Isolation of cowpox virus A-type inclusions and characterization of their major protein component.

A-type inclusions (ATI)2 are large well-defined structures that appear in the cytoplasm during the late stages of the multiplication cycles of many poxviruses. The ATIs produced by the CPRC1 strain of cowpox virus in strain 143 human osteosarcoma cells have been isolated and characterized. These inclusions which can be recovered in large amounts (about 20 mg per 10(9) cells), appear to consist entirely of a single protein species that has a molecular weight of 160,000. It is a late protein, stable in infected cells, and during the late stages of the multiplication cycle it is one of the most abundantly synthesized proteins. Very similar ATIs composed of a serologically closely related protein of the same size are formed in cells infected with raccoonpox virus. In cells infected with vaccinia virus strain WR much smaller inclusions are formed; further, in such cells large amounts of a late protein with a molecular weight of 94,000 are produced that is also closely related to the cowpox virus ATI protein.