Non Receptor Tyrosine Kinases Flashcards
Tyrosine kinases can be divided into 2 main groups:
Receptor Tyrosine Kinases (RTKs)
Non receptor tyrosine kinases. (NRTKs)
Non receptor tyrosine kinases:
10 sub families of non-receptor tyrosine kinases
7 subfamilies contain SH2 domains (Src, Abl, Btk, Fes, Syk, Csk and JAK families)
Tec, Frk and Fak families do not have SH2 domains
SH2 domains can regulate the activity of the kinase domain, but the mechanisms behind this vary depending on the family
Src family kinases:
Cause Cell proliferation, Cell survival, Cell adhesion, Cell migration. Src family kinases are oncogenes.
Two key phosphorylation sites:
- Kinase domain: activation, autophosphorylation site
- C-terminal peptide: inhibitory, catalysed by CSK
Myristoylation of "unique domain" SH3 domain SH2 domain Kinase Domain C-Terminal peptide
Myristoylation: attachment of a myristol group from myristic acid via an amide bond to an N-terminal glycine residue. Allows for a weak interaction between the protein and a lipid membrane
Src family kinases can activate further non receptor tyrosine kinases
FcEpsilon Receptor
Activation of mast cells occurs through this receptor
Tetramer
Binds IgE antibodies
alpha chain – binds IgE via Ig domains
beta chain –contain ITAM motif, binds the Src family tyrosine kinase Lyn
gamma chain – 2 copies bound by disulfide bridge, contains ITAMs that recruit Syk
Stimulation of degranulation:
FcEpsilon Receptor is activated by clustering
SYK is the key kinase that allows activation of downstream signaling
PKC and Ca2+ signaling drive degranulation
MAPK (and NFkappaB) activation induce cytokine production
MAPKs (via PLA2) promote prostaglandin and leukotriene production
Cytokines:
Key regulators of immunity
Cytokines mainly act to module the function of their target cells
Different cytokines have specific receptors – cytokines have been subdivided into families based on homology that the types of receptor they activate
Can act in autocrine or paracrine fashion
Some cytokines act to regulate immune cell development and hemostasis (eg IL-7) other act in pro-inflammatory (TNF, IL-6, IL-1) or anti-inflammatory (IL-10) responses
Some cytokines are monomeric (eg IL-1), others are dimers (eg IL-12) or trimers (eg TNF)
JAK STAT signalling
JAK (Janus kinase)
Tyrosine kinase
4 isoforms: JAK1, JAK2, JAK3, TYK2
STAT (Signal Transducer and Activator of Transcription)
Transcription factor
7 isoforms: STAT1, STAT2, STAT3, STAT4, STAT5A, STAT5B, STAT6
JAK activates STATs. Very important in the immune system.
Activated by cytokine receptors or some receptor tyrosine kinases
STAT function
STATs are transcription factors
Can act as homo- or heterodimers
While specificity exists between different STAT proteins, there is also a degree of functional redundancy
Two main response elements described
Most active GAS sites promote transcription (but can also inhibit or have no effect on transcription)
In both proximal and distal binding sites STAT may from complexes with other transcription factors
Interferon signalling:
Three main groups of interferon, type I, type II and type III:
Type I: IFNalpha, IFNbeta
- produced by many cells types in response to viral
infection
- induces an “anti-viral” state in nearby cells
- can also modulate immune cell function
Type II: IFNgamma
- produced by some immune cells (NK cells and T cells)
- activates macrophages
- modulates B cell and T cell function
STAT binding to GAS elements:
STATs can bind to GAS (Gamma interferon activation site) sequences
All STAT isoforms can bind GAS elements, although specific does exists at the level of individual promoters in cells
Original studies focused on identifying GAS elements associated with gene promoters
Next generation sequencing has highlighted additional roles for STATs
IRF: Interferon Regulatory Factor
9 isoforms
Involved in regulating transcription in innate and adaptive immune cells
Most act as dimers (eg IRF3 downstream of TLR3 signalling)
IRF9 is unusual in that it binds to STAT2
Negative regulation of JAK-STAT signalling:
SHP-1:
Protein tyrosine phosphatase
Recruited via its SH2 domains
Dephosphorylates receptor and/or JAK
PIAS (Protein Inhibitor of Activated STAT):
4 genes
Binds to STAT proteins and blocks their ability to bind DNA
Can act as an E3 ligase for Sumo (Sumo is a ubiquitin related protein)
SOCS (Suppressor of Cytokine Signalling):
8 genes (SOCS1 to 7 and CIS)
Have SH2 domain and SOCS box
SH2 binds to p-Tyr and blocks access to STATS
SOCS may recruit ubiquitination machinery and target JAKs for proteosomal degradation
SOCS 1 and 3 have a kinase inhibitory domain in their N terminus
Transcription of SOCS genes can be induced by JAK-STAT signalling
SOCS3 can determine signalling outcome
NRTK overview:
e.g.
IFN -> receptor -> JAK activity -> Phosphorylation of STATs -> Effect (such as GAS elements) to promote transcription
