A mutational analysis of the vaccinia virus B5R protein

A mutational analysis of the vaccinia virus (VV) B5R protein is presented. This protein is related to the regulators of complement activation (RCA) su perfamily, has four short consensus repeats (SCRs) that are typical of this superfamily and is present on extracellular enveloped virus (EEV) particle s. Here we have constructed VV mutants in which the cytoplasmic tail (CT) o f the B5R protein is progressively truncated, and domains of the B5R protei n [the SCR (short consensus repeat) domains, the transmembrane anchor regio n or the CT] are substituted by corresponding domains from the VV haemagglu tinin (HA), another EEV protein. Analysis of these mutant viruses showed th at loss of the B5R CT did not affect the formation of intracellular envelop ed virus (IEV), actin tails, EEV or virus plaque size. However, if the SCR domains of the B5R protein were replaced by the corresponding region of the HA, the virus plaque size was diminished, the formation of actin tails was decreased severely and the titre of infectious EEV released from cells was reduced approximately 25-fold compared to wild-type virus and 5-fold compa red to a virus lacking the entire B5R gene. Thus the linkage of HA to the B 5R transmembrane and CT is deleterious for the formation and release of EEV and for cell-to-cell virus spread. In contrast, deletion or substitution o f the B5R CT did not affect virus replication, although the amount of cell surface B5R was reduced compared to control.