Molecular and biochemical characterization of suppressor of cytokine signaling-3 (SOCS-3) and SH2-containing phosphatase 1 (SHP-1) in the context of G-CSF) and erythropoietin signaling

In this work interactions of SOCS-3 with the EpoR were studied since there was some evidence from the literature that SOCS-3 would bind to more than one recruitment site on the Epo receptor. For that purpose SOCS-3 was cloned, expressed and purified for subsequent biochemical studies. These experiments revealed two possible binding sites on the EpoR, however, sequence-homology alignments as well as a structure model suggest that a double-phosphotyrosine motif is most important for SOCS-3 binding. The phosphatase SHP-1 has been shown to be a negative regulator of red blood cell formation. Furthermore, SHP-1 has been demonstrated to be induced by G-CSF in granulocytic differentiation. These observations lead to the assumption that this protein may represent a potential drug target and an inhibitor of this phosphatase would be expected to promote erythropoiesis and granulopoiesis. Thus, SHP-1 was cloned, expressed and purified and a biochemical high-throughput screening assay for detection of low-molecular weight inhibitors was established. To get more insight into the association of SOCS-3 with the EpoR, co-crystallizing experiments with recombinant SOCS-3 and an EpoR peptide were set up. Recombinant SOCS-3 was expressed, purified to high purity and concentrated. A several hundred crystal screens were set up with the method of the hanging drop. Finally, the role of SOCS-3 in regard to G-CSF-mediated signal transduction was investigated. It could be shown that SOCS-3 mRNA is induced in the monocytic precursor cell line U937 as well as in primary polymorphonuclear neutrophils. Biochemical as well as cellular analyses revealed that SOCS-3 suppresses G-CSF-mediated signal transduction by binding to the G-CSFR in a stimulation-dependent manner. By means of SPR, peptide precipitation assays as well as mutational analyses, one tyrosine residue within the receptor could be addressed of being the major recruiting site of SOCS-3, namely tyrosine 729