Structural basis for c-KIT inhibition by the suppressor of cytokine signaling 6 (SOCS6) ubiquitin ligase

The c-Kit receptor tyrosine kinase mediates the cellular response to stem cell factor (SCF). Whereas c-Kit activity is important for the proliferation of hematopoietic cells, melanocytes and germ cells, uncontrolled c-Kit activity contributes to the growth of diverse human tumors. Suppressor of cytokine signaling 6 (SOCS6) is a member of the SOCS family of E3 ubiquitin ligases that can interact with c-Kit and suppress c-KIT-dependent pathways. Here, we analyzed the molecular mechanisms that determine SOCS6 substrate recognition. Our results show that the SH2 domain of SOCS6 is essential for its interaction with c-Kit pY568. The 1.45-Å crystal structure of SOCS6 SH2 domain bound to the c-Kit substrate peptide (c-Kit residues 564-574) revealed a highly complementary and specific interface giving rise to a high affinity interaction (K(d) = 0.3 μm). Interestingly, the SH2 binding pocket extends to substrate residue position pY+6 and envelopes the c-Kit phosphopeptide with a large BG loop insertion that contributes significantly to substrate interaction. We demonstrate that SOCS6 has ubiquitin ligase activity toward c-Kit and regulates c-Kit protein turnover in cells. Our data support a role of SOCS6 as a feedback inhibitor of SCF-dependent signaling and provides molecular data to account for target specificity within the SOCS family of ubiquitin ligases.