmed. Phylogenetic analysis reveals several conserved regions that might be potent drug targets.

Virus maturation by budding

Enveloped viruses mature by budding at cellular membranes. It has been generally thought that this process is driven by interactions between the viral transmembrane proteins and the internal virion components (core, capsid, or nucleocapsid). This model was particularly applicable to alphaviruses, which require both spike proteins and a nucleocapsid for budding. However, genetic studies have clearly shown that the retrovirus core protein, i.e., the Gag protein, is able to form enveloped particles by itself. Also, budding of negative-strand RNA viruses (rhabdoviruses, orthomyxoviruses, and paramyxoviruses) seems to be accomplished mainly by internal components, most probably the matrix protein, since the spike proteins are not absolutely required for budding of these viruses either. In contrast, budding of coronavirus particles can occur in the absence of the nucleocapsid and appears to require two membrane proteins only. Biochemical and structural data suggest that the proteins, which play a key role in budding, drive this process by forming a three-dimensional (cage-like) protein lattice at the surface of or within the membrane. Similarly, recent electron microscopic studies revealed that the alphavirus spike proteins are also engaged in extensive lateral interactions, forming a dense protein shell at the outer surface of the viral envelope. On the basis of these data, we propose that the budding of enveloped viruses in general is governed by lateral interactions between peripheral or integral membrane proteins. This new concept also provides answers to the question of how viral and cellular membrane proteins are sorted during budding. In addition, it has implications for the mechanism by which the virion is uncoated during virus entry.

Assembled coronavirus from complementation of two defective interfering RNAs

In the presence of an RNA- temperature-sensitive (ts) mutant helper virus, two coronavirus mouse hepatitis virus (MHV) defective interfering (DI) RNAs complemented each other, resulting in the assembly of MHV particles; we used this ability to complement as a means to study coronavirus assembly. One of the two DI RNAs was DIssA, a naturally occurring self-replicating DI RNA encoding N protein and the gene 1 proteins that encode RNA polymerase function; DIssA supports the replication and transcription of other non-self-replicating DI RNAs. The other DI was a genetically engineered DI RNA that encoded sM and M proteins. Coinfection of these two DIs at the nonpermissive temperature for the ts helper virus resulted in replication and transcription of both DI RNAs but not in synthesis of the helper virus RNAs. MHV particles containing DI RNAs, N protein, and M protein, all of which were exclusively derived from the two DI RNAs, were released from the coinfected cells; the amount of sM protein was below the limits of detection. Analyses of DI RNAs with mutations in the two envelope protein genes demonstrated that M and sM proteins appeared to be required for assembly and release of MHV particles that contained DI RNAs and N protein, while S protein was not required for assembly and release of MHV particles.

Oligomerization of a trans-Golgi/trans-Golgi network retained protein occurs in the Golgi complex and may be part of its retention

The mouse hepatitis virus M protein is a triple spanning membrane glycoprotein that, when expressed independently, localizes to trans-Golgi as well as to the trans-Golgi network (TGN). Passage of this protein from the endoplasmic reticulum through the intermediate compartment to the late Golgi and TGN can be conveniently followed by analyzing its O-linked sugars. Using pulse-chase analyses we studied the oligomerization of the M protein in sucrose gradients. The Golgi and TGN forms migrated as large heterogeneous complexes, whereas the endoplasmic reticulum and intermediate compartment forms of the protein appeared to migrate as monomer. Moreover, a mutant of the M protein lacking the 22 COOH-terminal amino acids, that is transported to the plasma membrane, gave rise to similar complexes, albeit smaller in size, that persisted at the plasma membrane. We propose that the trans-Golgi/TGN retention of the MHV-M protein is governed by two mechanisms: oligomerization possibly mediated by the transmembrane domains and binding of its cytoplasmic tail to cellular factors in trans Golgi/TGN.

Coexpression and association of the spike protein and the membrane protein of mouse hepatitis virus

The M and S envelope glycoproteins of mouse hepatitis virus associate in the process of virus assembly. We have studied the intrinsic properties of M/S heterocomplexes by coexpressing M and S in the absence of other coronaviral proteins. The formation of M/S complexes under these conditions indicates that M and S can interact independently of other coronaviral factors. Pulse-chase analysis revealed that M and S associate in a pre-Golgi compartment. M/S complexes are efficiently transported beyond the coronavirus budding compartment to the Golgi complex. The failure to detect complexes at the surface of coexpressing cells demonstrated that they are retained intracellularly. Thus, coexpression of the envelope glycoproteins drastically affects the intracellular transport of the S protein: instead of being transported to the cell surface, S is retained intracellularly by its association with M.