RNA localisation and transcriptional control

Question 1

What are the different mechanisms of mRNA localisation?

Answer 1

-random diffusion and anchoring
-active transport (mediated along actin or microtubules)
-negative regulation: selective degradation

Question 2

What are zip-code binding proteins?

Answer 2

Proteins which bind to zip codes (nt seqs within the mRNAs that dictate the localisation of the transcript)
-zip codes typically in 3’ UTR of mRNA

Question 3

What happens in the mRNA localisation of ASH1 mRNA in* S.cerevisiae*?

Answer 3

-during cell division ASH1 is localised to daughter cell via myosin-mediated transport
-localisation depends on zip code binding protein She2, adaptor protein She3 and motor protein Myo4
-Ash1 is a transcriptional repressor which inhibits the gene HO being expressed, so that daughter cells maintain their mother’s mating type (while the mother cell expresses HO so can change mating type)

Question 4

How is translation stimulated?

Answer 4

-through circularisation of mRNA
-by eIF4G subunit of cap-binding complex interacting with PolyA binding proteins

Question 5

How is the stimulation of translation regulated by eiF4 binding proteins?

Answer 5

-eIF4 binding proteins compete with eIF4G for interaction with eIF4E, disrupting its interaction
-prevents circularisation of mRNA, blocking translation

Question 6

What is the mTOR complex?

Answer 6

mTOR = mechanistic target of rapamycin (a growth inhibitor, immunosuppressant)
complex involved in signal transduction systems which activates protein kinases
-phosphorylates to stimulate translation initiation
-sensitivity of pathway to mTOR varies

Question 7

How is mTOR regulated?

Answer 7

By diverse range of signalling pathways
Activated by Ras/MAPK, P13K, aas
Repressed by rapamycin, energy depletion

Question 8

By what mechanisms can translation be regulated at a global level?

Answer 8

-regulation of cap binding complex status via eIF4 binding proteins preventing circularisation
-phosphorylation of various inhibitors and promotors via the mTOR complex
-alteration of concentrations of initiation factor eIF2 via phosphorylation

Question 9

How is translation downregulated in response to stress?

Answer 9

-the active pool of eIF2α is decreased by eIF2α being phosphorylated so that it is unable to disassociate from its GEF so can not initiate translation

Question 10

By what mechanisms can translation be regulated at a specific-transcript level?

Answer 10

-autoregulation
-dosage compensation

Question 11

How is autoregulation used to control eukaryotic iron homeostasis?

Answer 11

Iron regulatory proteins detect Fe2+ availability and regulate ferretin translation accordingly
-when Fe levels too low, IRPs bind to mRNA, which blocks scanning so that ferretin can’t be transcribed
-when Fe levels too high, IRPs bind to Fe, so that ferretin transcription can occur and ferretin can absorb excess Fe

Question 12

How is sex-chromosome dosage compensation carried out in flies?

Answer 12

-male specific lethal complex (Msl-2) is inhibited in females by sex lethal protein (sxl), which blocks splicing of introns in the msl-2 transcript and represses its translation
-males have the Msl-2 which allows dosage compensation of chromosomes