JAK2 ASSOCIATES WITH THE BETA(C) CHAIN OF THE RECEPTOR FAR GRANULOCYTE-MACROPHAGE COLONY-STIMULATING FACTOR, AND ITS ACTIVATION REQUIRES THE MEMBRANE-PROXIMAL REGION

The high-affinity receptor for granulocyte-macrophage colony-stimulati ng factor (GM-CSP) consists of a unique alpha chain and a beta(c) subu nit that is shared with the receptors for interleukin-3 (IL-3) and IL- 5. Two regions of the beta(c) chain have been defined; these include a membrane-proximal region of the cytoplasmic domain that is required f or mitogenesis and a membrane-distal region that is required for activ ation of Ras, Raf-1, mitogen-activated protein kinase, and S6 kinase. Recent studies have implicated the cytoplasmic protein tyrosine kinase JAK2 in signalling through a number of the cytokine receptors, includ ing the IL-3 and erythropoietin receptors. In the studies described he re, we demonstrate that GM-CSF stimulation of cells induces the tyrosi ne phosphorylation of JAK2 and activates its in vitro kinase activity. Mutational analysis of the beta(c) chain demonstrates that only the m embrane-proximal 62 amino acids of the cytosolic domain are required f or JAK2 activation. Thus, JAK2 activation is correlated with induction of mitogenesis but does not, alone, activate the Ras pathway. Carboxy l truncations of the alpha chain which inactivate the receptor for mit ogenesis, are unable to mediate GM-CSR-induced JAK2 activation. Using baculovirus-expressed proteins, we further demonstrate that JAK2 physi cally associated with the beta(c) chain but not with the alpha chain. Together, the results further support the hypothesis that the JAK fami ly of kinases are critical to coupling cytokine binding to tyrosine ph osphorylation and ultimately mitogenesis.