Signalling Pathways in Translation Regulation Flashcards
How is global upregulation effected?
Global regulation is unspecific, upregulating all translation that is currently occurring by targeting the initiation factors for stimulation or repression. This is especially significant given that this is a major rate limiting step in translation.
What are 5’ UTRs important?
The regulatory elements are often contained within the 5’UTR of the mRNA, hence why oncogene mRNAs often have giant 2-3kb long UTRs allowing for another layer of tight regulation post-transcription.
How can the mRNA sequence effect or be used to effect its regulation?
- Capping/decapping
- 5’UTR length and secondary structure
- IRES structure
- Short ‘uORF’s in front of the main ORF.
- Polyadenylation and removal thereof
- Binding sites for trans-regulatory factors (proteins, miRNA)
What are uORFs?
The short upstream open reading frames (uORFs) are caused by AUGs not associated with a Kodak sequence.
What is the primary purpose of the Cap and PolyA tail?
The cap and the polyA tail protect the mRNA from being damages, particularly due to their association with the protective proteins Cap Binding Complex and PAB1.
What is the main way of initiating mRNA degradation?
Degradation is usually initiated by the shortening of the PolyA tail.
This is not just for regulation, the mRNA must be degraded when it is no longer needed.
Why does shortening the PolyA tail lead to degradation?
This can lead to the protective enzymes of the polyA tail being replaces by 3’-5’ exonucleases such as POP2, CCR4 and NOT proteins. It is also the first step in decapping.
How does decapping lead to degradation?
The protective cap binding complex is replaced by a complex composed of LSM 1-7 (a kind of helicase), Decapping enzymes 1 and 2 (DCP1/2) and XRN1, a 5’-3’ exonuclease that degrades the RNA.
What is proofreading?
This is the simple matter that ternary complexes that do not have the perfect codon will likely dissociate out of the A site without hydrolysing GTP, so that amino acid is not added.
This requires translation to be a reasonable slow process, or the GTP hydrolysis may occur regardless. This is where there is a balance between speed and accuracy in translation.
What is Nonsense Mediated mRNA Decay?
When an mRNA has a premature stop codon or is the consequence is an incomplete protein, so the mRNA is targeted for degradation during its translation.
What is the mechanism of Nonsense Mediated mRNA Decay?
Premature stop codons (non-sense mutations) are recognised when not all of the exon junction bound proteins have been removed after translation ceases. These complexes are targeted by Upf proteins that activate decapping enzymes, allowing cytosolic exonucleases to degrade the mRNA.
What is Non-Stop Mediated mRNA decay?
If a mutation or error in transcription has led to the lack of a stop codon then this leads to the polyA tail being translated into a long chain of lysine residues.
What is the mechanism of Non-Stop Mediated mRNA decay?
Lack of a stop codon (non-stop mutation) is noticed by the binding of Ski7 to the polylysine tail, which interacts with the exosome – an RNA degrading protein complex. It also stimulates Lys specific proteases to break down the erroneous peptide.
What are miRNAs?
Micro RNAs (miRNAs) inhibit translation or reduce the mRNA level through degradation targeting or by directing the mRNA to processing bodies for storage.
The miRNA often resides within a larger RNA-Induced Silencing Complex (RISC).
How are miRNAs produced?
From an ss pri-miRNA transcript that folds to form an imperfectly base paired hairpin loop, the tails on either end of which are cleaved off by an enzyme complex called DGCR8/drosha.
This is translocated to the cytoplasm via exportin5 and the loop end is cleaved off by Dicer/TRBP. The selected strand is then loaded into an Ago protein to form a RISC.
What are the effects of miRNAs?
Micro RNAs are thought to prevent translation by binding with the RISC to the 3’ end of the mRNA transcript and curling it around to that the RISC directly interacts with the ribosome(s) and inhibits their activity.
Micro RNAs are also involved in gene regulation at the level of heterochromatin structure, often in an upregulatory manner.
How many miRNAs do humans have?
There are thought to be over a thousand miRNAs encoded in our genome that are involved in regulating approximately one third of human genes.
Expression of different miRNAs is often tissue specific or specific to a particular developmental stage.
What are siRNAs?
Small Inhibitory RNAs (siRNAs) are RNA duplexes that target mRNA for degradation. They are also known as small interfering RNA and silencing RNA.
These are often artificial or produced by viruses, but can also be endogenous in which case they are bi-directionally transcribed into a long dsRNA that is cut into small strands by a dicer enzyme. The resulting particulates are a perfect antisense for the target mRNA.
What is mTOR?
The mammalian Target of Rapamycin (mTOR) is a phosphatidylinositol 3-kinase-related kinase protein. It is an incredibly important regulatory molecule for many processes including ribosomal biogenesis, amino acid transport and initiation of translation.
Why is mTOR so important?
It is so important because its activity is controlled by a huge number of variables, allowing it to act as a sensor for the level of various nutrients and metabolites as well as the energy state of the cell.
What is Rheb?
Much of the regulation of mTOR goes through Rheb, which stimulated mTOR when GTP bound and inhibits it when GDP bound.`
How does mTOR increase the translation rates?
Protein Biosynthesis Upregulation
Ribosome Biogenesis Stimulation
How does mTOR upregulate protein biosynthesis upregulation?
mTOR can phosphoactivate S6 Kinase (S6K) which phosphorylates the ribosomal protein S6 and so upregulates the translation process.
mTOR also phosphorylates eIF4E-BP1, lowering its affinity for the initiation factor eIF4E. 4E-BP1 competes with eIF4G for eIF4E but unlike G it has no effect, effectively repressing eIF4E. Hence phosphorylation of 4E-BP1 promotes translation by allowing more eIF4E/G complexes to form.
How does mTOR upregulate ribosome biogenesis?
By even further phosphorylation of 4E-BP1 and by an un-elucidated interaction with a protein called UBF, mTOR can stimulate the production of more ribosomes.
4E-BP1 stimulates transcription of 5’TOP mRNA, which encodes a wide variety of proteins involved in translation, both ribosomal proteins and translation factors. UBF stimulation increases transcription of rRNAs.