Vaccinia virus encodes a novel inhibitor of apoptosis that associates with the apoptosome

Melissa R. Ryerson, Monique M. Richards, Marc Kvansakul, Christine J. Hawkins, Joanna L. Shisler

Research output: Contribution to journalArticlepeer-review


Apoptosis is an important antiviral host defense mechanism. Here we report the identification of a novel apoptosis inhibitor encoded by the vaccinia virus (VACV) M1L gene. M1L is absent in the attenuated modified vaccinia virus Ankara (MVA) strain of VACV, a strain that stimulates apoptosis in several types of immune cells. M1 expression increased the viability of MVA-infected THP-1 and Jurkat cells and reduced several biochemical hallmarks of apoptosis, such as PARP-1 and procaspase-3 cleavage. Furthermore, ectopic M1L expression decreased staurosporine-induced (intrinsic) apoptosis in HeLa cells. We then identified the molecular basis for M1 inhibitory function. M1 allowed mitochondrial depolarization but blocked procaspase-9 processing, suggesting that M1 targeted the apoptosome. In support of this model, we found that M1 promoted survival in Saccharomyces cerevisiae overexpressing human Apaf-1 and procaspase-9, critical components of the apoptosome, or overexpressing only conformationally active caspase-9. In mammalian cells, M1 coimmunoprecipitated with Apaf-1-procaspase-9 complexes. The current model is that M1 associates with and allows the formation of the apoptosome but prevents apoptotic functions of the apoptosome. The M1 protein features 14 predicted ankyrin (ANK) repeat domains, and M1 is the first ANK-containing protein reported to use this inhibitory strategy. Since ANK-containing proteins are encoded by many large DNA viruses and found in all domains of life, studies of M1 may lead to a better understanding of the roles of ANK proteins in virus-host interactions.

Original languageEnglish (US)
Article numbere01385-17
JournalJournal of virology
Issue number23
StatePublished - Dec 1 2017


  • Ankyrin repeat
  • Apaf-1
  • Apoptosis
  • Apoptosome
  • Caspase-9
  • M1L
  • Poxvirus
  • Vaccinia virus

ASJC Scopus subject areas

  • Microbiology
  • Immunology
  • Insect Science
  • Virology


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