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- Tyrosine Kinase Receptors - Not only a receptor - Also an enzyme: Tyrosine kinase - Cytokine superfamily receptors - No catalytic domain - Interact with non receptor protein-tyrosine kinases - Src family - JAK family
Enzyme-linked receptors fall into
3 categories:
- TGF-β receptors
What is Cytokine?
! Secreted polypeptide or low molecular weight
protein involved in cell-to-cell signaling.
! Acts in paracrine or autocrine fashion through
specific cellular receptors.
! Can be produced by cells of any tissue and act on
many cells involved in immune and inflammatory
response.
Cytokine receptors subfamilies have shared signaling subunits
Cytokine Receptor Signaling
- Similar to Receptor Tyrosine Kinase signaling
- Receptor clustering
- Cytokine receptors do NOT have any enzymatic activity, but bind cytosolic kinases
- Phosporylation and activation of JAK kinases
- Binding of STAT to p-Receptor via SH2 domain
- Phosphorylation of STAT by JAK kinase
- Translocation of p-STAT into nucleus
- Activation of transcription
- Feedback regulation: SHP1 and SOCS
The JAK-family of tyrosine kinases
- Family members
- JAK1 (135 kDa)
- JAK2 (130 kDa)
- JAK3 (120 kDa)
- Tyk2 (140 kDa)
- Common feature
- C-terminal kinase + pseudokinase
- ≠ RTK by lacking transmembrane domains and SH2, SH3, PTB, PH
- several regions homologous between JAK-members
- Associated with cytokine receptors (type in and II)
- Function
- Associated with cytokine receptors in non-stimulated cells in an inactive form
STAT proteins
STATs: Signal Transducers and Activators of Transcription
1. Transducers for signals from many cytokines
Broad spectrum of biological effects
2. Transcriptional activators
activation at the cell membrane, response in the nucleus
Rapid signal response
The activation/deactivation cycle of STAT molecules is quite short, about 15 min for an individual molecule.
STAT-family
members
- STAT3 - involved in response to several cytokines including IL6. It activates several genes involved in acute phase response - Important in growth regulation, embryonic development & organogenesis - Activation of STAT3 correlated with cell growth, link to cancer, binds c-Jun
- STAT4 - involved in IL12-response
- STAT5a & 5b - involved in response to several cytokines including prolactin, IL-2, and regulates expression of milk proteins in breast tissue in response to prolactin
- STAT6 - involved in IL4-response
- non-mammalian family members (e.g. Drosophila)
- STAT1 - involved in IFNα/β- and IFNγ-response
- STAT2 - involved in IFNα/β- response. Mainly acting as partner for STAT1/p
STAT proteins
- Once the STAT is bound to the receptor, the C-terminal
tyrosine is phosphorylated by an associated JAK kinase
- The phosphorylated STAT dissociates from the receptor, and
two activated STATs form a dimer and then enters the nucleus
STAT-DBD structure (^) – Symmetry-axis through DNA,
each monomer contacts a separate half site
- The dimer forms a C-shaped ”clamp” around DNA.
- The dimer is kept together by reciprocal SH2- Tyr P^ interactions between the SH2 domain in one monomer and the phosphorylated Tyr in the other.
- The SH2 domain in each monomer is closely linked to the core DBD and is itself close to DNA, and is assumed also to contribute to DNA-binding.
- N-terminal coiled-coil region not close to DNA, probably involved in prot-prot interaction with flexible position
Specificity in response
- each cytokine activates a subgroup STAT
- some cytokines activate only one specific STAT
What does mediate specificity?
1. the SH2 - receptor interaction specific for certain
combinations swaps-experiments of SH2 between STATs
change specificity affinity of the SH2-receptor interaction is
affected by the sequence context of the Tyr
2. different STAT-dimers bind different response elements in the
genome and turn on different genes
STAT1 knock-out mice illustrate biological specificity: STAT1-/-
phenotype: total lack of IFN-response → highly sensitive to virus-
infection
Through their SH2 domains, STAT1a and STAT1b bind to the tyrosine-phosphorylated receptor and become phosphorylated. They then form a dimer, (called a Sis-inducible factor, SIF) which translocates to the nucleus, where it binds to a Sis-inducible element (SIE) within the fos promoter.
Direct phosphorylation of STAT transcription factors.
Negative Regulation of the JAK-STAT pathway
- Signal-induced transcription of target genes can not last for too long and needs de-sensitized
- Signaling from cytokine receptor is usually dampened by two classes of proteins: short term regulation by SHP1 phosphatase and long term regulation by SOCS proteins
Signaling from Cytokine Receptors Is Modulated by
Negative Signals
CIS proteins act in two ways to negatively regulate cytokine receptor stimulated signaling:
The SH2 domain in several SOCS
proteins bind to phosphotyrosines on an activated receptor, preventing binding of other SH2-containing signaling proteins and thus inhibiting receptor signaling
SOCS-1 can bind to critical
phosphotyrosine in the activation lip of activated JAK2 kinase thereby inhibiting its catalytic activity
All SOCS proteins contain a SOCS box
that recruits components of E3 ubiquitin ligases. As a result of SOCS-1 binding, JAK2 becomes polyubiquitinated and then degraded in proteasomes and thus terminate the signaling permanently
STAT proteins induce a class of small proteins termed SOCS proteins. These negative regulators are also known as CIS proteins
SOCS4 and SOCS5 bind to the EGFR through their respective SH
domains, which share 87% sequence homology and a poorly defined
Nterminal region.
The SOCS box recruits an E3 ligase and thereby leads to EGFR
ubiquitylation.
