Cell Signalling IV Flashcards
What are enzyme-coupled receptors and what are they often associated with?
Enzyme-coupled receptors are transmembrane proteins with their ligand-binding domains on the outer surface of plasma membrane.
The cytosolic side is often associated with a kinase enzyme most common class of these proteins act through receptor tyrosine kinases
How do receptor tyrosine kinases carry out their functions?
Binding of ligand to receptor activates tyrosine kinase domain on cytosolic part of receptor creating phosphotyrosine docking sites for intracellular molecules.
What is a good example of a molecule that signals through receptor tyrosine kinases?
Insulin
Do enzyme coupled receptors have 7 transmembrane domains like GPCRs?
No they typically only have a single transmembrane domain with the exception of insulin receptor which contains 3 subunits held together by disulfide bonds
How are receptor tyrosine kinases activated and what are they like prior to activation?
RTKs exist as monomers where kinase domain is inactive
Binding of ligand brings 2 monomers together to form a dimer and so the 2 kinase domains phosphorylate each other and activate other domains.
Tyrosine phosphorylation also leads to phosphorylation of other tyrosine residues and generate docking sites for intracellular signalling proteins
How does IRS1 (docking protein) transmit signal from intracellular domain of RTK?
Phosphotyrosine binding (PTB) domain binds to RTK and binds to other proteins such as Grb2 adaptor protein which then binds to sos and scaffold proteins (via sh3 domains) and to RTK (via SH2 domain)
How do intracellular signals bind to each other and remain near the plasma membrane?
They bind to phosphoinositide docking sites which are in the plasma membrane
How do cytosolic proteins bind to RTKs?
via SH3 or SH2 domains
What does phospholipase C-gamma do?
Very similar function to phospholipase C-beta. They dock onto the phosphorylated RTK. They act as enzymes which have SH2 and SH3 domains
What domain is required to bind to phosphorylated tyrosines?
SH2 domains.
What are the common proteins that bind to RTKs at phosphorylated sites?
PI3-kinase
GTPase-activating protein (GAP)
phospholipase C-gamma (PLCgamma)
What protein mediates signalling by most RTKs?
the GTPase superfamily Ras
What does Ras do?
Functions as a molecular switch which is active with GTP bound and inactive when bound to GDP.
What does Grb2 do?
It recognizes a specific phosphotyrosine, binds via SH2 domain, recruits Sos which stimulates inactive Ras
What are the 2 types of regulatory proteins that control activity of monomeric GTP-binding proteins?
Guanine nucleotide exchange factors (GEFs)
GTPase-activating proteins (GAPS)
What do the regulatory proteins that control activity of monomeric GTP-binding proteins do?
GEFs promote exchange of GDP for GTP
GAPS stimulate hydrolysis of GTP to GDP to switch off GTP-binding proteins
How do RTKs activate Ras?
Ras-GEF (aka sos) is activated by Grb2. It does this by binding to the SH3 domains. Ras-GEF activates Ras by exchanging GDP for GTP which activates downstream signals.
What does Ras do?
Ras and Rho relay signals from cell-surface receptors.
How do GTP and GDP influence the action of Ras and other GTP-binding proteins?
Like other GTP-binding proteins Ras functions as a molecular switch active with GTP and inactive with GDP.
How is Grb2 activated?
It recognizes a specific phosphotyrosine which it binds to via SH2 domain
What does Grb2 do to Ras?
It recruits Ras-GEF (AKA Sos) which stimulates inactive Ras to become active Ras.
How are active GTPases inactivated?
GAPS or GTPase activating proteins stimulate hydrolysis of GTP to GDP to switch off GTP-binding proteins.
What do members of the Ras superfamily have in common?
The Ras and Rho family function from receptor tyrosine kinases
How does Ras relay a signal?
Active Ras activates Raf (MAP kinase kinase kinase)
Raf then phosphorylates Mek (MAP kinase kinase)
Mek then phosphorylates Erk (MAP kinase)
Erk then phosphorylates Protein X and Y which change protein activity and transcription factors which affect gene expression.
What does MAP stand for?
Mitogen-Activated Protein
How does PI3 Kinase get activated?
It binds to a phosphorylated tyrosine via an SH2 group
What happens when PI3K is activated?
Inositol phospholipid is activated by PI3K which provides it with a third phosphate group.
Phosphorylated inositol phospholipid binds to protein kinase 1 which then phosphorylates Akt
Protein kinase 2 then phosphorylates Akt which relays the signal.
How do cytokine receptors alter gene transcription?
They use enzyme-coupled receptors which also associate with cytoplasmic tyrosine kinases.
How do enzyme-coupled receptors alter gene transcription?
In the abscence of a bound ligand the JAK kinases are poorly active.
When ligand/cytokine binds a conformational change takes place to being JAK kinases close together and induces cross phosphorylation
Cross phosphorylation induces conformational change that activates kinases to phosphorylate other tyrosine residues in cytosolic domain of the receptor forming docking sites for STAT transcription factors.
STAT translocates to nucleus and induces gene transcription.
How are STAT and JAK associated with changes in gene expression?
Phosphotyrosine residues on the receptor recruit STATs
Bound STAT is phosphorylated by JAK
Phosphorylated STATs dimerize
Dimerized STAT moves into the nucleus and binds DNA to activate target gene transcription
How do RTKs and GPCRs activate signalling?
RTKs and GPCRs activate overlapping signaling pathways
