Characterization of a large, proteolytically processed cowpox virus membrane glycoprotein conserved in most chordopoxviruses

- Homologs of the cowpox virus 219 protein are encoded by most chordopoxviruses.
- The cowpox virus 219 homologs are the largest poxvirus proteins.
- The protein traffics the secretory pathway where glycosylation and cleavage occur.
- N- and C-terminal fragments remain associated and inserted into the plasma membrane.
- The 219 protein is not required for cowpox virus replication and virulence in mice.

Most poxvirus proteins are either highly conserved and essential for basic steps in replication or less conserved and involved in host interactions. Homologs of the CPXV219 protein, encoded by cowpox virus, are present in nearly all chordopoxvirus genera and some species have multiple copies. The CPXV219 homologs have estimated masses of greater than 200 kDa, making them the largest known poxvirus proteins. We showed that CPXV219 was expressed early in infection and cleaved into N- and C-terminal fragments that remained associated. The protein has a signal peptide and transited the secretory pathway where extensive glycosylation and proteolytic cleavage occurred. CPXV219 was located by immunofluorescence microscopy in association with the endoplasmic reticulum, Golgi apparatus and plasma membrane. In non-permeabilized cells, CPXV219 was accessible to external antibody and biotinylation. Mutants that did not express CPXV219 replicated normally in cell culture and retained virulence in a mouse respiratory infection model.

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