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Chapter 12 : Coronavirus Structural Proteins and Virus Assembly
Category: Viruses and Viral Pathogenesis
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This chapter reviews the processing, targeting, and assembly of the coronavirus structural proteins, and the release of assembled virions from infected cells. Both virus-like particles (VLPs) and virus assembly are sensitive to deletion and changes of the two C-terminal residues. The most striking difference between the crippled viruses and recovered viruses that grew similarly to the wild-type virus was the positions of four hydrophilic polar residues along one face of the predicted β-helix. It is interesting that when key charged residues are modified, these independently give rise to overlapping second-site suppressor or adaptive changes in the same region of M protein. Results from these studies strongly argue that M-N interactions are complex, indicating that more than just the single R227 and D440-D441 charges are important. In coronaviruses that induce syncytia, cellular alternations may also significantly impact virus release. Polarized epithelial cells are the first cells infected during coronavirus infection. The apical or basolateral localization of the virus receptor determines the site of coronavirus entry in polarized epithelial cells. In respiratory epithelial cells, severe acute respiratory syndrome coronavirus (SARS-CoV) and human coronavirus strain 229E preferentially enter and are exocytosed from the apical surface. Rapid progress in this area of coronavirus research is expected. The emergence of SARS-CoV sparked tremendous interest in and recognition of coronaviruses as intriguing for their molecular and cellular biology and as significant pathogens.
Coronavirus structure and intracellular assembly site. (A) Electron micrograph of purified IBV particle after negative staining (left). Bar, 50 nm. Virion schematic showing the major structural proteins (right). Note that some coronaviruses contain additional E proteins (e.g., HE in some group 2 viruses and several accessory proteins in SARS-CoV). (B) Schematic of virus assembly in cells (left). The E proteins are synthesized in the ER and transported to the ERGIC/Golgi complex. Independent targeting signals and interactions with the other E proteins allow accumulation in the ERGIC, and after interaction with the nucleocapsid, virions bud into the lumen of the ERGIC. They are released from infected cells by exocytosis. The right panel is an electron micrograph of Vero cells infected with IBV, showing a Golgi region with budded virions inside (arrows). Bar, 500 nm.
Coronavirus N-protein phosphorylation. A schematic illustrating the three-domain model for coronavirus N proteins with A and B spacer domains ( 137 ) is shown at the top. The relative positions of the phosphorylated sites identified on intracellular N protein from TGEV-infected cells ( 14 ) and MHV-infected cells ( 196 ) and IBV N protein expressed alone ( 22 ) are shown below. The positions of the RNA-binding domain that includes the SR region ( 129 ) and putative dimerization domain ( 208 ) are indicated for MHV. The positions of the SR regions are indicated for TGEV and IBV.
Topology of coronavirus E proteins. Two topologies are shown for M and E proteins, as supported by evidence discussed in the text. Small triangles represent glycosylation but are not meant to indicate the number or type of oligosaccharides, which differ in the proteins from different coronaviruses. S proteins and some E proteins are palmitoylated on their cytoplasmic tails (indicated by the squiggly line).
Potential roles for the E protein transmembrane domain in release of infectious virus. (A) Electron micrographs of Vero cells infected for 14 h with IBV or IBV containing an E protein with a heterologous transmembrane domain (IBV-EG3). Typical pleomorphic transport intermediates are present in cells infected with wild-type IBV, but large spherical vacuoles containing virions and degraded material are prominent in cells infected with IBV-EG3. Bars, 500 nm. (B) Results from mutations in the E protein transmembrane domain suggest that this domain could promote maturation of virions in late Golgi or post-Golgi compartments (a), promote formation of virus containing transport intermediates (b), promote fusion of transport intermediates with the plasma membrane (c), or prevent fusion of transport intermediates with lysosomes (d). The three last roles could be as a nonstructural protein. Ion channel activity or other interactions of the E protein transmembrane domain could be involved.