Oligomerization and scaffolding functions of the erythropoietin receptor cytoplasmic tail

Signal transduction by the erythropoietin receptor (EPOR) is activated by l igand-mediated receptor homodimerization. However, the relationship between extracellular and intracellular domain oligomerization remains poorly unde rstood. To assess the requirements for dimerization of receptor cytoplasmic sequences for signaling, we overexpressed mutant EPORs in combination with wild-type (WT) EPOR to drive formation of heterodimeric (i.e. WT-mutant) r eceptor complexes. Dimerization of the membrane proximal portion of the EPO R cytoplasmic region was found to be critical for the initiation of mitogen ic signaling. However, dimerization of the entire EPOR cytoplasmic region w as not required. To examine this process more closely, we generated chimera s between the intracellular and transmembrane portions of the EPOR and the extracellular domains of the interleukin-2 receptor beta and gamma(c), chai ns. These chimeras allowed us to assess more precisely the signaling role o f each receptor chain because only heterodimers of WT and mutant receptor c himeras form in the presence of interleukin-2. Coexpression studies demonst rated that a functional receptor complex requires the membrane-proximal reg ion of each receptor subunit in the oligomer to permit activation of JAK2 b ut only one membrane-distal tail to activate STAT5 and to support cell prol iferation. Thus, this study defines key relationships involved in the assem bly and activation of the EPOR signal transduction complex which may be app licable to other homodimeric cytokine receptors.