Constitutive association of JAK1 and STAT5 in pro‐B cells is dissolved by interleukin‐4‐induced tyrosine phosphorylation of both proteins

Abstract

The bipartite human interleukin-4 (IL-4) receptor was functionally expressed in murine pro-B cells and activated by human IL-4 to evoke intracellular signaling. Mutual association of signal transducing proteins within the receptor complex was then studied in dependence of ligand stimulation. Besides ligandinduced receptor heterodimerization and contacts of the two IL-4 receptor subunits K K and Q Q with Janus kinases JAK1 and JAK3 a prominent constitutive binding between JAK1 and signal transducer and activator of transcription STAT5 was detected. Since both these proteins become phosphorylated in response to IL-4 receptor stimulation, the influence of tyrosine phosphorylation on their mutual contact was analyzed. Association of JAK1 and STAT5 was found to occur exclusively between unphosphorylated proteins.z 1998 Federation of European Biochemical Societies.

JAK/STAT: Why choose a classical or an alternative pathway when you can have both?

“… 39 , 40 This implies that the membrane‐proximal cytoplasmic portion of IL‐4Rα is sufficient for STAT activation. While this is coherent with previous data showing a direct interaction of STAT5 with JAK1, 36 no such interaction has yet been detected for STAT6. It is also worth noting that activation of STAT5 and STAT‐6 is strictly dependent on JAK1, since when the box 1 domain required for JAK1 binding is truncated on both chains of the receptor (IL‐4Rα and γc), activation of these STATs is abolished.…”

“…Only a few studies have been able to confirm a direct ‘physical’ interaction between JAK and STAT proteins 35,36 . Most of these interactions were demonstrated through GST pull down using cells overexpressing proteins of interest.…”

“…Here again, a direct interaction between JAK1 and STAT5 has been described. By performing immunoprecipitations, Erhard et al 36 observed a direct link between JAK1 and STAT5 even in unphosphorylated conditions. In other studies, when a truncated IL‐4Rα lacking all tyrosine residues but retaining the box 1 domain is stimulated with IL‐4, STAT5 and STAT‐6 activation is still observed and comparable to that obtained with the WT receptor 39,40 .…”

“…Some studies have been able to demonstrate a noncanonical direct interaction between JAK and STAT proteins. Indeed, the interaction between the JH2 (pseudokinase) domain of JAK1, JAK2, JAK3 and the carboxy‐terminal region of STAT5, but not STAT1 or STAT3, was demonstrated using the yeast two‐hybrid system, co‐immunoprecipitation and GST pull down 35,36 . Thus, studies showing that the proximal regions of receptors (containing box domains) are sufficient for STAT activation suggest that JAK and STAT molecules could interact directly.…”

“…Indeed, the interaction between the JH2 (pseudokinase) domain of JAK1, JAK2, JAK3 and the carboxy‐terminal region of STAT5, but not STAT1 or STAT3, was demonstrated using the yeast two‐hybrid system, co‐immunoprecipitation and GST pull down. 35 , 36 Thus, studies showing that the proximal regions of receptors (containing box domains) are sufficient for STAT activation suggest that JAK and STAT molecules could interact directly. The mechanisms by which STATs interact with JAKs are not fully understood, but seem to be independent of phosphotyrosine.…”