Suppressors of cytokine signalling (SOCS) are key regulators of cytokine signalling, inhibiting the JAK-STAT signal cascade in a negative feedback loop. Aberrant regulation of JAK-STAT signalling is implicated in cancer. 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 . 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 of these proteins remain poorly characterized .
We have recently identified a highly conserved 70-residue fragment within the long and apparently disordered N-termini of SOCS4 and SOCS5 . We have further shown that SOCS5 can interact directly with Janus kinase (JAK) via this unique and conserved region in its N-terminus, inhibiting JAK catalytic activity . In order to investigate the structure and function of this region, residues 175-244 of mouse SOCS5 (mSOCS5175-244) were expressed in E. coli. The solution structure and backbone dynamics of mSOCS5175-244 were characterized using multidimensional heteronuclear NMR spectroscopy. NMR data indicate that mSOCS5175-244 adopts an ordered conformation in solution, with a well-defined helix being present in the C-terminal region of the molecule.
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. Therefore, the interactions of this conserved region of SOCS5 with JAK were also investigated using NMR spectroscopy and surface Plasmon resonance. The results from this study provide insights into the function of the N-terminal region of SOCS5 and its role in the regulation of cytokine signalling.