The vaccinia virus I5L open reading frame encodes a 79-amino-acid protein, with two predicted transmembrane domains, conserved among all sequenced members of the chordopoxvirus subfamily. No nonpoxvirus homologs or functional motifs have been recognized, and the role of the I5 protein remains unknown. I5 synthesis was dependent on viral DNA replication and occurred exclusively at late times, consistent with a consensus late promoter motif adjacent to the start of the open reading frame. I5 was present in preparations of purified virions and could be extracted with nonionic detergent, suggesting membrane insertion. Transmission electron microscopy of immunogold-labeled thawed cryosections of infected cells revealed the association of an epitope-tagged I5 with the membranes of immature and mature virions. Viable I5L deletion and frameshift mutants were constructed and found to replicate like wild-type virus in a variety of cell lines, indicating that the protein was dispensable for in vitro cultivation. However, mouse intranasal challenge experiments indicated that a mutant virus with a frameshift resulting in a stop codon near the N terminus of I5 was attenuated compared to control virus. The attenuation correlated with clearance of mutant viruses from the respiratory tract and with less progression and earlier resolution of pathological changes. We suggest that I5 is involved in an aspect of host defense that is evolutionarily conserved although a role in cell tropism should also be considered.

The vaccinia virus A43R open reading frame encodes a 168-amino acid protein with a predicted N-terminal signal sequence and a C-terminal transmembrane domain. Although A43R is conserved in all sequenced members of the orthopoxvirus genus, no non-orthopoxvirus homolog or functional motif was recognized. Biochemical and confocal microscopic studies indicated that A43 is expressed at late times following viral DNA synthesis and is a type-1 membrane protein with two N-linked oligosaccharide chains. Neither mature nor enveloped virions contained appreciable amounts of A43, which was detected in Golgi and plasma membranes. Loss of A43R expression had no discernible effect on plaque size or virus replication in cell culture and little effect on virulence in a mouse intranasal infection model. Although the A43 mutant produced significantly smaller lesions in the skin of mice than the control, the amounts of virus recovered from the lesions were similar.