Grb10 interacts differentially with the insulin receptor, insulin-like growth factor I receptor, and epidermal growth factor receptor via the Grb10 Src homology 2 (SH2) domain and a second novel domain located between the pleckstrin homology and SH2 domains

The JAK-binding protein JAB inhibits Janus tyrosine kinase activity through binding in the activation loop

The Janus family of protein tyrosine kinases (JAKs) regulate cellular processes involved in cell growth, differentiation and transformation through their association with cytokine receptors. However, compared with other kinases, little is known about cellular regulators of the JAKs. We have recently identified a JAK-binding protein (JAB) that inhibits JAK signaling in cells. In the studies presented here we demonstrate that JAB specifically binds to the tyrosine residue (Y1007) in the activation loop of JAK2, whose phosphorylation is required for activation of kinase activity. Binding to the phosphorylated activation loop requires the JAB SH2 domain and an additional N-terminal 12 amino acids (extended SH2 subdomain) containing two residues (Ile68 and Leu75) that are conserved in JAB-related proteins. An additional N-terminal 12-amino-acid region (kinase inhibitory region) of JAB also contributes to high-affinity binding to the JAK2 tyrosine kinase domain and is required for inhibition of JAK2 signaling and kinase activity. Our studies define a novel type of regulation of tyrosine kinases and might provide a basis for the design of specific tyrosine kinase inhibitors.

Mutational analyses of the SOCS proteins suggest a dual domain requirement but distinct mechanisms for inhibition of LIF and IL-6 signal transduction

SOCS-1 (suppressor of cytokine signaling-1) is a representative of a family of negative regulators of cytokine signaling (SOCS-1 to SOCS-7 and CIS) characterized by a highly conserved C-terminal SOCS box preceded by an SH2 domain. This study comprehensively examined the ability of several SOCS family members to negatively regulate the gp130 signaling pathway. SOCS-1 and SOCS-3 inhibited both interleukin-6 (IL-6)- and leukemia inhibitory factor (LIF)-induced macrophage differentiation of murine monocytic leukemic M1 cells and LIF induction of a Stat3-responsive reporter construct in 293T fibroblasts. Deletion of amino acids 51-78 in the N-terminal region of SOCS-1 prevented inhibition of LIF signaling. The SOCS-1 and SOCS-3 N-terminal regions were functionally interchangeable, but this did not extend to other SOCS family members. Mutation of SH2 domains abrogated the ability of both SOCS-1 and SOCS-3 to inhibit LIF signal transduction. Unlike SOCS-1, SOCS-3 was unable to inhibit JAK kinase activity in vitro, suggesting that SOCS-1 and SOCS-3 act on the JAK-STAT pathway in different ways. Thus, although inhibition of signaling by SOCS-1 and SOCS-3 requires both the SH2 and N-terminal domains, their mechanisms of action appear to be biochemically different.

Identification of the rat adapter Grb14 as an inhibitor of insulin actions

We cloned by interaction with the beta-subunit of the insulin receptor the rat variant of the human adapter Grb14 (rGrb14). rGrb14 is specifically expressed in rat insulin-sensitive tissues and in the brain. The binding of rGrb14 to insulin receptors is insulin-dependent in vivo in Chinese hamster ovary (CHO) cells overexpressing both proteins and importantly, in rat liver expressing physiological levels of proteins. However, rGrb14 is not a substrate of the tyrosine kinase of the receptor. In the two-hybrid system, two domains of rGrb14 can mediate the interaction with insulin receptors: the Src homology 2 (SH2) domain and a region between the PH and SH2 domains that we named PIR (for phosphorylated insulin receptor-interacting region). In vitro interaction assays using deletion mutants of rGrb14 show that the PIR, but not the SH2 domain, is able to coprecipitate insulin receptors, suggesting that the PIR is the major binding domain of rGrb14. The interaction between rGrb14 and the insulin receptors is almost abolished by mutating tyrosine residue Tyr1150 or Tyr1151 of the receptor. The overexpression of rGrb14 in CHO-IR cells decreases insulin stimulation of both DNA and glycogen synthesis. These effects are accompanied by a decrease in insulin-stimulated tyrosine phosphorylation of IRS-1, but insulin receptor autophosphorylation is unaltered. These findings suggest that rGrb14 could be a new downstream signaling component of the insulin-mediated pathways.

Inhibition of hGrb10 binding to the insulin receptor by functional domain-mediated oligomerization

hGrb10 is a newly identified Src homology 2 (SH2) and pleckstrin homology (PH) domain-containing protein that binds to autophosphorylated receptor tyrosine kinases, including the insulin and insulin-like growth factor receptors. To identify potential downstream proteins that interact with hGrb10, we screened a yeast two-hybrid cDNA library using the full-length hGrb10gamma as bait. A fragment of hGrb10, which included the IPS (insert between the PH and SH2 domain) and the SH2 domains, was found to bind with high affinity to the full-length protein. The interaction between the IPS/SH2 domain and the full-length hGrb10 was further confirmed by in vitro glutathione S-transferase fusion protein binding studies. Gel filtration assays showed that hGrb10 underwent tetramerization in mammalian cells. The interaction involved at least two functional domains, the IPS/SH2 region and the PH domain, both of which interacted with the NH2-terminal amino acid sequence of hGrb10gamma (hGrb10gamma DeltaC, residues 4-414). Competition studies showed that hGrb10gamma DeltaC inhibited the binding of hGrb10 to the tyrosine-phosphorylated insulin receptor, suggesting that this region may play a regulatory role in hGrb10/insulin receptor interaction. We present a model for hGrb10 tetramerization and its potential role in receptor tyrosine kinase signal transduction.

Grb10 identified as a potential regulator of growth hormone (GH) signaling by cloning of GH receptor target proteins

The cloning of receptor targets procedure, used so far to identify proteins associated with tyrosine kinase receptors was modified to clone SH2 proteins able to bind to the growth hormone receptor (GHR). The cytoplasmic region of GHR, a member of the cytokine receptor superfamily does not contain tyrosine kinase activity. It was thus phosphorylated in bacteria by the Elk tyrosine kinase and radiolabeled to screen a mouse expression library. With this probe, we identified Shc and the p85 subunit of phosphatidylinositol 3-kinase as direct targets of the receptor. The other proteins identified, Csk, Shb, Grb4, and Grb10 are new potential transducers for cytokine receptors. We show in Huh-7 hepatoma cells that Grb10 and GHR associate under GH stimulation. Co-transfections in 293 cells further show that Grb10 interacts with both the GHR and Jak2. Functional tests demonstrate that Grb10 inhibits transcription of two reporter genes containing, respectively, the serum response element of c-fos and the GH response element 2 of the Spi2.1 gene, whereas it has no effect on a reporter gene containing only Stat5 binding elements. Our results suggest that Grb10 is a new target for a member of the cytokine receptor family that down-regulates some GH signaling pathways downstream of Jak2 and independently of Stat5.