lec 13- cell signalling and signal transduction part 2 Flashcards
what does phosphorylation do to a protein?
changes its charge and generally leads to a conformational change which can alter ligand binding or cause an increase or decrease in activity
does GTP turn into GDP?
yes
does GDP turn into GTP?
no, it is exchanged
what does GAPS, RGS, GDI, and GEF stand for?
GAPs = GTPase-activating protein
RGS = regulators of G protein signalling
GDI = guanine nucleotide dissociation inhibitors
GEF = Guanine nucleotide exchange factors
what are enzyme coupled receptors?
they are transmembrane proteins that bind ligands, cytosolic domain either contains an intrinsic kinase activity or associates directly with kinase
what is a non-receptor kinase?
A kinase that associates with a receptor or is involved in a signal transduction pathway downstream
what is the most common type of enzyme coupled receptors?
receptor tyrosine kinase (RTKs)
what is the most common type of non-receptor kinase?
non-receptor tyrosine kinase
how many RTKs and non-receptor RTKs ae encoded by humans?
60 and 32
is tyrosine kinase the ligand for RTKs?
no, the RTKs have tyrosine kinase activity instead of tyrosine kinase binded as a ligand
what are the 4 steps of receptor dimerization?
- inactive RTKs are binded to signalling protein
- binding of signalling protein causes trans-autophosphorylation resulting in activation of kinase domains
- the trans-autophosphorylation also generates binding sites for signalling proteins
- then the activated signalling proteins relay signal downstream
what does a monomeric ligand interact with?
two receptor monomers
what promotes dimerization?
two monomeric ligands binding independently to two receptor monomers
what is trans-autophosphorylation?
when the tyrosine kinase from one monomer phosphorylates the tyrosine residues on the other creating identical monomers
what can phosphorylated tyrosine act as?
docking sites for other proteins
what do phosphorylated tyrosines bind to?
effector proteins
what do effector proteins consist of?
-Src homology 2 (SH2) domain
-phosphotyrosine-binding (PTB) domain
what is a SH2 domain composed of?
roughly 100 amino acids and contain a conserved binding-pocket that accommodates a phosphorylated tyrosine residue
what does the PTB domain bind to and what is it a part of?
bind to phosphorylated tyrosine residues and are usually present as a part of Asn-Pro-X-Tyr motif
what must each molecule recognize to bind?
the phosphorylated tyrosine in the context of other specific amino acids
what are the 4 things that SH2 and PTB domain proteins do?
- act as adaptor proteins that bind other proteins
- docking proteins that supply receptors with other tyrosine phosphorylation sites
- signalling enzymes (kinases) that lead to changes in the cell
- transcription factors
what terminates the signal transduction by RTKs?
the internalization of the receptor primarily through clatherin-mediated endocytosis
what is the fate of the internalized RTKs?
either degrade in lysosomes or return to the plasma membrane
what are other things that can happen to RTKs during termination?
- the binding of RTks to clatherin adaptor protein AP-2
-may be targeted by ubiquitin ligases to undergo ubquitination through SH2 domains or adaptor proteins
what is the best characterized signalling cascade?
Ras-MAP kinase
what is SRC?
a tyrosine kinase
is SRC a RTK?
no its a non-receptor tyrosine kinase
how many domains does SRC have and what do some of them do?
4 domains, 2 are involved in binding to other proteins
what are the 2 domains that commonly bind in SRC called?
SH2 and SH3
what are cancer-critical genes?
genes whose alteration frequently contributes to causing cancer
what is a gain-of-function mutation?
proto-oncogenes that mutate into oncogenes
what is a proto-oncogene?
a normal gene that helps cells grow and divide
what is an oncogene?
when a normal gene mutates into a gain-of-function gene
what are oncoproteins and proto-oncoproteins?
proteins made from oncogenes and proto-oncogenes
what does the receptor do once trans-autophosphorylation occurs?
it recruit cytosolic proteins
how do the cytosolic proteins recognize the receptor?
they have amino acid stretches (domains) that recognize the phosphotyrosine and nearby residues on the receptor
(one such domain is called the SH2)
what is cross talk between signalling pathways?
when multiple pathways are activated
what activates the Ras/MAP kinase pathway?
RTKs
what type of mutant gene makes up 30% of all cancers?
RAS genes
what are the three types of RAS genes that mutate into cancers?
H-ras, N-ras, and K-ras
how does signal transduction of Map Kinase pathway occur?
- RTK is acivated by trans-autophosphorylation which lets it bind to SH2 domain on adaptor protein
- SH3 on adaptor protein binds to Ras-GEF which exchanges GDP on inactive Ras protein for GTP
- GTP on Ras protein makes it activate helping it recruit MAP kinase kinase kinase (Raf)
- Raf then activates MAP kinase kinase (Mek)
- Mek activates MAP kinase (Erk)
- Map kinase then phosphorylates a variety of downstream proteins including other kinases
how does Ras activate Raf?
by associating with kinase and changing its conformation
what do scaffolding proteins do?
function to tether members of a signalling pathway in a specific orientation to enhance their mutual interactions (e.g. AKAPs)
can scaffolding proteins induce a conformational change in signalling proteins?
yes, causing activation or inhibition
what can scaffolding proteins prevent other proteins from doing?
participating in other pathways, resulting in higher specificity
what do Rho family GTPases do to surface receptors?
couple cell surface receptors to the cytoskeleton
what does Rho ATPase do?
regulate actin and microtubules, helps regulate gene transcription, regulates membrane transport and more
what is Rho GTPase bound too before attaching to the membrane?
GDI in the cytosol
what does GDI do to Rho?
prevents it from interacting with its GEF at the plasma membrane
