Multiple tyrosine residues inthecytosolic domainofthe erythropoietin receptor promoteactivation ofSTAT5

Signaling through theerythropoietin recep- tor(EPO-R) iscrucial forproliferation, differentiation, and survival oferythroid progenitor cells. EPO induces ho- modimerization oftheEPO-R,triggering activation ofthe receptor-associated kinase JAK2andactivation ofSTAT5. By mutating theeight tyrosine residues inthecytosolic domain oftheEPO-R,weshowthateither Y343 orY41issufficient to mediate maximal activation ofSTAT5; tyrosine residues Y429 andY431 canpartially activate STAT5.Comparison ofthe sequences surrounding these tyrosines reveals YXXLasthe probable motif specifying recruitment ofSTAT5totheEPO-R. Expression ofa mutantEPO-Rlacking alleight tyrosine residues inthecytosolic domainsupported alowbutdetect- ablelevel ofEPO-induced STAT5activation, indicating the existence ofanalternative pathway forSTAT5activation independent ofanytyrosine intheEPO-R.Thekinetics of STAT5activation andinactivation werethesame, regardless ofwhichtyrosine residue intheEPO-Rmediated its activation orwhether thealternative pathway wasused. Theability of mutant EPO-Rstoactivate STAT5didnotdirectly correlate withtheir mitogenic potential. Erythropoietin (EPO), ahormone synthesized bythekidney, istheprimary regulator ofmammalian erythropoiesis. Hor- monebinding toits cognate receptor iscrucial forprolifera- tion, differentiation, andsurvival oferythroid progenitor cells.