The signal transducers and activators of transcription (STAT) proteins were originally discovered through the biochemical purification of factors involved in interferon (IFN) regulated gene transcription. The STAT family consists of seven members (STAT1, STAT2, STAT3, STAT4, STAT5a, STAT5b and STAT6). They are localized in three chromosomal clusters, suggesting that this family of proteins has evolved by gene duplication: STAT1 and STAT4 are found on chromosome 2, STAT2 and STAT6 on chromosome 12 and STAT3 and STAT5a and STAT5b on chromosome 17. Homologous proteins have been found in non-vertebrates such as Drosophila. Since no STAT-like genes have been identified in unicellular organisms, including yeast, STAT might have evolved with the development of multicellular organisms.
All STATs share conserved domains that were first recognized by sequence analysis and mutagenesis studies. Starting from the amino terminus, they are the N-terminal domain, the coiled- coil domain, the DNA binding domain, the linker domain, the SH2 domain and the transactivation domain. This conserved structural organization was confirmed and greatly advanced by X-ray crystallography of the core amino acids of STAT1 and STAT3 bound to DNA.
Regulation of STAT transcriptional activity
The ultimate biological effect of activated STATs lies in their ability to rapidly increase the transcriptional activity of genes in response to extracellular cues. The ability of different STAT proteins to regulate a wide variety of genes is due, at least in part, to different binding affinities for natural DNA binding sites and to the strength of transcriptional activation. The transcriptional activity of all STATs depends on the carboxy-terminal transcriptional activation domain (TAD). STAT1, STAT3 and STAT4 undergo alternative splicing to generate distinct C-termini lacking a TAD. The truncated forms of these STATs function as dominant negatives when over-expressed in cultured cells. Targeted disruption of truncated STAT3β in mice that still have a functional full length STAT3α results in significant changes in the pattern of gene induction and impaired recovery from endotoxic shock. More recent studies have shown that each isoform of STAT3 has unique and distinct functions.
Further regulation by the TAD is achieved through modifications by serine phosphorylation and by the recruitment of co-activators. The TAD of STAT1, STAT3, STAT4 and STAT5 contain a P(M)SP motif that was found to be phosphorylated. Mutation o this serine to alanine resulted in reduced cytokine-stimulated transcription. Experiments by Kovarik et al. suggested that STAT C-termini contribute to the specificity of cellular responses by linking STATS to different serine kinase pathways and through intrinsically different requirements for serine phosphorylation at distinct promoters. Serine phosphorylation can be targeted independently of tyrosine phosphorylation and therefore may represent synergy between two activating signals.
All STATs, except STAT4, have been shown to associate with the histone acetyltransferase coactivaors p300 and CBP. This interaction has been found to increase STAT transcriptional activity. In addition, STAT2 has been shown to recruit the acetyltransferase GCN5 which results in acetylation of histones in the promoters of IFN-α regulated genes. NMI, another cofactor shown to interact with coiled-coil domain of STAT5 has been proposed to facilitate interaction of STAT5 with p300/CBP, resulting in enhanced transcriptional activity. Other transcription factors shown to be required for maximal STAT-dependent transcription include Spl, USF1, glucocorticoid receptor, CCAAT/enhancer binding protein, c-jun, androgen receptor, and SMAD.
Negative regulation of STATs
The duration of STAT activation is regulated by the balance of receptor associated JAK activity in the cytoplasm and dephosphorylation in the nucleus. SH2 containing phosphatases (SHP) proteins are constitutively expressed and can attenuate cytokine signal transduction by dephosphorylating JAKs and receptors. Members of the protein inhibitors of activated STATs family are also constitutively expressed and mediate repression of STAT activity. The suppressors of cytokine signaling (SOCS) family of proteins are generally expressed at low level in the unstimulated cells but are rapidly induced by cytokines and inhibit JAK-STAT signaling in a classic negative feedback loop. SOCS proteins can directly inhibit cytokine signaling by inhibiting JAK activity, competing with STATs for receptor docking sites, and by targeting signaling molecules for degradation through the ubiquitin-proteosome pathway. Studies of SOCS gene targeting in mice has revealed critical roles for these proteins in negatively regulating certain cytokines and demonstrates the disastrous biological consequences of rampant cytokine signaling.
Negative regulation also occurs in the nucleus and STAT dephosphorylation is an important signal for export back to the cytoplasm. There is evidence that TC45, a nuclear tyrosine phosphatase, is responsible for STAT1 and STAT3 dephosphorylation. Cells lacking TC45 retain tyrosine phosphorylated STAT1 longer that normal cells and overex