Receptor Tyrosine Kinases 1 Flashcards
20 RTK subfamilies
TRUE OR FALSE
TRUE
Structure of RTKs:
Ligand Binding Domain (Extracellular)
Transmembrane Domain
Tyrosine Kinase Domain (Cytoplasmic)
RTK Activation:
Most RTKs are expressed an inactive monomers
Ligand binding promotes dimerization and receptor activation
- Insulin / IGF receptors are an exception: exist as a “dimer” joined by a disulphide bridge
- Different RTKs form dimers in different ways
- Dimerisation promotes activation of the tyrosine kinase domain; precise mechanism varies in different receptors
3 Different ways in which RTKs form dimers
- Each ligand binds only one receptor, Ligand does not contribute to dimer interface e.g. EGF Receptor
- Both ligand and receptor contribute to dimer interface e.g. C-Kit Receptor
- Dimeric ligands results in dimer – no direct contribution of ligand binding domain in dimer e.g. Trk (NGF) receptor
EGF / ErbB receptor activation:
Transmembrane receptor, found on the plasma membrane
Inactive ErbB receptors are present as monomers on the plasma membrane
Can dimerise, however ligand binding drives dimerization and subsequent activation
Can activate signaling as a homodimer or as a heterodimer with another ErbB family member
EGF binding leads to activation of the tyrosine kinase domain
This results in autophosphorylation on multiple tyrosines in the cytoplasmic domain of the receptor
This creates binding sites signaling adaptors such as Shc and Grb which process phosphotyrosine binding motifs
EGF receptor:
Member of the ErbB family of RTKs:
- EGFR
- ErbB2
- ErbB3
- ErbB4
ErbB receptors can bind multiple ligands
ErbB2 is an orphan receptor; can dimerise with other ErbB receptors
ErbB3 is kinase inactive – does not signal as a homodimer
Signaling adaptors
Protein interactions are mediated by distinct, highly discerning, independently folded domains
These domains can recognize post translational modification such as phosphorylation
Signaling adaptors use combinations of these protein recognition domains to assemble complexes of signaling enzymes
Typically adaptors do not have any catalytic activity
Shc and Grb are the key adaptors that link EGF receptor activation to MAPK pathways
Bind to phospho-tyrosine residues
SH2 domains
Examples found in many proteins
All bind phospho-Tyr, binding also affected by the amino acid sequence around the p-Y residue: different SH2 domains recognize different p-Y motifs
The ability of SH2 and PTB domains to bind to specific pTyr residues depending on the surrounding sequence means that all SH2 / PTB domain containing proteins will not be recruited to individual tyrosine phosphorylated proteins
PTB: “phosphotyrosine binding domain”
Examples found in several proteins
Binds to phospho-Tyr; preferred recognition motif is N-P-x-pY
The ability of SH2 and PTB domains to bind to specific pTyr residues depending on the surrounding sequence means that all SH2 / PTB domain containing proteins will not be recruited to individual tyrosine phosphorylated proteins
Shc:
ShcA was one of the first adaptors to be described
Alternate start codons results in 3 different isoforms
ShcA widely expressed, but downregulated in mature CNS neurons
ShcB and C expressed in the CNS and may replace ShcA
ShcD differentially expressed during development
Contain PTB and SH2 domains
ShcA binds to p-Y of the EGF receptor
Y992 binds the SH2 domain while Y1148 or Y1173 binds the PTB domain
Other p-Y may also contribute
PTB binding appears to be more important than SH2 in order to promote a strong MAPK activation
Once bound, ShcA is phosphorylated by the EGFR domain.
Phosphorylated ShcA recruits a 2nd adaptor Grb2
Grb2 can also bind to the EGFR independently of ShcA
While Gbr2 can bind to the receptor without ShcA, the ShcA pathway is more efficient and promotes signaling at low concretions of EGF receptor ligands
Grb2
Grb2 is a small adaptor protein containing an SH2 domain flanked by SH3 domains
SH3 domains bind Pro – X – X – Pro motifs
Grb2 activates the classical MAPK cascade
TRUE OR FALSE
TRUE
Grb2 is constitutively bound to SOS. Recruitment of Grb2 to the EGFR brings SOS to the membrane where is acts as a GEF for Ras
MAPK signalling:
MAPKs are present in all eukaryotes
In addition to RTKs, other receptors (eg GPCRs, cytokine receptors and ligand gated ion channels) also activate MAPKs
Mammalian cells have 14 MAPK genes:
- “Classical MAPKs”; ERK1, ERK2
- P38 MAPKs; p38a, p38b, p38g, p38d
- Jun N-terminal kinases; JNK1, JNK2, JNK3
- Atypical MAPKs; ERK3, ERK4, ERK5, ERK7, NLK
Wide range of substrates and functions:
- eg: cell proliferation, survival, development immune function, neuronal function, regulation of metabolism
- ERK1/2 and p38 activate further downstream kinases
Each MAPK cascade has specific MAP2K and MAP3K enzymes
MAP3K: MAPK kinase kinases phosphorylate and activate MAP2Ks. They can be activated by other kinases or by interactions with other signaling proteins such as small GTPases
MAP2K: MAPK kinases phosphorylate both the Tyr and Thr residues in the TXY motif. This makes them unusual as they have both Ser/Thr and Tyr kinase activity
MAPK: MAPKs are activated by phosphorylation of a TXY motif in the activation loop of their kinase domain
Gab proteins mediate PI3K and PLCgamma activation
3 isoforms, Gab1 ,2 and 3
Have an N terminal PH domain and P-X-X-P motifs (SH3 binding)
Recruited to membranes via their PH domain and via interaction with Grb2
Following recruitment to the EGF receptor complex, Gab1 is phosphorylated by the EGFR tyrosine kinase domain
PI 3 Kinase and PLGg are recruited and activated by the pTyr motifs of Gab
Gab proteins mediate PI3K and PLCgamma activation
3 isoforms, Gab1 ,2 and 3
Have an N terminal PH domain and P-X-X-P motifs (SH3 binding)
Recruited to membranes via their PH domain and via interaction with Grb2
Following recruitment to the EGF receptor complex, Gab1 is phosphorylated by the EGFR tyrosine kinase domain
PI 3 Kinase and PLCgamma are recruited and activated by the pTyr motifs of Gab
