Eukaryotic Protein Translation Flashcards
What is the kozac consensus sequence ?
includes the AUG (start codon) sequence to initiate translation in eukaryotes
Major stages of protein translation
- Load amino acids into tRNAs
- Initiation
- Elongation
- Termination
Key design principles of Translation (5)
- Load ribosomes onto correct start site of mRNA
- Prevent ribosomes from starting before it’s fully assembled
- Ribosome moves along mRNA to continue translation
- Ensure all components are fully removed when translation is finished
- Similar shaped components (tRNAs, elongation factors, termination factors fit into A-site)
How does eukaryotic translation differ from prokaryotes ? (4)
- Transcription and translation is DISCONTINUOUS (different times)
- mRNAs are more stable (days/ weeks)
- Cap-dependent and cap-independent
- NINE initiations factors: e1F, 2, 3, 4A, 4B, 4C, 4D, 5 and 6
List the 4 complexes of initiation + their purpose
- 43S pre-initiation complex (PIC): assembled to accept mRNA
- E1F4-mRNA complex: ensures mRNA is positioned properly for 43S PIC
- 48S initiation complex: 40S ribosome binds mRNA and scans for start codon (AUG)
- 80S elongation complex: brings in 60S to start translation
Describe how the 43S PIC is assembled
43S Pre-initiation Complex:
- 40S ribosomal unit
- elF1 (E site) and 1A (A site) = opens conformation
- elF2-GTP delivers Met-tRNA to P site
- elF3: gives structure, recruits proteins, and shields entry/exit
- elF5: GTPase activating protein hydrolyses eIF2-GTP once eIF1 leaves
T or F: the same Met-tRNA is used for both initiation and elongation
FALSE; Initiator Met-tRNA and elongation Met-tRNA are different
In translation, 5’ end of mRNA is linked to methylated guanosine via __ linkage
In translation, 5’ end of mRNA is linked to methylated guanosine via 5’—5’ triphosphate linkage
What is the purpose of the 5’ cap on mRNA ?
a). To regulate nuclear export of mRNAs
b). To inhibit degradation of mRNA by exonucleases
c). To assist in the assembly of an mRNA-ribosome complex during translation initiation
d). All of the above
What is the purpose of the 5’ cap on mRNA ?
a). To regulate nuclear export of mRNAs
b). To inhibit degradation of mRNA by exonucleases
c). To assist in the assembly of an mRNA-ribosome complex during translation initiation
d). All of the above
Review 5’ capping of mRNA
- 5’ triphosphate end of mRNA hydrolyzed = 5’- di-phosphate
- Guanosine monophosphate attaches to 5’- diphosphate
- Guanine is methylated
Describe components + purpose of eIF4 in mRNA complex
- eIF4A: RNA helicase
- eIF4B enhances eIF4A
- eIFE: binds 5’-cap of mRNA
- eIF4G: scaffold that binds PABP = circular structure of mRNA and later eIF3 (of 43S PIC)
PABP = polyA binding protein
How does eIF4-mRNA complex and 43S Complex come together ?
- (eIF4G-mRNA complex) eIF4G interacts with own eIF4E and eIF3 (43S Complex)
EIF4E = binds 5’-cap of mRNA
How does Ribosome recognize AUG codon ?
- 43S complex (eIF3) binds eIF4-mRNA complex at 5’-cap (eIF4E) and eIF4G
- moves from 5’ to 3’ scanning the 5’-UTR for AUG start codon
- 48S Initiation Complex = start codon identified when anti-codon of initiation tRNA (Met) base pairs with correct AUG codon
What subtle change happens when the correct AUG is bound ?
- eIF5B mediates joining of 60S and 40S subunits
- eIF5B hydrolyzes eIF2-GTP = 40S-bound factors dissociates
- eIF1A is removed with EIF5B = 80S Elongation Complex forms
How do Type IV viruses fool translation machinery into translating its own RNA ?
- remove 5’-cap of host mRNA to free up ribosomes
- virus RNA resembles tRNA composition; doesn’t need methylated tRNA to bind P site
- virus RNA shape forces open ribosome APE site
