Induction of Stat1 tyrosine phosphorylation in insect cells. (A) Sf9 cells were infected with Stat1 baculovirus alone or together with Bmx or Jak2 viruses or left uninfected. The cells were lysed 72 hours following infection, and the lysates were immunoprecipitated using anti–ISGF-3(p91) antibodies. The immunoprecipitates were electrophoresed in 7.5% SDS-PAGE, followed by Western immunoblotting using anti-phosphotyrosine antibodies or anti-Stat1 antibodies as indicated. (B) Equal amounts of protein from the lysates used in (A) were electrophoresed in 7.5% SDS-PAGE and Western immunoblotted with anti-phosphotyrosine antibodies. The tyrosine phosphorylated Bmx and Jak2 kinases are indicated by asterisks.