24: Initiation and elongation of protein synthesis: comparing us with our resident bacteria Flashcards
Describe the role of IF-3
Binds to the small ribosomal subunit, keeping the two subunits apart
Describe the role of IF-1
Blocks the “A” site at the active centre, ensuring initiating tRNA is in the “P” site
Describe the role of IF-2
Bonds initiating tRNA delivering it to the ribosome (courier van)
What are the steps in the formation of the initiation complex
Draw this out (look through slides for image)
- Ribosomal subunits are kept apart (IF-3)
- A complex initiator tRNA (fMet-tRNA) GTP and IF-2 forms in the cytoplasm and binds to both small ribosomal subunit and mRNA
- The 3 factors are ejected -IF-2 with GTP hydrolysis
- With IF-3 gone, the large ribosomal subunits can find the small subunit again
- Resulting in the formation of the initiation complex
How many initiation factors do bacteria have?
Three (IF 1-3)
How many initiation factors do archaebacteria have?
The three that bacteria have (IF 1-3) plus 2 more
How many initiation factors do Eukaryotes have?
The core 3, archaebacteria 2 extra, and 7 more (12 in total)
Describe the stages for cap recognition (eukaryotes)
Look through slides for image
eIF = Eukaryotic Initiation Factor
- eIF4E binds to the m7G cap (also known as cap binding factor)
- This recruits the small ribosomal subunit (40S)
- Associated with two other factors eIF4G and eIF4A (helicase -unwinds secondary structure in mRNA for the ribosome)
Where do eukaryotic (human) ribosomes attach to the mRNA?
Ribosomes attach at the m7 capped end of the mRNA and then scan along looking for the start codon ‘AUG’
What is the ‘Kozak consensus sequence’?
In eukaryotic organisims, there is a recognition consensus sequence around the AUG marking the start of the coding sequence in human mRNA
gcc(a/g)ccAUGg
Not all AUGs have the exact same sequence -if it’s a ‘weak Kozak sequence’ the ribosome may instead skip to the 2nd AUG in the mRNA
Where do bacterial ribosome attach to the mRNA?
The ribosome binds directly at the start AUG (doesn’t have to scan along the mRNAs length)
What is the Shine-Dalgarno sequence?
Another consensus sequence. Where the bacterial ribosome finds the start AUG codon
Describe how cellular events can be regulated through eIF4
Look through slides for diagram
- Signals such as hormones, growth factors etc bind to the surface of the cell and typically activate kinases (which add a phosphate)
- eIF4E becomes phosphorylated and therefore more active, which allows it to bind the cap more efficiently
- eIF4 binding protein inhibits the activity of eIF4E
- However, when its phosphorylated, the eIF4E binding protein becomes inactive
Describe how eIF-2B can be inactivated
By phosphorylation at its alpha subunit
(look through notes for image)
- eIF-2 (the courier van) delivers the initiating tRNA to the ribosome -the GTP hydrolysis ejects it as an eIF2-GDP complex
- GDP has to be exchanged with GTP for it to deliver again. This is done in us by exchange factor eIF-2B
- If eIF-2B is phosphorylated (i.e. now eIF-2B-P) the factor gets stuck on eIF-2 and initiation of protein synthesis (and hence protein synthesis) stops
- This can be a response to a viral infection, resulting in programmed cell death (apoptosis) due to protein synthesis being stopped
What are the three ribosomal sites for tRNA at the active site of the ribosome
A, P and E
