Suppressors of cytokine signalling (SOCS) are key regulators of cytokine signalling, inhibiting the JAK-STAT signal cascade in a negative feedback loop. Dysregulation of the JAK-STAT signalling is central to the pathophysiology of myeloproliferative disorders. The SOCS family, consisting of eight intracellular proteins, shares a common domain organization, with a central SH2 domain, a conserved C-terminal SOCS box and a variable N-terminal region [1]. The structure and role of the SH2 domain and SOCS box motif of the SOCS proteins are well defined, but the structure and function of the N-terminal regions remain poorly characterized [2].
We have demonstrated that SOCS5 can inhibit JAK1 activity and the N-terminus of SOCS5 is critical for this inhibition [3]. We have recently identified a highly conserved 70-residue fragment within the apparently disordered N-terminus of SOCS5 [4] and have further shown that SOCS5 can interact directly with the kinase domain of JAK via this unique conserved region in its N-terminus [3]. In order to investigate the structure and function of this region, residues 175-244 of mouse SOCS5 (mSOCS5_175-244) were expressed in E. coli and the solution structure and backbone dynamics were characterized using NMR spectroscopy. NMR data indicate that mSOCS5_175-244 adopts an ordered conformation in solution, with a well-defined α-helix spanning residues 223-233.
Pre-formed structural elements in intrinsically unstructured regions of proteins are known to serve as initial contact points that facilitate binding with their partner proteins. Such nascent structural elements, often present in isolation, are likely to be stabilized upon binding to the target. Therefore, the interactions of this conserved region of SOCS5 with JAK were also investigated using NMR spectroscopy and surface plasmon resonance. Inhibition of JAK via the N-terminus appears to be unique to SOCS5 and its mechanism of action is likely to be distinct from that of SOCS1 and SOCS3.