Identification of SH2-B beta as a potent cytoplasmic activator of the tyrosine kinase Janus kinase 2

Janus kinases (JAKs) are cytoplasmic tyrosine kinases critical for signalin g by growth hormone (GH) and many other ligands that bind to members of the cytokine receptor superfamily. SH2-B beta was previously identified as a J AK2-interacting protein that is tyrosyl phosphorylated in response to GH an d other cytokines that activate JAK2. In this stud, we examined whether SH2 -B beta alters the activity of JAK2. SH2-B beta, when coexpressed with JAK2 , significantly increased the tyrosyl phosphorylation of JAK2 and multiple other cellular proteins and stimulated the kinase activity of JAK2 by appro ximate to 20-fold. Coexpression of SH2-B beta with JAK2 dramatically increa sed tyrosyl phosphorylation of signal transducer and activator of transcrip tion (Stat)5B and Stat3, physiological substrates of JAK2. SH2-B beta(R555E ) with a defective; Src homology 2 domain was unable to stimulate JAK2 and JAK2-mediated tyrosyl phosphorylation of Stat5B and Stat3. More importantly , SH2- B beta enhanced GH induced tyrosyl phosphorylation of endogenous JAK 2 and ligand-induced tyrosyl phosphorylation of Stat5B by endogenous JAK2. In contrast, SH2-B beta did not potentiate the activation of other tyrosine kinases including the receptors for platelet-derived grow th factor, epide rmal growth factor, or nerve growth factor (TrKA), tyrosine kinases that al so bind SH2-B beta. These data demonstrate that SH2-B beta is a potent cyto plasmic activator of JAK2 and is thereby expected to be an important cellul ar regulator of signaling by GH and other hormones and cytokines that activ ate JAK2.