Translation Process Flashcards
Translation process
- initiation
- elongation
- termination
formation of a complex where the mRNA, ribosomes, and initiator tRNA form the translation machinery
initiation
forms the translation machinery
- mRNA
- ribosomes
- initiator tRNA
synthesis of peptide in a 3-step cycle that repeats each time an amino acid is added
elongation
cycle in elongation
3-step cycle
halting of translation upon encountering a stop codon
termination
- ribosome-binding site in prokaryotes
- consensus 5’ AGGAGGU 3’
- 3-10 nucleotides upstream of the initiation codon
- complementary to a region at the 3’ end of the 16S RNA
Shine Dalgarno sequence
consensus of Shine Dalgarno sequence
5’ AGGAGGU 3’
location of Shine Dalgarno sequence
3-10 nucleotides upstream of initiation codon
Shine Dalgarno sequence is complementary to where
3’ end of 16S RNA
- usually AUG
- sometimes GUG or UUG
start/initiation codons
other start codons in prokaryotes
- GUG
- UUG
Prokaryotic initiation factors
- IF-1
- IF-2
- IF-3
actively promotes dissociation of ribosomal subunits
IF-1
IF-1 function
promotes dissociation of ribosomal subunits
binds to 30S subunits to prevent re-association with 50S subunit
IF-3
IF-3 function
binds to 30S subunits to prevent re-association with 50S subunit
is a GTPase that directs the attachment of fmet-tRNA
IF-2
IF-2 function
GTPase that directs attachment of fmet-tRNA
- formulated methionine
- activated tRNA
fmet-tRNA
prokaryotic translation: elongation
- aminoacyl-tRNA binding at A site
- peptide bond formation
- translocation from A to P site and from P to E site
aminoacylation
EF-Tu with GTP
EF-Tu
elongation factor-thermo unstable
function of EF-Tu
brings aa-tRNA to ribosome during elongation
activates GDP to GTP
EF-Ts
enzyme involved in peptide bond formation
peptidyl transferase
translocation from A to P and P to E site
EF-G w/ GTP or translocase
what happens to GTP during translocation from A to P and P to E
cleaved and ribosome moves along mRNA
when any of 3 stop codons is reached
termination
creates UAG stop codon
amber mutation
where is amber mutation named after
Harris Bernstein (Bernstein = amber in German)
amber mutation
UAG
creates UAA stop codon
ochre mutation
ochre mutation
UAA
creates UGA stop codon
opal mutation
opal mutation
UGA
recognizes stop codons
release factor (protein)
different prokaryotic release factor
- RF-1
- RF-2
- RF-3
recognizes UAG and UAA
RF-1
recognizes UGA and UAA
RF-2
assists RF1 or RF2
RF-3
what do RFs activate
hydrolysis of peptidyl chain from the tRNA
what are released from the ribosome once it reaches stop codon
polypeptide chain and tRNA
Prokaryotic translation inhibitors
- tetracycline
- streptomycin
- erythromycin
- chloramphenicol
- puromycin
inhibits binding of tRNAs to ribosome
tetracycline
- changes shape of 30s and interferes with normal codon-anticodon pairing
- misreading
streptomycin
binds 50S and prevents translocation
erythromycin
binds 50S and inhibits peptidyl transferase activity
chloramphenicol
- resembles 3’ end of aa-tRNA
- premature chain termination
puromycin
prokaryotic:
binding to ribosome subunits
- IF1
- IF3
eukaryotic:
binding to ribosome subunits
- eIF3
- eIF4C
- eIF6
prokaryotic:
- binding to mRNA
- initiator tRNA delivery
- displacement of other factors
IF2
eukaryotic:
- binding to mRNA
- eIF4B
- eIF4F
eukaryotic:
- initiator tRNA delivery
- eIF2
- eIF2B
eukaryotic:
- displacement of other factors
eIF5
difference between prokaryote and eukaryote initiation based on where ribosome is located
pro: directly on start codon
eu: indirectly locate start codon
facilitates prokaryotic initiation
Shine-Dalgarno sequence
absent in eukaryotes (initiation)
RBS (ribosome binding site)
what mechanism is involved in eukaryotic initiation
scanning mechanism starting at 5’ cap
- formation of pre-initiation complex
- pre-initiation complex binds to 5’ cap
- pre-init complex scans mRNA until it locates AUG
- start codon is usually contained in Kozak consensus
- once pre-init complex is properly positioned, 60S binds to form 80S initiation complex
The Scanning Model
pre-initiation complex
40S + met-tRNA + eIF2 + GTP
assists the binding of pre-init complex to 5’cap
- eIF3 and
- cap binding complex
