In situ exokinase activities of GST/(NA661)rJAK2 mutants containing serine or alanine substitutions at four critical cysteine residues. Sf21 cells were co-infected with the inactive GST/rJAK2(K882E) substrate and GST/(NA661 )rJAK2, K882E, C866S, C917S, C1094S, C1105S, C866A, C917A C1094A or C1105A mutants (lanes 1–10, respectively). (A) Immunoprecipitated proteins were analyzed via Western immunoblot with anti-JAK2, then (B) the PVDF membranes were “stripped” and reprobed with anti-phosphotyrosine antibodies. The upper arrows indicate the 140-kDa GST/ rJAK2(K882E) substrate and the lower arrows indicate the various 84-kDa GST/ (NΔ661)rJAK2 variants.