MBB 267 Week 11: Mitchel 10 Flashcards
What are the 3 methods that mRNA can be localised?
mechanisms include;
- random diffusion of the mRNA after it has been exported to the cytoplasm and interaction with an anchoring RNA-binding protein
- active transport of the mRNA along microtubules or actin filaments
- the differential distribution of factors within the cell that either promote degradation of the mRNA or protect the mRNA against degradation.
What are zip codes?
-What are zip code binding proteins?
The nucleotide sequences within the mRNAs that dictate the localization of the transcript are known as zip codes
-The proteins that bind to these sequences are known as zip code binding proteins. Zip code sequences are typically found in the 3’ UTR region of the mRNA.
What is the ASH1 transcript?
-How is ASH1 localised in the cell?
ASH1 (asymmetric silencing of HO) encodes a transcriptional repressor of the HO gene that mediates mating type switching. mRNA localization in the yeast S. cerevisiae is exemplified by the ASH1 transcript, which is specifically localised to the daughter cell during cell division. sh1 protein binds to an upstream repression sequence (URS) of the HO gene and recruits the Rpd3 histone deacetylase (HDAC) complex, causing hypoacetylation of neighbouring nucleosomes and a more condensed chromatin structure, leading to transcriptional repression
-ASH1 mRNA is localised during cell division to the daughter cells by activated transport along actin cables. Transport requires the zip code binding protein She2, the adaptor protein She3 and the motor protein Myo4.
Why does the cell sometimes down regulate translation?
Translation of most cellular mRNA is down-regulated in response to stress (e.g. nutrient deprivation, ER stress, heme deficiency, viral infection)
How does translation initiation start?
translation initiation involves the formation of a ternary complex between the initiation factor eIF2 (charged with GTP) and the initiator tRNA, tRNAiMet. This complex interacts with the small ribosomal subunit, which is then recruited to the 5’ end of the mRNA through interaction with the initiation factor eIF4F. The 48S preinitiation complex is then able to scan along the mRNA until it encounters an initiation codon within a specific sequence context, where upon eIF2 hydrolyses GTP and the large ribosomal subunit is recruited. In order for eIF2 to function in another round of translation initiation, the bound GDP is released by the guanine exchange factor eIF2B.
What happens to phosphorylated eIF2(alpha)?
-When is eIF2(alpha) phosphorylated?
eIF2 is a trimeric complex, containing the subunits a, b and g. The regulatory subunit eIF2a is a major target for phosphorylation by various protein kinases that are stimulated upon stress responses. Once phosphorylated, eIF2 binds to the GEF eIF2B but remains stably bound. As eIF2B levels are much lower than eIF2, the GEF is rapidly titrated out, ternary complex levels are depleted and translation initiation is blocked.
-Stress-activated protein kinases phosphorylate eIF2α.
What does the cytoplasmic cap binding complex comprise?
the cap-binding subunit eIF4E, the RNA helicase eIF4A and the molecular bridging factor eIF4G
How does mRNA circulise?
-What impact does it have on translation rate?
eIF4G interacts directly with eIF4E and with the poly(A) binding protein PABP, causing the mRNA to circularise.
-stimulates translation.
What are 4EBPs?
-What is their function?
eIF4E binding proteins
-compete with eIF4G for interaction with eIF4E and therefore disrupt the eIF4E/eIF4G interaction and block translation
What is Rapamycin?
Rapamycin is a potent immunosuppressor and growth inhibitor.
What is mTOR?
-What is mTOR’s function?
The mTOR (mechanistic target of rapamycin) pathway regulates translation in response to nutrient and stress signals. -mTOR promotes eIF4F assembly by phosphorylating (inhibiting) 4EBPs and (activating) ribosomal protein S6 kinases (S6Ks).
How do some proteins autoregulate their own expression?
mRNA and rRNA binding sites have similar structures. Binding to the mRNA blocks ribosome binding
What is ferritin?
-How is ferritin expression regulated?
mRNA encoding the iron-binding protein
-The mRNA contains a stem-loop in the 5’ leader known as the iron response element. This can be bound by iron regulatory proteins (IRPs). Binding of IRPs blocks scanning of 48S pre-ribosomes through the 5’ leader of the ferritin mRNA and impedes translation initiation. IRPs bind excess Fe2+, which blocks their ability to interact with the iron response element. Thus, at high concentrations of Fe2+ (when more ferritin is required) translation of the ferritin mRNA is not inhibited, while at low concentrations of Fe2+ (when more ferritin is not required), translation is blocked.
What are the functions of the 2 RRM domains in Sxl
Sex lethal (Sxl) is an RNA binding protein that contains two RRM domains. It is a translational supressor, as well as a splice site supressor
How does Sxl inhibit translation of its target mRNAs?
it must bind to both the 5’ leader sequence and to the 3’ UTR. Sxl binding impedes recruitment of the 48S preinitiation complex to the cap binding complex and blocks scanning of the preinitiation complex along the 5’ leader sequence
