Characterization and functional analysis of Serp3: a novel myxoma virus-encoded serpin involved in virulence.

Myxoma virus (MV), a member of the family Poxviridae, is the causative agent of myxomatosis, a fatal disease of the European rabbit. The MV genome is a linear, double-stranded DNA molecule that encodes several factors important for evasion of the host immune system. Sequencing the right-end region of the MV genome identified an 801 bp open reading frame (ORF) encoding a polypeptide that belongs to the serpin superfamily. To date, two MV-encoded serpins have been characterized: SERP-1 binds to several targets and is an anti-inflammatory molecule, whereas Serp2 is essential for virus virulence and has both anti-inflammatory and anti-apoptotic effects. Thus, Serp3 is the third MV-encoded serpin. DNA sequence analysis of Serp3 indicated a similarity to poxvirus late promoters, which was confirmed by mRNA expression analysis. Serp3 has an atypical serpin motif and has significant sequence deletions as compared to most cellular and viral serpins. However, molecular modelling studies suggested that Serp3 can retain the overall serpin fold. Insertional inactivation of the serp3 ORF led to a significant attenuation of virulence in vivo (as measured by the increase in survival of infected rabbits) and limited dissemination of the virus to secondary sites of infection. In rabbits infected with a Serp3 deletion mutant (MV-Serp3(-)), the main histopathological feature is the absence of secondary myxomas. Both wild-type MV and MV-Serp3(-) replicate at comparable levels in vivo. Serp3 may represent a significant virulence factor of MV and probably acts in synergy with other viral proteins.