The Suppressor Of Cytokine Signalling (SOCS) proteins were, as their name suggests, first described as inhibitors of cytokine signalling. While their actions clearly now extend to other intracellular pathways, they remain key negative regulators of cytokine and growth factor signalling. In this review we focus on the mechanics of SOCS action and the complexities of the mouse models that have underpinned our current understanding of SOCS biology.

The four members of the recently identified suppressor of cytokines signaling family (SOCS-1, SOCS-2, SOCS-3, and CIS, where CIS is cytokine-inducible SH2-containing protein) appear, by various means, to negatively regulate cytokine signal transduction. Structurally, the SOCS proteins are composed of an N-terminal region of variable length and amino acid composition, a central SH2 domain, and a previously unrecognized C-terminal motif that we have called the SOCS box. By using the SOCS box amino acid sequence consensus, we have searched DNA databases and have identified a further 16 proteins that contain this motif. These proteins fall into five classes based on the protein motifs found N-terminal of the SOCS box. In addition to four new SOCS proteins (SOCS-4 to SOCS-7) containing an SH2 domain and a SOCS box, we describe three new families of proteins that contain either WD-40 repeats (WSB-1 and -2), SPRY domains (SSB-1 to -3) or ankyrin repeats (ASB-1 to -3) N-terminal of the SOCS box. In addition, we show that a class of small GTPases also contains a SOCS box. The expression of representative members of each class of proteins differs markedly, as does the regulation of expression by cytokines. The function of the WSB, SSB, and ASB protein families remains to be determined.Cytokines act by binding to and inducing dimerization of members of the hemopoietin receptor family expressed on the surface of responsive cells (1). Although the cytoplasmic proteins that then transduce the signal are relatively welldefined and include the Janus kinase (JAK) family of kinases and signal transducers and activators of transcription (STAT) transcription factors (2, 3), the proteins involved in limiting signal transduction are not well characterized.The four known members of the suppressor of cytokine signaling (SOCS) family (CIS, SOCS-1͞SSI-1͞JA B-1, SOCS-2, and SOCS-3, where CIS is cytokine-inducible SH2-containing protein) represent a family of negative regulators of cytokine signal transduction (4-9). The SOCS proteins appear to form part of a classical negative feedback loop that regulates cytokine signal transduction. Transcription of each of the SOCS genes occurs rapidly in vitro and in vivo in response to cytokines, and once produced, the various members of the SOCS family appear to inhibit signaling in different ways. For SOCS-1, inhibition of signal transduction appears to occur by binding to and inhibiting the catalytic activity of members of the JAK family of cytoplasmic kinases (4-6), while CIS appears to act by competing with signaling molecules such as the STATs for binding to phosphorylated receptor cytoplasmic domains (7, 9).The SOCS proteins share structural similarities. Each has an N-terminal region of variable length and highly variable amino acid sequence, a central SH2 domain, and a striking region of C-terminal homology that we designated the SOCS box (4). Given the sequence similarity evident in the SOCS box of the four SOCS proteins and its conserved position at the C terminus of each protein, ...

Toll-like receptor (TLR) signals that initiate innate immune responses to pathogens must be tightly regulated to prevent excessive inflammatory damage to the host. The adaptor protein Mal is specifically involved in signaling via TLR2 and TLR4. We demonstrate here that after TLR2 and TLR4 stimulation Mal becomes phosphorylated by Bruton's tyrosine kinase (Btk) and then interacts with SOCS-1, which results in Mal polyubiquitination and subsequent degradation. Removal of SOCS-1 regulation potentiates Mal-dependent p65 phosphorylation and transactivation of NF-kappaB, leading to amplified inflammatory responses. These data identify a target of SOCS-1 that regulates TLR signaling via a mechanism distinct from an autocrine cytokine response. The transient activation of Mal and subsequent SOCS-1-mediated degradation is a rapid and selective means of limiting primary innate immune response.

The suppressors of cytokine signaling (SOCS) family of proteins act as intracellular inhibitors of several cytokine signal transduction pathways. Their expression is induced by cytokine activation of the Janus kinase/signal transducer and activator of transcription (JAK/STAT) pathway and they act as a negative feedback loop by subsequently inhibiting the JAK/STAT pathway either by direct interaction with activated JAKs or with the receptors. These interactions are mediated at least in part by the SH2 domain of SOCS proteins but these proteins also contain a highly conserved C-terminal homology domain termed the SOCS box. Here we show that the SOCS box mediates interactions with elongins B and C, which in turn may couple SOCS proteins and their substrates to the proteasomal protein degradation pathway. Analogous to the family of F-box-containing proteins, it appears that the SOCS proteins may act as adaptor molecules that target activated cell signaling proteins to the protein degradation pathway.

Suppressor of cytokine signaling-3 (SOCS-3) is one member of a family of intracellular inhibitors of signaling pathways initiated by cytokines that use, among others, the common receptor subunit gp130. The SH2 domain of SOCS-3 has been shown to be essential for this inhibitory activity, and we have used a quantitative binding analysis of SOCS-3 to synthetic phosphopeptides to map the potential sites of interaction of SOCS-3 with different components of the gp130 signaling pathway. The only high-affinity ligand found corresponded to the region of gp130 centered around phosphotyrosine-757 (pY757), previously shown to be a docking site for the tyrosine phosphatase SHP-2. By contrast, phosphopeptides corresponding to other regions within gp130, Janus kinase, or signal transducer and activator of transcription proteins bound to SOCS-3 with weak or undetectable affinity. The significance of pY757 in gp130 as a biologically relevant SOCS-3 docking site was investigated by using transfected 293T fibroblasts. Although SOCS-3 inhibited signaling in cells transfected with a chimeric receptor containing the wild-type gp130 intracellular domain, inhibition was considerably impaired for a receptor carrying a Y3 F point mutation at residue 757. Taken together, these data suggest that the mechanism by which SOCS-3 inhibits the gp130 signaling pathway depends on recruitment to the phosphorylated gp130 receptor, and that some of the negative regulatory roles previously attributed to the phosphatase SHP-2 might in fact be caused by the action of SOCS-3.