MULTIPLE TYROSINE RESIDUES IN THE CYTOSOLIC DOMAIN OF THE ERYTHROPOIETIN RECEPTOR PROMOTE ACTIVATION OF STAT5

Signaling through the erythropoietin receptor (EPO-R) is crucial for p roliferation, differentiation, and survival of erythroid progenitor ce lls, EPO induces homodimerization of the EPO-R, triggering activation of the receptor-associated kinase JAK2 and activation of STAT5. By mut ating the eight tyrosine residues in the cytosolic domain of the EPO-R , we show that either Y-343 or Y-401 is sufficient to mediate maximal activation of STAT5: tyrosine residues Y-429 and Y-431 can partially a ctivate STAT5. Comparison of the sequences surrounding these tyrosines reveals YXXL as the probable motif specifying recruitment of STAT5 to the EPO-R. Expression of a mutant EPO-R lacking all eight tyrosine re sidues in the cytosolic domain supported a low but detectable level of EPO-induced STAT5 activation, indicating the existence of an alternat ive pathway for STAT5 activation independent of any tyrosine in the EP O-R. The kinetics of STAT5 activation and inactivation were the same, regardless of which tyrosine residue in the Ii;PO-R mediated its activ ation or whether the alternative pathway was used, The ability of muta nt EPO-Rs to activate STAT5 did not directly correlate with their mito genic potential.