p27^Kip1 interacts with Grb2 and G-CSF receptor and is tyrosine-phosphorylated in NB4 cells. (A) Lysates of NB4 cells untreated or stimulated with G-CSF for 60 minutes were precipitated with antibody to the G-CSF receptor or Grb2 and probed as indicated. Control immunoprecipitations with an antibody to human IgG excluded the possibility of unspecific binding. (B) Lysates of different cell lines were precipitated with anti-p27^Kip1 and the Western blot was probed as indicated. (C) Immunoprecipitated p27^Kip1 from NB4 cells was incubated with 10 U LAR-PTP for 30 minutes at 30°C. Tyrosine phosphorylation of LAR-treated p27^Kip1 was compared to untreated p27^Kip1.