Flashcards in Signal Transduction III Deck (27)
How many times do receptor kinase tyrosines span a membrane?
receptor kinase tyrosines are primarily involved in what kinds of functions of the cell?
proliferation, differentiation, and cell movement
What happens when a signal binds to a RKT?
The signal attaches to two receptors in the membrane, which then align close to each other. The kinase portion of the receptor (inside the cell) then cross-phosphorylate each other, resulting in active RTKs
What do the 'activated' RTKs form?
'scaffolds'- where proteins associate on the length of the RTK
Are the protein scaffolds on RTKs dynamic or once a protein binds, is it stuck there?
They are dynamic. Association and dissociation of proteins along the RTK is rapid and constant (in response to the needs of the cell)
What forms the basis of 'scaffolds' on RTKs?
Is binding to phospho-tyrosines specific or non-specific?
How did mutagenesis prove that phospho-tryosines are specific?
When you add alanine to replace tyrosine on a scaffold, no binding occurs
All proteins that bind to phospho-tyrosine residues contain what?
an adaptor that contains an SH2 domain (Src Homology)
How do SH2 domains bind to phospho-tyrosines in RTKs?
They recognize and bind to a five AA stretch at the N-terminus (aka very specific stretches). So the binding of specific proteins to specific phospho-tyrosines is not only dependent on the tyrosine being phosphorylated, but also on the sequence of the next five AAs at the N-terminus.
What results from this is specificity along the RTK to certain proteins at certain phosphor-tyrosine residues
RTKs signal through which pathway?
the Ras-GEF pathway
What is the function of adaptor proteins?
no enzymatic function, they act as mediators- sticking two things together
What are the steps of the RTK signal pathway up to as protein activation?
1) extracellular signal activats RTKs as dimers by cross-phosphorylation
2) SH2 domains of the Grb2 protein bind to phospho-tyrosines and have two C-terminus SH3 domains
3) SH3 domains recognize poly-proline (PXXP) sequences on Ras GEF protein
4) Ras GEF gives a Ras protein a guanyl nucelotide to allowing the Ras protein to exchange GDP for GTP (thus, activating it)
What type of protein are Ras proteins?
low molecular weight G protein (basically an a subunit alone and have intrinsic GTPase ability)
that require axillary proteins to function
What do Ras proteins need to exchange GDP for GTP?
How do Ras proteins get the guanyl nucleotide exchange factor that they need to exchange GDP for GTP?
from Ras GEF (aka guanyl nucleotide exchange factor)
Are activated Ras proteins bound to GDP or GTP?
What does an activated Ras protein do?
1) activates RAF (activated MAP kinase kinase kinase)
2) RAF DOUBLE phosphorylates (the double phosphorylation is required- can't have just 1) MEK (activated MAP kinase kinase) to activate it
3) 2) MEK DOUBLE phosphorylates (the double phosphorylation is required- can't have just 1) ERK (activated MAP kinase) to activate it
all phosphorylations occur using ATP
What does activated (aka double phosphorylated) ERK do?
some stays in the nucleus to phosphorylate proteins/transcription factors and some go to cytosol to phosphorylate proteins
What is a dominant-negative swap? What did it prove?
When you swap out a tyrosine and make a mutant RTK, then over-express it on the membrane. The result is that normal RTKs cannot find each other to dimerize
This proved that dimers were involved in the RTK pathway
What are cytokine receptors?
They are RTKs without intrinsic kinase activity
Do cytokine receptors dimerize?
Yes (called JAK-STAT receptors)
What is the name of the kinases recruited by cytokine receptors?
JAK (remember, RTKs have intrinsic kinase activity and do not require recruitment)
What are the two main differences between cytokine and RTK receptors?
cytokine must recruit tyrosine kinase and STAT proteins transduce signal to nucleus
How do cytokine receptors work?
1) receptor binds cytokine and dimerizes
2) JAX kinase is recruited, and tyrosine residues are cross-phosphorylated on the receptor, thus activating the receptor
3) STATs (signal transducers and activators of transcription) bind to phospho-tyrosine residues through SH2 domain and are themselves phosphorylated by JAK
4) STATs then release and dimerize to each other through their SH2 domains
5) dimerized STATs move to nucleus to influence transcription
What are the domains of Janus Kinases (JAK)?
domain 1- functional tyrosine kinase
domain 2- regulation of domain 1