Coronaviruses (CoVs) assemble by budding into the lumen of the early Golgi complex prior to exocytosis. The small CoV envelope (E) protein plays roles in assembly, virion release, and pathogenesis. CoV E has a single hydrophobic domain (HD), is targeted to Golgi complex membranes, and has cation channel activity in vitro. However, the precise functions of the CoV E protein during infection are still enigmatic. Structural data for the severe acute respiratory syndrome (SARS)-CoV E protein suggest that it assembles into a homopentamer. Specific residues in the HD regulate the ion-conducting pore formed by SARS-CoV E in artificial bilayers and the pathogenicity of the virus during infection. The E protein from the avian infectious bronchitis virus (IBV) has dramatic effects on the secretory system which require residues in the HD. Here, we use the known structural data from SARS-CoV E to infer the residues important for ion channel activity and the oligomerization of IBV E. We present biochemical data for the formation of two distinct oligomeric pools of IBV E in transfected and infected cells and the residues required for their formation. A high-order oligomer of IBV E is required for the production of virus-like particles (VLPs), implicating this form of the protein in virion assembly. Additionally, disruption of the secretory pathway by IBV E correlates with a form that is likely monomeric, suggesting that the effects on the secretory pathway are independent of E ion channel activity. IMPORTANCE CoVs are important human pathogens with significant zoonotic potential, as demonstrated by the emergence of SARS-CoV and Middle East respiratory syndrome (MERS)-CoV. Progress has been made toward identifying potential vaccine candidates in mouse models of CoV infection, including the use of attenuated viruses that lack the CoV E protein or express E-protein mutants. However, no approved vaccines or antiviral therapeutics exist. We previously reported that the hydrophobic domain of the IBV E protein, a putative viroporin, causes disruption of the mammalian secretory pathway when exogenously expressed in cells. Understanding the mechanism of this disruption could lead to the identification of novel antiviral therapeutics. Here, we present biochemical evidence for two distinct oligomeric forms of IBV E, one essential for assembly and the other with a role in disruption of the secretory pathway. Discovery of two forms of CoV E protein will provide additional targets for antiviral therapeutics. (1). There has been some success in the development of mouse models of SARS-CoV and MERS-CoV infection, and candidate vaccines where the envelope (E) protein is deleted or mutated have been described (2-7). However, there is still much that is unclear regarding the role of the CoV E protein in infection.CoVs acquire their membrane envelope by budding into the endoplasmic reticulum (ER)-Golgi complex intermediate compartment (ERGIC), a characteristic that sets CoVs apart from other well-studied enveloped viruses (8). The infect...

Emerin is a conserved membrane component of nuclear lamina structure. Here, we report an advance in understanding the molecular basis of emerin function: intermolecular emerin-emerin association. There were two modes: one mediated by association of residues 170-220 in one emerin molecule to residues 170-220 in another, and the second involving residues 170-220 and 1-132. Deletion analysis showed residues 187-220 contain a positive element essential for intermolecular association in cells. By contrast, deletion of residues 168-186 inactivated a proposed negative element, required to limit or control association. Association of GFP-emerin with nuclear BAF in cells required the LEM domain (residues 1-47) and the positive element. Emerin peptide arrays revealed direct binding of residues 170-220 to residues 206-225 (the proposed positive element), residues 147-174 (particularly P 153 MYGRDSAYQSITHYRP 169 ) and the LEM domain. Emerin residues 1-132 and 159-220 were each sufficient to bind lamin A or B1 tails in vitro, identifying two independent regions of molecular contact with lamins. These results, and predicted emerin intrinsic disorder, support the hypothesis that there are multiple 'backbone' and LEM-domain configurations in a proposed intermolecular emerin network at the nuclear envelope.

Coronaviruses (CoVs) assemble by budding into the lumen of the early Golgi complex prior to exocytosis. The small CoV envelope (E) protein plays roles in assembly, virion release, and pathogenesis. CoV E has a single hydrophobic domain (HD), is targeted to Golgi membranes, and has cation channel activity in vitro. The E protein from avian infectious bronchitis virus (IBV) has dramatic effects on the secretory system, which require residues in the HD. Mutation of the HD of IBV E in a recombinant virus background results in impaired growth kinetics, impaired release of infectious virions, accumulation of IBV spike (S) protein on the plasma membrane compared to wild-type (WT) IBV-infected cells, and aberrant cleavage of IBV S on virions. We previously reported the formation of two distinct oligomeric pools of IBV E in transfected and infected cells. Disruption of the secretory pathway by IBV E correlates with a form that is likely monomeric, suggesting that the effects on the secretory pathway are independent of E ion channel activity. Here, we present evidence suggesting that the monomeric form of IBV E correlates with an increased Golgi luminal pH. Infection with IBV or expression of IBV E induces neutralization of Golgi pH, promoting a model in which IBV E alters the secretory pathway through interaction with host cell factors, protecting IBV S from premature cleavage and leading to the efficient release of infectious virus from the cells. This is the first demonstration of a coronavirus-induced alteration in the microenvironment of the secretory pathway. IMPORTANCE Coronaviruses are important human pathogens with significant zoonotic potential. Progress has been made toward identifying potential vaccine candidates for highly pathogenic human CoVs, including the use of attenuated viruses that lack the CoV E protein or express E mutants. However, no approved vaccines or antiviral therapeutics exist. Understanding the role of the CoV E protein in virus assembly and release is thus an important prerequisite for potential vaccines as well as in identifying novel antiviral therapeutics.