APS, an adaptor protein containing Pleckstrin homology (PH) and Src homology-2 (SH2) domains inhibits the JAK-STAT pathway in collaboration with c-Cbl

We cloned a novel adaptor protein, APS (adaptor molecule containing Pleckst rin homology (PH) and Src Homology-2 (SH2) domains), which was tyrosine pho sphorylated in response to c-kit or B cell receptor stimulation. Here, we r eport that APS was tyrosine phosphorylated by Janus kinase-2 (JAK2) at its G-terminal tyrosine residue and interacted with c-Cbl. Forced expression of APS in an erythropoietin (EPO)-dependent hematopoietic cell line resulted in reduced activation of STAT5 but not cell proliferation in response to EP O. APS bound to the phosphorylated tyrosine residue, Y343 of the erythropoi etin receptor cytoplasmic domain. Go-expression of APS and c-Cbl, but not e xpression of either alone inhibited EPO-dependent STAT5 activation in 293 c ells. This required the G-terminal phosphorylation site, as well as PH and SH2 domains of APS. Therefore, one of the major functions of APS is in recr uitment of c-Cbl into the receptor/JAK complex, thereby inhibiting JAK sign aling activity.