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Src homology 2 (SH2) domain found in signal transducer and activator of transcription (STAT) 6 proteins STAT6 mediate signals from the IL-4 receptor. Unlike the other STAT proteins which bind an IFNgamma Activating Sequence (GAS), STAT6 stands out as having a unique binding site preference. This site consists of a palindromic sequence separated by a 3 bp spacer (TTCNNNG-AA)(N3 site). STAT6 is able to bind the GAS site but only at a low affinity. STAT6 may be an important regulator of mitogenesis when cells respond normally to IL-4. There is speculation that the inappropriate activation of STAT6 is involved in uncontrolled cell growth in an oncogenic state. IFNgamma is a negative regulator of STAT6 dependent transcription of target genes. Bcl-6 is another negative regulator of STAT6 activity. Bcl-6 is a transcriptional repressor normally expressed in germinal center B cells and some T cells. IL-4 signaling via STAT6 initially occurs unopposed, but is then dampened by a negative feedback mechanism through the IL-4/Stat6 dependent induction of SOCS1 expression. The IL-4 dependent aspect of Th2 differentiation requires the activation of STAT6. IL-4 signaling and STAT6 appear to play an important role in the immune response. Recently, it was shown that large scale chromatin remodeling of the IL-4 gene occurs as cells differentiate into Th2 effectors is STAT6 dependent. STAT proteins mediate the signaling of cytokines and a number of growth factors from the receptors of these extracellular signaling molecules to the cell nucleus. STATs are specifically phosphorylated by receptor-associated Janus kinases, receptor tyrosine kinases, or cytoplasmic tyrosine kinases. The phosphorylated STAT molecules dimerize by reciprocal binding of their SH2 domains to the phosphotyrosine residues. These dimeric STATs translocate into the nucleus, bind to specific DNA sequences, and regulate the transcription of their target genes. However there are a number of unphosphorylated STATs that travel between the cytoplasm and nucleus and some STATs that exist as dimers in unstimulated cells that can exert biological functions independent of being activated. There are seven mammalian STAT family members which have been identified: STAT1, STAT2, STAT3, STAT4, STAT5 (STAT5A and STAT5B), and STAT6. There are 6 conserved domains in STAT: N-terminal domain (NTD), coiled-coil domain (CCD), DNA-binding domain (DBD), alpha-helical linker domain (LD), SH2 domain, and transactivation domain (TAD). NTD is involved in dimerization of unphosphorylated STATs monomers and for the tetramerization between STAT1, STAT3, STAT4 and STAT5 on promoters with two or more tandem STAT binding sites. It also plays a role in promoting interactions with transcriptional co-activators such as CREB binding protein (CBP)/p300, as well as being important for nuclear import and deactivation of STATs involving tyrosine de-phosphorylation. CCD interacts with other proteins, such as IFN regulatory protein 9 (IRF-9/p48) with STAT1 and c-JUN with STAT3 and is also thought to participate in the negative regulation of these proteins. Distinct genes are bound to STATs via their DBD domain. This domain is also involved in nuclear translocation of activated STAT1 and STAT3 phosphorylated dimers upon cytokine stimulation. LD links the DNA-binding and SH2 domains and is important for the transcriptional activation of STAT1 in response to IFN-gamma. It also plays a role in protein-protein interactions and has also been implicated in the constitutive nucleocytoplasmic shuttling of unphosphorylated STATs in resting cells. The SH2 domain is necessary for receptor association and tyrosine phosphodimer formation. Residues within this domain may be particularly important for some cellular functions mediated by the STATs as well as residues adjacent to this domain. The TAD interacts with several proteins, namely minichromosome maintenance complex component 5 (MCM5), breast cancer 1 (BRCA1) and CBP/p300. TAD also contains a modulatory phosphorylation site that regulates STAT activity and is necessary for maximal transcription of a number of target genes. The conserved tyrosine residue present in the C-terminus is crucial for dimerization via interaction with the SH2 domain upon the interaction of the ligand with the receptor. STAT activation by tyrosine phosphorylation also determines nuclear import and retention, DNA binding to specific DNA elements in the promoters of responsive genes, and transcriptional activation of STAT dimers. In addition to the SH2 domain there is a coiled-coil domain, a DNA binding domain, and a transactivation domain in the STAT proteins. In general SH2 domains are involved in signal transduction. They typically bind pTyr-containing ligands via two surface pockets, a pTyr and hydrophobic binding pocket, allowing proteins with SH2 domains to localize to tyrosine phosphorylated sites. |
Jiyao W et al.(2023). "The conserved domain database in 2023.",