Us3 protein incorporation into PRV634 virions. (A) Diagram of the unique short regions of Bartha, Becker, and PRV634. PRV634 is isogenic relative to Becker and expresses Bartha Us3. Bartha sequences are shown in black, and Becker sequences are shown in gray. (B) Extracts from Becker- and PRV634-infected cells were prepared at 6 h postinfection. Extracts were subjected to Western blot analysis with Us3-specific antiserum. Note the shift in Us3 mobility between Becker and PRV634, a recombinant Becker virus expressing Bartha Us3. Numbers at right are in kilodaltons. (C) Purified Becker and PRV634 virions were treated with NP-40 and PK, separated on a 12% polyacrylamide gel, and analyzed by Western blotting with Us3-specific antiserum. The mock band is the top band (resistant to both NP-40 and PK), while Us3 is the lower band. The Western blot for gB served as a loading control and as a control for protease activity. pgB, preprocessed form of gB.