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

Mutation of N15 or V25 abolishes ion-channel activity of SARS-CoV E in artificial membranes (11, 21) . We previously reported that the E protein of IBV is expressed in mammalian cells is found in two pools by velocity gradient analysis: a low molecular weight pool (LMW) and a high molecular weight pool (HMW) (26). The LMW pool represents IBV E in a monomeric state while the HMW pool correlates with a homo-oligomer of IBV E. When mutations corresponding to the conserved HD residues of SARS-CoV E that inhibit ion channel activity were made in IBV E (T16A and A26F), we found that the HD mutants segregated primarily into one oligomeric pool or the other. The E T16A mutant was primarily in the HMW pool while the E A26F mutant was found primarily in the LMW pool. The presence of the LMW pool of IBV E, the predominant and likely monomeric form found when E A26F is present, correlates with the secretory pathway disruption associated with the WT IBV E protein (26). This was surprising in that it suggested an E ion channel-independent role for IBV E associated with manipulation of the secretory pathway. It was recently reported that that these HD mutants do abolish ion channel activity of IBV E in artificial membranes, and virus titers are reduced by a log in the supernatant of infected cells, suggesting a defect in virion release (27). Our data on the IBV-EG3 virus corroborates this study (23).