As a c-fms-interacting protein, we cloned a novel adaptor molecule, signal-transducing adaptor protein-2 (STAP-2), which contains pleckstrin homology-and Src homology 2-like (PH and SRC) domains and a prolinerich region. STAP-2 is structurally related to STAP-1/ BRDG1 (BCR downstream signaling-1), which we had cloned previously from hematopoietic stem cells. STAP-2 is a murine homologue of a recently identified adaptor molecule, BKS, a substrate of BRK tyrosine kinase. STAP-2 was tyrosine-phosphorylated and translocated to the plasma membrane in response to epidermal growth factor when overexpressed in fibroblastic cells. To define the function of STAP-2, we generated mice lacking the STAP-2 gene. STAP-2 mRNA was strongly induced in the liver in response to lipopolysaccharide and in isolated hepatocytes in response to interleukin-6. In the STAP-2 ؊/؊ hepatocytes, the interleukin-6-induced expression of acute-phase (AP) genes and the tyrosinephosphorylation level of STAT3 were reduced specifically at the late phase (6 -24 h) of the response. These data indicate that STAP-2 plays a regulatory role in the AP response in systemic inflammation. STAP-2 contains a YXXQ motif in the C-terminal region that is a potential STAT3-binding site. Overexpression of wild-type STAP-2, but not of mutants lacking this motif, enhanced the AP response element reporter activity and an AP protein production. These data suggest that STAP-2 is a new class of adaptor molecule that modulates STAT3 activity through its YXXQ motif.Tyrosine kinases play an important role in regulating cell growth, differentiation, and transformation. Activated receptor tyrosine kinases trans-phosphorylate several tyrosines in their cytoplasmic domains, which provide recognition sites for various adaptor and effector proteins in multiple signal transduction pathways (1, 2). These adaptor proteins utilize their Src homology-2 (SH2) 1 and SH3 domains to mediate the interactions that link different proteins involved in signal transduction. For example, the adaptor protein Grb2 links a variety of surface receptors to the Ras/MAP kinase signaling cascade. Grb2 interacts with activated receptor tyrosine kinases via its SH2 domain and recruits the guanine nucleotide-releasing factor, SOS (Son of Sevenless), close to its target protein, Ras, at the cell membrane. Phosphoinositide-3-OH kinase (PI3K) and phospholipase C␥ (PLC␥) are also recruited to receptor tyrosine kinases through their SH2 domains. Growth factor-induced membrane recruitment of signaling proteins is also mediated by a family of docking proteins. These docking proteins contain an N-terminal membrane-targeting domain, such as the PH domain, and C-terminal multiple tyrosine phosphorylation sites for recruiting SH2 domain-containing proteins. A significant effort has been made to search for novel adaptor and docking proteins, because these molecules will uncover the unique signal transduction and modulation mechanisms of receptor tyrosine kinases.Signal transducer and activator of transcription (STAT) fam...

Signal transducing adaptor protein‐2 (STAP‐2) is a recently identified adaptor protein, that contains pleckstrin and Src homology 2 (SH2)‐like domains as well as a YXXQ motif in its C‐terminal region. Our previous studies have demonstrated that STAP‐2 binds to STAT3 and STAT5, and regulates their signaling pathways. In the present study, STAP‐2 was found to positively regulate LPS/TLR4‐mediated signals in macrophages. Disruption of STAP‐2 resulted in impaired LPS/TLR4‐induced cytokine production and NF‐κB activation. Conversely, overexpression of STAP‐2 enhanced these LPS/TLR4‐induced biological activities. STAP‐2, particularly its SH2‐like domain, bound to both MyD88 and IKK‐α/β, but not TRAF6 or IRAK1, and formed a functional complex composed of MyD88‐STAP‐2‐IKK‐α/β. These interactions augmented MyD88‐ and/or IKK‐α/β‐dependent signals, leading to enhancement of the NF‐κB activity. These results demonstrate that STAP‐2 may constitute an alternative LPS/TLR4 pathway for NF‐κB activation instead of the TRAF6‐IRAK1pathway.

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Interleukin-6 (IL-6) is a multifunctional cytokine playing roles in the immune system, hematopoiesis and acute phase reactions. IL-6 also regulates the growth of in various types of human malignant tumors. Here we demonstrate that IL-6-induced gene expression was suppressed by a specific Heat-shock protein 90 (Hsp90) inhibitor, geldanamycin (GA) in human hepatoma Hep3B cells. GA also suppressed the IL-6-induced activation of signal transducer and activator of transcription 3 (STAT3) in a human embryonic kidney carcinoma 293T cells. This inhibitory effect of GA on STAT3 activation was reversed by overexpression of Hsp90. Furthermore, Hsp90 directly bound to STAT3 via its N-terminal region, which interacted with GA. We provide evidence that the action of GA on IL-6 functions was due to the inhibition of direct physical interactions between STAT3 and Hsp90, which represents a novel role of Hsp90 in the IL-6 signaling pathways.

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