L1 : PTMs in Signalling and Metabolism Flashcards
What are the fates of newly translated proteins?
- Translocation
- PTMs
- Activation
- Degradation
What are the main functions of PTMs in cells?
- Creation of docking sites in signalling complexes
- Activation/inhibition of enzymatic activities
- Modification of subcellular localisation
- Targeting for degradation
Main types of PTMs?
Phosphorylation
Acetylation
Methylation
Ubiquitination
CoAlation
Must name and describe how each PTM acts in answers
Explain how protein phosphorylation occurs
Kinases directly transfer the gamma phosphate from ATP onto specific AAs (usually Ser/Thr, Tyr)
ATP binding pocket
Substrate binding pocket holds target protein in place
How does kinase activation lead to signal transduction?
Ligand binding triggers dimerisation, causing conformational changes and activating kinases
Transphosphorylation on specific Tyr residues, serve as docking sites for signalling proteins
Proteins with SH2 or PTB domains bind pTyr, dictated by consensus sequence flanking residue
Docked proteins link kinase to downstream pathways (eg. GRB2 links to ERK)
How is kinase c-Src regulated by phosphorylation?
Phospho-Tyr-530 acts as an intramolecular docking site for Src SH2 domain, causing closed, inactive conformation
Full enzyme activity achieved by phosphorylation at Y419
Example of how PTMs can modulate subcellular localisation in FOXO?
FOXO (TF) regulates stress responses
Ubiquitination of FOXO targets for proteasomal degradation
Removal of ubiquitin allows it to regulate the stress response
What are the details of protein ubiquination?
Ubiquitin is highly stable 76aa protein
Conjugated to Lys residues by Ub ligases (E3)
How are plasma membrane proteins like EGFR degraded in lysosomes?
Degradation triggered by monoubiquitination or Lys63 polyubiquitination
Tagging signals internalisation and lysosomal targeting
What is PTEN and its role in PI3K signalling?
Tumour suppressor and lipid phosphatase
Acts on products of PI3K by dephosphorylating PIP3 to PIP2
Implicated in cell survival and proliferation
How do different mechanisms of ubiquitin binding affect PTEN?
Polyubiquitination -> proteasomal degradation
Monoubiqutination -> translocation to nucleus, functions to induce cell death (apoptosis) and prevent uncontrollable proliferation
How does ubiquitination regulate protein-protein interactions in TGF-beta pathway?
Ubiqutination of Smad4 by Smurf2 blocks its interaction with Smad2
Disrupts signal transduction
How does blocking ubiquitin-proteasome pathways act as an anti-cancer strategy?
IkB-alpha is normally ubiqutinated and degraded
When removed, free to bind to TFs, potentially reducing pro-survival gene expression in cancer cells
Bortezomib (Velcade) used in clinic to block IkBa degradation and treat multiple myeloma
How does acetylation occur?
KAT (Lys acetyltransferases) transfer acetate from acetyl CoA onto target substance
HDAC removes acetate
How does addition of acetyl to histones affect gene expression?
Acetylation causes repulsion of histones, releasing the DNA
Allows TFs to bind and initiate gene expression
How are proteins methylated?
Methylases and demethylases
Methyl group donated by S-adenosylmethionine (SAM)
Methyl does not introduce charge, mainly associated with inhibition or prevention of induction of gene expression (promotes tightly packed DNA)
How is gene expression regulated by acetylation and methylation?
Bromodomains bind to modified acetylated histones
Chromodomains bind to modified methylated histones
