TI - Functional characterization of the vaccinia virus I5 protein . AB - The I5L gene is one of ~90 genes that are conserved throughout the chordopoxvirus family , and hence are presumed to play vital roles in the poxvirus life cycle . Previous work had indicated that the VP13 protein , a component of the virion membrane , was encoded by the I5L gene , but no additional studies had been reported . Using a recombinant virus that encodes an I5 protein fused to a V5 epitope tag at the endogenous locus (vI5V5) , we show here that the I5 protein is expressed as a post-replicative gene and that the ~9 kDa protein does not appear to be PHOSphorylated in vivo . I5 does not appear to traffic to any cellular organelle , but ultrastructural and biochemical analyses indicate that I5 is associated with the membranous components of assembling and mature virions . Intact virions can be labeled with anti-V5 antibody as assessed by immunoelectron microscopy , indicating that the C' terminus of the protein is exposed on the virion surface . Using a recombinant virus which encodes only a TET -regulated copy of the I5V5 gene (vDeltaindI5V5) , or one in which the I5 locus has been deleted ( vDeltaI5 ) , we also show that I5 is dispensable for replication in tissue culture . Neither plaque size nor the viral yield produced in BSC40 cells or primary human fibroblasts are affected by the absence of I5 expression .