Ectromelia virus virulence factor p28 acts upstream of caspase-3 in response to UV light-induced apoptosis

Ectromelia virus (EV) virulence factor p28 (EVp28) is a member of a family of poxvirus proteins that are defined largely by the presence of a C-termin al RING finger motif and localization to virus factories within the cytopla sm of infected cells. Previously, overexpression of the Shope fibroma virus (SFV) homologue, N1R, in vaccinia virus (VV)-infected BGMK cells was found to inhibit virus-induced apoptosis. Here, we report that both EVp28 and ov erexpression of SFV N1R in poxvirus-infected HeLa cells protect specificall y from UV light-induced apoptosis, but not from apoptosis induced by Fas or TNF, Further, we report that both VV and EV protect from apoptosis induced by UV, Fas and TNF, Immunoblot analysis indicates that EVp28 acts upstream of caspase-3, blocking activation of the protease in response to UV irradi ation. Although no difference was found in replication of an EVp28(-) mutan t virus, which expresses a truncated p28 protein lacking the RING motif, co mpared to EV wild-type in HeLa cells, UV irradiation of infected HeLa cells reduced the replication of the EV mutant compared with wild-type EV.