The SH2 inositol 5-phosphatase Ship1 is recruited in an SH2-dependent manner to the erythropoietin receptor

Ship1 ((S) under bar H2 (i) under bar nositol 5-(p) under bar hosphatase (1 ) under bar) has been shown to be a target of tyrosine phosphorylation down stream of cytokine and immunoregulatory receptors. In addition to its catal ytic activity on phosphatidylinositol substrates, it can serve as an adapto r protein in binding Shc and Grb2. Erythropoietin (EPO), the primary regula tor of erythropoiesis, has been shown to activate the tyrosine phosphorylat ion of Shc, resulting in recruitment of Grb2, However, the mechanism by whi ch the erythropoietin receptor (EPO-R) recruits Shc remains unknown. EPO ac tivates the tyrosine phosphorylation of Ship1, resulting in the interdepend ent recruitment of Shc and Grb2, Ship1 is recruited to the EPO-R in an SH2- dependent manner. Utilizing a panel of EPO-R deletion and tyrosine mutants, we have discovered remarkable redundancy in Ship1 recruitment. EPO-R Tyr(4 01) appears to be a major site of Ship1 binding; however, Tyr(429) and Tyr( 431) can also serve to recruit Ship1, In addition, we have shown that EPO s timulates the formation of a ternary complex consisting of Ship1, Shc, and Grb2. Ship1 may modulate several discrete signal transduction pathways. EPO -dependent activation of ERK1/2 and protein kinase B (PKB)/Akt was examined utilizing a panel of EPO-R deletion mutants. Activation of ERK1/2 was obse rved in EPO-R Delta 99, which retains only the most proximal tyrosine, Tyr( 343). In contrast, EPO-dependent PKB activation was observed in EPO-R Delta 43, but not in EPO-R Delta 99. It appears that EPO-dependent PKB activatio n is downstream of a region that indirectly couples to phosphatidylinositol 3-kinase.