Myxoma virus (MYX) induces extensive immunosuppression in infected rabbits and is associated with high levels of mortality. The virus encodes multiple gene products designed to circumvent the cellular immune response to the viral infection. Deletion analysis has shown that the M11L open reading frame (ORF) is an important virulence factor which downregulates leukocyte infiltration of MYX-induced tumors. To investigate the role of the M11 L protein in viral pathogenesis, we sequenced the MYX M11L ORF and showed that the sequence has motifs consistent with a 166-aa class III membrane-spanning molecule possessing a single transmembrane helix near the C-terminus and a 142-aa N-terminal extracellular domain that has six cysteine residues plus two consensus N-glycosylation sites. Transcription analysis indicates that M11 L is expressed as an early gene, and surface immunofluorescence studies with anti-M11L antibodies reveal that M11 L protein is transported to the infected cell surface. Immunoprecipitation analysis of an attenuated viral recombinant, vMYX-GF-ΔM11L, indicates that an M11 L variant protein with an altered C-terminus is synthesized at about 45% of wild type levels; however, it is not detectable on the cell surface, suggesting that proper M11L function requires localization at the infected cell membrane. We propose that M11L is a virulence factor whose function is to recognize an extracellular ligand essential for the cellular inflammatory response.
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