The vaccinia virus soluble interferon-c receptor is a homodimer

The vaccinia virus (VV) interferon (IFN)-c receptor (IFN-cR) is a 43 kDa soluble glycoprotein that is secreted from infected cells early during infection. Here we demonstrate that the IFN-cR from VV, cowpox virus and camelpox virus exists naturally as a homodimer, whereas the cellular IFN-cR dimerizes only upon binding the homodimeric IFN-c. The existence of the virus protein as a dimer in the absence of ligand may provide an advantage to the virus in efficient binding and inhibition of IFN-c in solution. Vaccinia virus (VV) is an orthopoxvirus that expresses many proteins that modulate the host response to infection (Smith et al., 1997b). One strategy to suppress the host response to infection employed by VV and other poxviruses is to express proteins that are secreted from the infected cell and which bind host cytokines, chemokines, complement factors or interferons (IFNs). These secreted virus proteins often share amino acid similarity with host proteins that bind the same ligands, although this is not always the case. To date, VV has been shown to express soluble proteins that bind tumour necrosis factor (TNF) (Alcamı! et al., 1999), interleukin (IL)-1