FIG. 6.

Phosphorylation of Syk by Jak1 but not by Jak3. (A) COS-7 cells were transfected with one or more cDNAs encoding catalytically active or inactive versions of Syk, Jak1, or Jak3 as indicated. Two days later, cells were lysed and immunoprecipitated with an anti-Syk antibody. The immune complexes were separated by SDS-PAGE, transferred to nitrocellulose, and immunoblotted with antiphosphotyrosine (anti PTyr) MAb 4G10 (upper panel) or anti-Syk antibody (lower panel). (B) U4A cells were transfected with one or more cDNAs encoding catalytically active or inactive versions of Jak1 and Jak3 as indicated. The immune complexes were separated by SDS-PAGE, transferred to nitrocellulose, and immunoblotted with antiphosphotyrosine MAb 4G10 (upper panels), followed by immunoblotting with anti-Jak3 (lanes 1 to 3, lower panel) and anti-Jak1 antibody (lanes 4 to 6, lower panel).