Phosphorylation Signalling Flashcards
(48 cards)
What cellular processes are facilitated by signalling pathways?
• Mitogenic signalling
o Membrane-nucleus signalling, eg hormones that affect transcription
• Sensing cellular stress
o Response to nutrients/ATP/AA/O2 levels
• Chromatin organisation and transcription control
• Cell Cycle control
• Sensing DNA Damage
What is the simplest kind of signalling pathway?
Two componenet signalling, where a single protein has a sensing domain that responds to stimulus and activates or inactivates a signalling domain that acts upon another protein with a reciever domain, causing the effector protein to output a response to the change in stimulus.
What kind of phosphorylation signalling do bacteria use that eukaryotes don’t?
Autophosphorylation of Histidine residues
What is the relative frequency of use of the different PTMs used in signalling?
Phosphorylation Acetylation Hydroxylation Methylation Ubiquitination SUMOylation
How much more common is phosphorylation than acetylation?
A full order of magnitude
Why is phosphorylation commonly used in signalling?
Adding a phosphate group to a protein can have a large effect on it, which is useful for causing large conformational changes or altering catalytic/binding ability.
Phosphate groups introduce a large steric change to the protein as well as being a large concentration of negative charge which can affect all the surrounding protein structure.
How many human kinases are there?
450
Which two phosphorylable residues are very similar?
Serine and threonine are very similarly structured amino acids, differing only by an extra carbon in the serine side chain before the hydroxyl group.
Which phosphorylable residue is dissimilar to the others?
Tyrosine, having a very different shape. As such, tyrosines are very rarely phosphorylated by the same kinases as serine and threonine (which often are done by the same unspecific kinase) but there are exceptions to this, notably in MAPKKs such as MEK1 and 2 which phosphorylate serine and tyrosine on the same target.
Which residue is phosphorylated least?
Tyrosine phosphorylation is much rarer than the others, accounting for 1 in 2000 phosphorylation events. It does however play a key role in mitogenic signalling.
What are some examples of RTKs?
These include receptors for EGF2, ErbBs, IGF2 and insulin as well as HER2 receptors.
How do RTKs transmit the signal?
The RTK intracellular domains usually consist of two arms, each with kinase domains that phosphorylate each other (transphosphorylation) at multiple tyrosine residues.
These phosphate groups act as docking sites, allowing the binding and activation of various adaptor proteins that go on to activate signalling pathways.
Of the signalling pathways initiated by RTKs, why is STAT5 notable?
STAT5 is activated by the RTKs directly, and itself directly regulated the genes with no intermediate cascade.
What domains bind second messengers to the phosphorylated RTK arms?
SH2 or PTB.
What characterises PTB phosphotyrosine binding?
Not all members of the PTB domain family actually bind pY, their structure is more similar to that of the PH domains responsible for binding to phospholipids in the plasma membrane and this is in fact the role of many of them. The PTB domains that are pY-binding only bind pY in a specific sequence context: N-P-x-pY.
What are RTK receptor substrates?
Instead of only transphosphorylating the two arms, some RTKs such as the insulin and IGF-1 receptor recruit a substrate whose tyrosines are phosphorylated to recruit the adaptor proteins. Some cross-phosphorylation does still tend to occur.
What is the RTK receptor substrate for insulin and IGF-1 receptors?
IRS1/2 (insulin receptor substrate)
What is the RTK receptor substrate for the fibroblast growth factor receptors?
FRS1/2
What are HER3 RTKs?
Also known as ErbB3 receptors, these are a member of the EGFR family. They are often found to be overactive in breast cancer, hence antibody drugs have been used to disable them.
What is the mechanism of HER3 signal transduction?
Upon binding the ligand HER3 must form heterodimers with other RTKs, often HER2 or EGFR (AKA ErbB1 or HER1), to compensate for its own lack of kinase ability. Because HER3 cannot phosphorylate itself it must recruit other receptors to do it for it, but when it is activated it creates a very potent signal that can have strong downstream effects.
What is the role of HER2?
HER2 is an active kinase receptor, but does not bind any known ligands so may only be present in order to function as a heterodimer with HER3.
What different effects can phosphorylation have on a protein?
Creation of protein docking sites - RTK signalling
Disruption of Complexes – E2F/pRb/HDAC
Enzyme Activation – MAP Kinase Pathways
Enzyme Inactivation – c-SRC and CSK
De/Stabilisation of Proteins – Myc Level
Modulation of Subcellular Location -FOXO
What is E2F involved in?
E2F is a transcription factor which causes progression from the G1 to S phase, at which cells make a commitment to divide. Its targets include DNA pol.
What represses E2F?
It is repressed by a tumour suppressor called pRb which binds E2F with a long C-terminal tail domain. It also binds a HDAC, which provides a level of regulation we will not look at.
