The Infectious Bronchitis Virus Coronavirus Envelope Protein Alters Golgi pH to Protect Spike Protein and Promote Release of Infectious Virus
Coronaviruses (CoVs) are important human pathogens with significant zoonotic potential. Progress has been made toward identifying potential vaccine candidates for highly pathogenic human CoVs, including use of attenuated viruses that lack the CoV envelope (E) protein or express E mutants. However, no approved vaccines or anti-viral therapeutics exist. CoVs assemble by budding into the lumen of the early Golgi prior to exocytosis. The small CoV 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 the avian infectious bronchitis virus (IBV) has dramatic effects on the secretory system, which requires residues in the HD. Mutation of the HD of IBV E during infection results in impaired growth kinetics, impaired release of infectious virions, accumulation of IBV S protein on the plasma membrane when compared IBV WT infected cells, and aberrant cleavage of IBV S on the surface of 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 a rise in the pH of the Golgi lumen. We demonstrate that infection with IBV induces neutralization of Golgi luminal pH, promoting a model in which IBV E alters the secretory pathway through interaction with host cells factors, protecting IBV spike protein (S) from premature cleavage and leading to the efficient release of infectious virus from the cells.