ASSOCIATION OF JAK2 AND STAT5 WITH ERYTHROPOIETIN RECEPTORS - ROLE OFRECEPTOR PHOSPHORYLATION IN ERYTHROPOIETIN SIGNAL-TRANSDUCTION

Cytokine receptors act at least partially by associating with Janus ty rosine protein kinases at the conserved box one moth of the receptor. These receptor-associated kinases then activate STAT transcription fac tors through phosphorylation. me found that the 78-kDa erythropoietin receptor (EPOR), a highly modified form of the 66-kDa receptor which i s abundant in HCD57 cells, was phosphorylated on serine residues witho ut EPO stimulation, Coprecipitation experiments showed the 78-kDa EPOR but not the more abundant 66-kDa EPOR was associated with JAK2, a Jan us protein kinase, in both the presence and absence of EPO. Solubilize d 78-kDa EPOR bound to purified, genetically engineered JAK2 better th an the 62-76-kDa receptor proteins, and additional phosphorylation of tyrosine residues further increased the binding of the 78-kDa EPOR to JAK2-agarose beads. STAT5 DNA binding was activated by 10-100-fold low er concentrations of EPO in HCD57 cells than in primary erythroid cell s, and STATE associated with the EPOR in an EPO dependent manner. Thes e data suggest that phosphorylation of either serine or tyrosine resid ues of the EPOR can enhance the association of the receptor with JAK2, possibly increasing the sensitivity to EPO.