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STATs (signal transducers and activators of transcription) are a family of transcription factors that are specifically activated to regulate gene transcription when cells encounter cytokines and growth factors. The crystal structure of an NH 2 -terminal conserved domain (N-domain) comprising the first 123 residues of STAT-4 was determined at 1.45 angstroms. The domain consists of eight helices that are assembled into a hook-like structure. The N-domain has been implicated in several protein-protein interactions affecting transcription, and it enables dimerized STAT molecules to polymerize and to bind DNA cooperatively. The structure shows that N-domains can interact through an extensive interface formed by polar interactions across one face of the hook. Mutagenesis of an invariant tryptophan residue at the heart of this interface abolished cooperative DNA binding by the full-length protein in vitro and reduced the transcriptional response after cytokine stimulation in vivo.

STAT proteins (signal transducers and activators of transcription) are latent cytoplasmic transcription factors that are phosphorylated by Janus kinases in response to cytokines. Phosphorylated STAT proteins translocate to the nucleus, where they transiently turn on specific sets of cytokine-inducible genes. The mechanism that controls the amounts of activated STAT proteins is not understood. STAT1 proteins activated by interferon-gamma treatment in HeLa cells were shown to be stabilized by a proteasome inhibitor and ubiquitinated in vivo. Thus, the amount of activated STAT1 may be negatively regulated by the ubiquitin-proteasome pathway.

Signal transducers and activators of transcription (STAT) proteins can be conditionally activated in response to epidermal growth factor (EGF) and interferon (IFN)-gamma. STAT activation was correlated with cell growth inhibition in response to EGF and IFN-gamma. Activated STAT proteins specifically recognized the conserved STAT-responsive elements in the promoter of the gene encoding the cyclin-dependent kinase (CDK) inhibitor p21 WAF1/CIP1 and regulated the induction of p21 messenger RNA. IFN-gamma did not inhibit the growth of U3A cells, which are deficient in STAT1, but did inhibit the growth of U3A cells into which STAT1 alpha was reintroduced. Thus, STAT1 protein is essential for cell growth suppression in response to IFN-gamma. The STAT signaling pathway appears to negatively regulate the cell cycle by inducing CDK inhibitors in response to cytokines.

In response to specific ligands, various STAT proteins (signal transducers and activators of transcription) are phosphorylated on tyrosine by Jak protein kinases and translocated to the nucleus to direct gene transcription. Selection of a STAT at the interferon gamma receptor as well as specific STAT dimer formation depended on the presence of particular SH2 groups (phosphotyrosine-binding domains), whereas the amino acid sequence surrounding the phosphorylated tyrosine on the STAT could vary. Thus, SH2 groups in STAT proteins may play crucial roles in specificity at the receptor kinase complex and in subsequent dimerization, whereas the kinases are relatively nonspecific.

Through the study of transcriptional activation in response to interferon alpha (IFN-alpha) and interferon gamma (IFN-gamma), a previously unrecognized direct signal transduction pathway to the nucleus has been uncovered: IFN-receptor interaction at the cell surface leads to the activation of kinases of the Jak family that then phosphorylate substrate proteins called STATs (signal transducers and activators of transcription). The phosphorylated STAT proteins move to the nucleus, bind specific DNA elements, and direct transcription. Recognition of the molecules involved in the IFN-alpha and IFN-gamma pathway has led to discoveries that a number of STAT family members exist and that other polypeptide ligands also use the Jak-STAT molecules in signal transduction.

The STAT family of proteins carries out a dual function: signal transduction and activation of transcription. A new family member, Stat3, becomes activated through phosphorylation on tyrosine as a DNA binding protein in response to epidermal growth factor (EGF) and interleukin-6 (IL-6) but not interferon gamma (IFN-gamma). It is likely that this phosphoprotein forms homodimers as well as heterodimers with the first described member of the STAT family, Stat91 (renamed Stat1 alpha), which is activated by the IFNs and EGF. Differential activation of different STAT proteins in response to different ligands should help to explain specificity in nuclear signaling from the cell surface.