The Infectious Bronchitis Virus Coronavirus Envelope Protein Alters Golgi pH to Protect Spike Protein and Promote Release of Infectious Virus

Jason W. Westerbeck; Carolyn E. Machamer

One of the more fascinating and relatively enigmatic aspects of CoV biology is that CoV virions bud into the lumen of the secretory pathway at the endoplasmic reticulum-Golgi intermediate compartment (ERGIC) , and then must navigate through the Golgi and the anterograde endomembrane system to be efficiently released from the host cell. It has been demonstrated that the structure and function of the Golgi depends upon an acidic pH gradient that decreases from the lumen of the cis-Golgi to the lumen of the trans-Golgi. This pH gradient is produced by a balance maintained by proton influx into the lumen of the Golgi, proton leak, and counter-ion conductance (4) . Pharmacological manipulations of the pH gradient, resulting in neutralization of the lumen, have all been shown to cause slow trafficking of cargo through the Golgi as well as alteration in Golgi morphology (4) (5) (6) (7) (8) (9) (10) . A class of small viral membrane proteins with ion channel activity, called viroporins, have been shown to have dramatic effects on the secretory pathway, similar to those elicited by pharmacological manipulation of luminal pH (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) .Several well-studied members of this viroporin family of proteins include the Influenza A M2 protein (IAV M2), hepatitis C virus (HCV) p7 protein, and the CoV envelope (E) protein. These representative viroporins demonstrate several common functional features despite differences in viral assembly and budding locations. It has been suggested the role of M2 in the secretory . CC-BY-NC-ND 4.0 International license is made available under a The copyright holder for this preprint (which was not peer-reviewed) is the author/funder. It . https://doi.org/10.1101/440628 doi: bioRxiv preprint pathway is to neutralize luminal pH to protect the HA fusion protein of influenza from premature activation (5) (6) (7) . Overexpression of M2 causes secretory pathway disruption where the rate of intracellular trafficking is slowed and Golgi morphology is altered (8). HCV p7 is also thought to play a protective role by allowing egress of viral structural proteins through the secretory pathway. HCV can be partially rescued by both pharmacological neutralization of the luminal spaces by bafilomycin A1 and by in trans expression of IAV M2 (9, 10) . Similar to M2, the infectious bronchitis virus (IBV) coronavirus E protein elicits multiple secretory pathway disruption phenotypes when overexpressed in mammalian cells.