Translation Flashcards
Stop Codons
UGA, UAA, UAG
First Proofreading Step of Translation
Happens within the tRNA, there is editing and synthesis site. If wrong AA is incorporated into synthesis site, AA is incorporated into editing site
Formation of Pre-Initiation Complex
Step 1: ____ puts Met at the beginning of the protein, binds to ____, which binds to _____. This all binds to ____ subunit.
Cap of mRNA is bound to _____, which binds _____, which has interactions with _____, circularizing mRNA, making it a good substrate for other initiation factors to bind to 5’ end of mRNA.
Step 1: itRNA puts Met at the beginning of the protein, binds to eIF2, which binds to GTP. This all binds to small subunit.
Cap of mRNA is bound to eIF4E, which binds eIF4G, which has interactions with poly-a BPs, circularizing mRNA, making it a good substrate for other initiation factors to bind to 5’ end of mRNA.
EIF2
Binds to GTP and itRNA
Complex binds to small ribosomal subunit in pre-initiation complex
Delivers initiator tRNA
-bac counterpart is IF2
EIF4E
Binds cap of mRNA, which binds EIF4G
-no bacterial counterpart
EIF4G
Binds EIF4E and poly-A BPs, circularizing mRNA, making it a good substrate for other initiation factors to bind to 5’ end of mRNA
-no bacterial counterpart
What happens when small ribosomal subunit scans for AUG codon?
ATP is expended to allow for helicase activity
What happens when 60S large subunit joins?
EIF2 and other initiation factors dissociate
Hydrolysis of GTP, causing EIF2 release
Step 1 of Elongation
Delivery of aa-tRNA to _____ site and ____ site release
-____ is bound to GTP, and _____, which can basepair to mRNA via anticodon loop
Delivery of aa-tRNA to A side and E site release
-EEF1A is bound to GTP, and aminoacyl transferase, which can basepair to mRNA via anticodon loop
Step 2 of Elongation
GTP Hydrolysis and ____ release, which ensures fidelity, that the right tRNA is there, ribosome proofreads in A site, where codon/anticodon pairs are checked by ribosome conformation
GTP Hydrolysis and eEF1A release, which ensures fidelity, that the right tRNA is there, ribosome proofreads in A site, where codon/anticodon pairs are checked by ribosome conformation
Step 3 of Elongation
_____ promotes sliding of large subunit to next codon, with GTP attached
Catalyzes ______
EEF2 promotes sliding of large subunit to next codon, with GTP attached
Catalyzes TRANSLOCATION
Step 4 of elongation
GTP hydrolysis and ____ release, marking completion of cycle
GTP hydrolysis and eEF2 release, marking completion of cycle
EEF1A
Binds all canonical tRNAs except for iTRNA and selenocysteine tRNA
G protein
-bacterial counterpart is EFTu
EEF2
G protein that is required for ribosome translocation
-EFG is bacterial counterpart
Termination
ERF1 looks like a tRNA, binds A site, promotes release of completed polypeptide, ribosomal subunits dissociate
-NO TERMINATOR tRNA
Differences between bacterial and eukaryotic translation
- Transcription and translation are coupled in bac because there is no nuclear membrane
- mRNAs are polycistronic in bac
- No 5’ cap is necessary in bac
- In bac, shine-dalgarno sequence is upstream of start codon, which directly bps to ribosomal RNA
- Bac ribosomes are smaller and divergent from mammalian ribosomes to allow selective inhibition by antibiotics
- Ribosome inhibitors typically do have residual toxicity due to theit effects on mitochondrial ribosomes
Apo B editing
Intestinal (B-48) is shorter, C gets deaminated to U (enzymatically), creating premature stop codon
miRNA
Inhibit translation via usually binding to 3’ UTR
RISC complex is thought to physically impede translation initiation
-many variations of bping bw miRNA and mRNA make target identification difficult
IRE and IRP
- in low [Fe], IRP binds IRE and prevents formation of initiation complex, halting translation of ferritin (Fe storage protein)
- in high [Fe], IRP cannot bind IRE because IRP is bound to Fe, so Fe changes the protein’s conformation
Phosphorylation of eIF2 Leads To….
Translation inhibition in response to
- Low [AA] due to poor nutrition
- Cellular stress
- Immune response
- Unfolded proteins
eIF2 recycling requires…
eIF2B GEFs because affinity for GDP is much higher for than that for GTP
The Different Nutritional States and Regulation of eIF2
Normal conditions: GDP is exchanged for GTP via ______, causing active eIF2
Poor nutrition conditions: _____ phosphorylates eIF2, which sequesters all of eIF2B as an inactive complex, which _______.
Normal conditions: GDP is exchanged for GTP via eIF2b, causing active eIF2
Poor nutrition conditions: protein kinase phosphorylates eIF2, which sequesters all of eIF2B as an inactive complex, which slows protein synthesis dramatically
How Hypoxia Regulates Translation
mTOR signaling pathway represses translation in response to hypoxia by regulating funciton of eIF4E.
mTOR pathway is like a cellular rheostat, it up-regulates translation during growth and down-regulates during stress
mTOR is constitutively active
Normoxia: induces mTOR signaling
-mTOR is a kinase, it phosphorylates 4EBP, which keeps 4EBP from interfering with translation
Hypoxia: mTOR does not phosphorylate 4EBP, so it binds 4E, precluding translation
Selenocysteine Incorporation and it’s importance
Requires tRNA, SBP2, eEFSec, UGA codon, SECIS element in 3’UTR
- selenocysteine is encoded by UGA stop codon that has been re-coded to allow Sec-tRNA binding
- SBP2 protein is required for recoding process
- Some rare forms of hypothyroidism are caused by SBP2 mutations in humans because reduced Sec incorporation leads to reduced production of deiodinases