cap binding complex
eIF4F
eIF4F
eIF4A + eIF4E + eIF4G
where is the start codon usually contained in
Kozak consensus
Kozak consensus
5’ ACCAUGG 3’
The Scanning Model
- formation of pre-initiation complex
- pre-initiation complex binds to 5’ cap
- pre-init complex scans mRNA until it locates AUG. Start codon is usually contained in Kozak consensus
- once pre-init complex is properly positioned, 60S binds to form 80S initiation complex
binds to form the 80S initiation complex
60S
what happens after 60S binds and forms 80S
- GTP hydrolysis
- release of eIFs
eukaryotic translation:
elongation
- eEF-1α
- eEF-1βγ
- eEF-2
- no E site in ribosome
eEF-1α : __ (prokaryotes)
EF-Tu
eEF-1βγ : __ (prokaryotes)
EF-Ts
eEF-2 : __ (prokaryotes)
EF-G
eukaryote:
- recognizes all 3 stop codons
- requires ATP
eRF1
function of eRF1
recognizes all 3 stop codons
- stimulates release of eRF1 from the ribosome after termination
- assists eRF1
eRF2
function of eRF2
stimulates release of eRF1 from ribosome after termination
Accuracy of Protein Synthesis
- charging tRNA with correct aa
- codon-anticodon
- specific contacts between tRNA, mRNA, and rRNA within A site
- proof-reading by rejecting an incorrect aminoacyl-tRNA before it can donate its aa
what is involved in protein targeting in eukaryotes
- signal peptide
- signal recognition particle (SRP)
found in the N-terminal of the nascent protein
signal peptide
immediate product of translation that is inactive
nascent protein
recognizes signal peptide
signal recognition particle (SRP)
what does the SRP do
- recognize signal peptide
- binds to ribosome to arrest translation
where does the SRP with arrested ribosome binds to
SRP receptor
where is the SRP receptor found
Cytosolic face of rough ER membrane
where the ribosome attaches to trigger the release of SRP
ribosome receptor protein
what happens when ribosome attaches to ribosome receptor protein
release of SRP for re-use
once, SRP is released, what happens to the ribosome
continues translation and nascent protein is pushed through rough ER’s lumen
what happens as the nascent protein passes through the rough ER’s lumen
signal peptidase cleaves off signal peptide
what happens to the protein inside the rough ER’s lumen
glycosylated
control the final location of the protein
pattern of glycosylation
Events in Protein Targeting
- SRP recognizes singal peptide, binds to ribosome to arrest translation
- SRP with arrested ribosome binds to SRP receptor on cytosolic face of rough ER
- ribosome attaches to ribosome receptor protein, triggers release of SRP
- ribosome continues translation, nascent protein pushed through rough ER’s lumen
- as nascent passes through, signal peptidase cleaves off signal peptide
- protein is glycosylated to control its final location
Some Post-Translational Modifications
- glycosylation
- hydroxylation
- acetylation
- phophorylation
addition of -OH group like in the formation of 4-hydroxyproline in collagen
hydroxylation
addition of -COCH3 group such as in histone proteins
acetylation
addition of -PO43- group in tyrosine, serine, and threonine residues
phosphorylation
- reversible
- activation/inactivation of enzyme activity
- modulation of molecular interactions
- signaling
phosphorylation
- protein stability
- protection of N terminus
- regulation of protein-DNA interactions (histones)
acetylation
regulation of gene expression
methylation
- cellular localizatio and targeting signals
- membrane tethering
- mediator of protein-protein interactions
acylation, fatty acid modification
- excreted proteins
- cell-cell recognition/signaling O-GlcNAc
- reversible
- regulatory functions
glycosylation (N-linked, O-linked)
protein stability and protein-ligand interactions
hydroxyproline
modulator of protein-protein and receptor-ligand interactions
sulfation (sTyr)
- intra- and intermolecular crosslink
- protein stability
disulfide bond formation
- possible regulator of protein-ligand and protein-protein interactions
- common chemical artifact
deamidation
- protein stability
- blocked N terminus
pyroglutamic acid
- destruction signal
- after tryptic digestion
- site is modifed with Gly-Gly dipeptide
ubiquitination
oxidative damage during inflammation
nitration of tyrosine