Role of the coronavirus E viroporin protein transmembrane domain in virus assembly

Coronavirus envelope (E) proteins are small (approximately 75- to 110-amino-acid) membrane proteins that have a short hydrophilic amino terminus, a relatively long hydrophobic membrane domain, and a long hydrophilic carboxy-terminal domain. The protein is a minor virion structural component that plays an important, not fully understood role in virus production. It was recently demonstrated that the protein forms ion channels. We investigated the importance of the hydrophobic domain of the mouse hepatitis coronavirus (MHV) A59 E protein. Alanine scanning insertion mutagenesis was used to examine the effect of disruption of the domain on virus production in the context of the virus genome by using a MHV A59 infectious clone. Mutant viruses exhibited smaller plaque phenotypes, and virus production was significantly crippled. Analysis of recovered viruses suggested that the structure of the presumed alpha-helical structure and positioning of polar hydrophilic residues within the predicted transmembrane domain are important for virus production. Generation of viruses with restored wild-type helical pitch resulted in increased virus production, but some exhibited decreased virus release. Viruses with the restored helical pitch were more sensitive to treatment with the ion channel inhibitor hexamethylene amiloride than were the more crippled parental viruses with the single alanine insertions, suggesting that disruption of the transmembrane domain affects the functional activity of the protein. Overall the results indicate that the transmembrane domain plays a crucial role during biogenesis of virions.