Activation of JAK2 by wild-type and constitutively active hβc mutants. Lysates from FDC-P1 cells expressing wild-type hβc plus hGMR or the indicated constitutive mutants were subject to immunoprecipitation with anti-JAK2 antibodies followed by immununoblotting with (A) antiphosphotyrosine or (B) anti-JAK2 antibodies. Cells expressing wild-type hβc plus hGMR were either factor-deprived (−GM), restimulated for 10 minutes (+GM), or grown continuously in hGM-CSF (cont. GM). Because the loading of JAK2 for the I374N and V449E mutants was higher than that for the other samples in the upper panels, a second experiment showing these two mutants (plus wild-type controls) is shown in the lower panels.