Localization of the African swine fever virus attachment protein P12 in the virus particle by immunoelectron microscopy.

The African swine fever virus attachment protein p12 was localized in the virion by immunoelectron microscopy. Purified virus particles were incubated, before or after different treatments, with p12-specific monoclonal antibody 24BB7 and labeled with protein A-colloidal gold. Untreated virus particles showed labeling only in lateral protrusions that followed the external virus envelope. Mild treatment of African swine fever virions with the nonionic detergent octyl-glucoside or with ethanol onto the electron microscope grid resulted in a heavier and more homogeneous labeling of the virus particles. In contrast, the release of the external virus proteins by either octyl-glucoside or Nonidet-P40 and beta-mercaptoethanol generated a subviral fraction that was not labeled by 24BB7. Preembedding, labeling, and thin-sectioning experiments confirmed that the antigenic determinant recognized by 24BB7 was localized into the external region of the virus particle but required some disruption to make it more accessible. From these results we conclude that protein p12 is situated in a layer above the virus capsid with, at least, one epitope predominantly not exposed in the virion surface; this epitope may not be related to the virus ligand-cell receptor interaction.