No abstract

SOCS (suppressors of cytokine signaling) proteins are negative regulators of cytokine signaling that function primarily at the receptor level. Remarkably, in vitro and in vivo observations revealed both inhibitory and stimulatory effects of SOCS2 on growth hormone signaling, suggesting an additional regulatory level. In this study, we examined the possibility of direct crossmodulation between SOCS proteins and found that SOCS2 could interfere with the inhibitory actions of other SOCS proteins in growth hormone, interferon, and leptin signaling. This SOCS2 effect was SOCS box-dependent, required recruitment of the elongin BC complex, and coincided with degradation of target SOCS proteins. Detailed mammalian protein-protein interaction trap (MAPPIT) analysis indicated that SOCS2 can interact with all members of the SOCS family. SOCS2 may thus function as a molecular bridge between a ubiquitin-protein isopeptide ligase complex and SOCS proteins, targeting them for proteasomal turnover. We furthermore extended these observations to SOCS6 and SOCS7. Our findings point to a unique regulatory role for SOCS2, SOCS6, and SOCS7 within the SOCS family and provide an explanation for the unexpected phenotypes observed in SOCS2 and SOCS6 transgenic mice.Cytokine signaling is typically a transient event, implying rapid and finely tuned attenuation. Receptor binding leads to rapid activation of receptor-associated members of the JAK 3 family. Subsequent phosphorylation of tyrosine residues in the receptor tails enables recruitment of downstream signaling molecules whereby STATs play a prominent role. Activated STATs translocate to the nucleus, where they control cytokineregulated gene transcription. Negative control occurs at many levels and involves receptor down-regulation, protein-tyrosine phosphatases, protein inhibitors of activated STATs, and members of the SOCS (suppressors of cytokine signaling) protein family.The SOCS family consists of eight different members (SOCS1-7 and CIS (cytokine-inducible SH2 domain-containing protein)) characterized by conserved structural features. All SOCS proteins consist of a central SH2 domain flanked by a variable N-terminal region and a conserved C-terminal SOCS box (1, 2). The SH2 domain can inhibit STAT activation by direct competition for the phosphorylated receptor recruitment sites (3-8). SOCS1 and SOCS3 carry an additional kinase inhibitory region (KIR) in their N-terminal domains that acts as a pseudosubstrate for the JAK kinase, thereby blocking signaling (5). The SOCS box was shown to act as an interaction domain for the elongin BC complex (9, 10), which, in turn, is a component of an ubiquitin-protein isopeptide ligase (E3) complex (11). This way, the SOCS box can control protein turnover by marking target proteins for proteasomal degradation (12). However, the significance of the interaction between SOCS proteins and the elongin BC complex is not totally clarified, as some reports propose that elongin association targets SOCS molecules for proteasomal degradation (10,(12)(13)...

Hypothalamic leptin receptor signalling plays a central role in weight regulation by controlling fat storage and energy expenditure. In addition, leptin also has direct effects on peripheral cell types involved in regulation of diverse body functions including immune response, bone formation and reproduction. Previous studies have demonstrated the important role of SOCS3 (suppressor of cytokine signalling 3) in leptin physiology. Here, we show that CIS (cytokine-inducible SH2 protein) and SOCS2 can also interact with the leptin receptor. Using MAPPIT (mammalian protein-protein interaction trap), a cytokine receptor-based two-hybrid method operating in intact cells, we show specific binding of CIS with the conserved Y985 and Y1077 motifs in the cytosolic domain of the leptin receptor. SOCS2 only interacts with the Y1077 motif, but with higher binding affinity and can interfere with CIS and STAT5a prey recruitment at this site. Furthermore, although SOCS2 does not associate with Y985 of the leptin receptor, we find that SOCS2 can block interaction of CIS with this position. This unexpected interference can be explained by the direct binding of SOCS2 on the CIS SOCS box, whereby elongin B/C recruitment is crucial to suppress CIS activity.