Vaccinia Virus A6 Is a Two-Domain Protein Requiring a Cognate N-Terminal Domain for Full Viral Membrane Assembly Activity.

Poxvirus virion biogenesis is a complex, multistep process, starting with the formation of crescent-shaped viral membranes, followed by their enclosure of the viral core to form spherical immature virions. Crescent formation requires a group of proteins that are highly conserved among poxviruses, including A6 and A11 of vaccinia virus (VACV). To gain a better understanding of the molecular function of A6, we established a HeLa cell line that inducibly expressed VACV-A6, which allowed us to construct VACV mutants with an A6 deletion or mutation. As expected, the A6 deletion mutant of VACV failed to replicate in noncomplementing cell lines with defects in crescent formation and A11 localization. Surprisingly, a VACV mutant that had A6 replaced with a close ortholog from the Yaba-like disease virus YLDV-97 also failed to replicate. This mutant, however, developed crescents and had normal A11 localization despite failing to form immature virions. Limited proteolysis of the recombinant A6 protein identified an N domain and a C domain of approximately 121 and 251 residues, respectively. Various chimeras of VACV-A6 and YLDV-97 were constructed, but only one that precisely combined the N domain of VACV-A6 and the C domain of YLDV-97 supported VACV replication albeit at a reduced efficiency. Our results show that VACV-A6 has a two-domain architecture and functions in both crescent formation and its enclosure to form immature virions. While a cognate N domain is not required for crescent formation, it is required for virion formation, suggesting that interactions of the N domain with cognate viral proteins may be critical for virion assembly.