The JAK-STAT signaling pathway delivers information from extracellular chemical signals that regulate growth, survival, differentiation and pathogen resistance. The JAK-STAT signaling cascade consists of three major components: the cell surface receptor, Janus kinase (JAK), and two signal transduction and transcriptional activator proteins (STAT). Destruction or dysregulation of JAK-STAT function can lead to immunodeficiency syndrome and cancer. Cell surface receptors, usually cytokines such as interferons, interleukins and growth factors, activate the associated JAK and increase their kinase activity. In mammals, the JAK family includes four members: JAK1, JAK2, JAK3 and Tyk2. The tyrosine residue on the activated JAK phosphorylation receptor then creates a binding site for the protein having the SH2 domain. The SH2 domain containing STAT is recruited to the receptor where they are also phosphorylated by JAK tyrosine. STAT is a latent transcription factor that is present in the cytoplasm until it is activated. These activated STAT form heterologous or homodimers and translocate to the nucleus where they induce transcription of the target gene. Phosphorylation sites on the receptor and JAK serve as docking sites for SH2-containing Stats and SH2-containing proteins and adaptors that link receptors to MAP kinase, PI3K/Akt, and other cellular pathways.
Structure of JAKs and STATs
The mammalian JAK family has four members JAK1, JAK2, JAK3 and Tyk2(tyrosine kinase 2). Of these, Jakl, Jak2, and Tyk2 are ubiquitously expressed, while Jak3 is predominately expressed in immune and hematopoietic tissues. Jaks contain four conserved domains, which can be further divided into 7 Jak homology (JH) domains. The N-terminal FERM domain (named after its homology to band four point one, ezrin, radixin, and moesin; JH7 and JH6) is required for interaction with cytokine receptors and regulates catalytic activity. Jaks also have a SH2-like segment or JH4 domain whose function has not yet been characterized. The pseudo-kinase domain or JH2 negatively regulates the catalytic activity of Jaks, mediated by SOCS3-independent mechanism and mutations in JH2 domain of Jak3 have been detected in Jak3-SCID (severe combined immunodeficiency) patients. The C-terminal catalytic domain or JH1 is a conserved functional kinase domain. While the function of Jaks is nonredunant, Jaks all share the conserved structure. Jaks bind not only cytokine receptors but also other proteins such as STAMs. Jaks become activated secondary to tyrosine phosphorylation in response to cytokines. Subsequently Jaks initiate the signal transduction cascade by phosphorylation of tyrosine motifs present in receptor cytoplasmic domains and in receptor associated proteins. Jak are phosphorylated on multiple sites during cytokine signaling, the identity and function of most of these sites remain unknown.
STATs are a group of latent cytoplasmic transcription factors. To date, seven mammalian STATs have been reported STAT1, STAT2, STAT3, STAT4, STAT5a, STAT5b, and STAT6. All seven mammalian STATs share six conserved domain-like structures. The N-terminal domain is a stabilizing domain, which stabilizes dimer or tetramer formation. The coiled-coil domain plays an important role in protein interaction. The DNA-binding domain has several conserved residues throughout all STATs. The linker domain has a helical structure and is important for transcription. The SH2 domain is required for receptor binding and contains a tyrosine residue that is phosphoryted by Jaks, leading to activation and dimerization of STATs. The transactivation domain (TAD) mediates interactions of the STATs with a number of transcriptional coactivators, which also facilitates chromatin modifications, and transcriptional activation. In addition, phosphorylation of a conserved serine residue in TAD is required for full transcriptional activation of STATs.
Phosphotyrosine-containing motifs in cytoplasmic domains of cytokine receptor act as docking sites for many SH2 signaling proteins, including STATs. Dimerization occurs immediately after tyrosine phosphorylation through reciprocal phosphotyrosine - SH2 interactions. STAT1, STAT2, STAT3, STAT4, STAT5a, STAT5b form homodimers, whereas STAT1 and STAT2 or STAT1 and STAT3 can form heterodimers. In addition, several STATs can form tetramers. STATs are able to be phosphorylated in both a Jak-dependent and independent manner. STATs are phosphorylated by Jaks in response to cytokines. Some hormones receptors such as EGFR and PDGFR function as a receptor tyrosine kinase to phophorylate STATs. STATs can also be phophorylated by several non-receptor oncogenic tyrosine kinases, such as v-Src and BCR-ABL. The targeted disruption of individual STATs in mice has revealed the physiological importance of STATs in the immune system. Only STAT3-deficiency in mice is embryonic lethal, indicating that STAT3 plays a pivotal role in development. Several different kinases have been suggested to mediate Ser727 phosphorylation of STAT1, including p38, ERKs, JNKs, PI3K, PDK1 (3-phosphinositide-dependent protein kinase I), p90 ribosomal S6 kinase (p90RSK2), PKC8, CaMKII, and CK2, depending on the stimulus. It has been proposed that ISGylation is used to regulate the IFN-inducible Jak-STAT pathway in vitro. In contrast to in vitro studies, ISG15 deficient mice have intact responses to IFN signaling. Recent studies have shown that STATs can form stable dimers in the absence of phosphorylation, although these dimers cannot bind to DNA. These STAT1 or STAT3 homodimers dynamically shuttle between the cytoplasm and nucleus independent of their phosphorylated status. Monomeric STAT1, STAT3 and STAT5b also continuously shuttle between cytoplasm and nucleus. The biological function of unphosphorylated STATs remains to be investigated but it has been suggested that unphosphorylated STATs can form complex with other proteins (STAT1/IRF1 or STAT2/IRF9 or STAT3/JUN) and may play an important role in mediating distinct gene expression.
Binding of various ligands (usually cytokines such as interferons and interleukins) to cell surface receptors dimerizes the receptor, bringing the receptor-associated JAK into proximity. JAK then phosphorylates each other on a tyrosine residue located in a region called the activation loop by a process called transphosphorylation, which increases the activity of their kinase domains. The tyrosine residue on the activated JAK phosphorylation receptor then creates a binding site for the protein having the SH2 domain. STAT then uses their SH2 domain to bind to phosphorylated tyrosine on the receptor, which are then phosphorylated by JAK tyrosine, resulting in dissociation of STAT from the receptor. These activated STATs form heterologous or homodimers in which the SH2 domain of each STAT binds to phosphorylated tyrosine of the opposite STAT, and then the dimer translocates to the nucleus to induce transcription of the target gene. STATs may also be directly tyrosine phosphorylated by receptor tyrosine kinases - but because most receptors lack built-in kinase activity, JAK signaling is usually required.
Figure 1. Key steps of the JAK-STAT pathway
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