ACTIVATION OF RECEPTOR-ASSOCIATED TYROSINE KINASE JAK2 BY PROLACTIN

JAK family tyrosine kinases have recently been implicated in intracell ular signal transduction by transmembrane cytokine receptors of the in terferon (IFN) and hematopoietin receptor families. Using the prolacti n (PRL) dependent rat pre-T cell line Nb2, a PRL receptor-associated, candidate tyrosine kinase of 120-130 kDa was recently characterized (1 ). In the present work this protein is identified as JAK2, based upon reciprocal anti-JAK2 and anti-phosphotyrosine immunoprecipitation and immunoblotting. JAK2 underwent rapid and transient tyrosine phosphoryl ation in response to receptor activation, reaching peak levels within 5 min of exposure to 100 nm PRL at 37 degrees C. In vitro tyrosine kin ase assays using either [gamma-P-32]ATP and autoradiography or unlabel ed ATP combined with anti-phosphotyrosine immunoblotting, demonstrated that the activity of JAK2 was stimulated by PRL. Phosphoamino acid an alysis of JAK2 after in vitro tyrosine kinase assay revealed that the majority of phosphate was incorporated into tyrosine residues. Further more, JAK2 was associated with PRL receptors to a comparable extent be fore and after PRL binding, as demonstrated by anti-receptor immunopre cipitation and subsequent anti-JAK2 immunoblotting. We propose that bi nding of ligand to the PRL receptor activates preassociated JAK2, and that this enzyme generates the initial signal in the intracellular com munication cascade.