Protein synthesis, modification and trafficking Flashcards
central dogma explain (basic protein synthesis
dna to mrna attach to ribosome then protein synthesis
why the need for proteins?
cell growth,proliferation,survival
diff between prokaryotic and eukaryotic
Prokaryotic : polycistronic - multiple genes, unstable, translation can occur on nascent transcript
eukaryotic: monocistronic,capped and polyadenylated, 5’amd 3’ untranslated region
structure of cap
cap found at 5’ end, sealls end of mRNA to protect it from nuclease digestion
landing site for cap binding protein
polya tail at 3’ end
protects from enzymatic degradation,
aids in transcription termination, export of mrna from nucleus and translation
general characteristics of tRNA
single stranded
gc rich can fold backwards on itself
helps decode mrna into protein
function at specific sites in ribosome
80s ribosome
consist of large and small subunits each which contain 50% protein and 50% rRNAs by mass
small and large subunits bind together during initation-60s,40s - constanly breaking and remerging =equilibrium
peptydl transferase activity associated with large 60s sub unit
three binding sites for tRNA : E P A
three stages of cap-dependant protein synthesis
Initiation
Elongation
Termination
main points of initation
rate limitng step - tightly regulated
more than ten soluble eukaryotic iniation factors eIF
aim to bring mRNA to ribosome and initator tRNA to AUG start codon
detail of initation
1.Assembly of 43s pre-iniation complex
association met-RNA with eIF2/GTP
40S trapped as a monomer by eIF3(43S)
binding of complexes to form 43s pre-iniation complex
2.Binding of mRNA to the 43S complex
eif43 bind to CAP on mRNA
eif4G binds to eiF4e
eif4a unwinds mrna secondary structure
43s preinitation complexx binds to eif4g via eif3 - 48s preiniation complex formed
scanning
3.Assembly of 80S iniation complex
the 40S/43S preiniation complex scans AUG codon
met tRNA occupies the P site on ribosome
60S subunit associated to form 80S complex
aka 60s binds to 43s.mRNA hydrolysis of gtp bound to eIF2
release most of factors
requires eIF5
structure of eIF2
three subunits
gtp binding site - on gamma
phosphorylation site - on alpha
kboxes - on beta - involved in eIF2B and eIF5 interaction
info on EIF4G
binds to eIF4A and Eif3
has multiple domains for interaction with other iniation factors
acts as scaffold that brings other iniation factors together
one example is poly(a) binding oriteun = circlisation
strong vs weak messages
Messages with many secondary structures near the 5’ end are weak messengers as they intefere with the attatchment of 43s pre complex
CAP-independent Translation
Need Internal ribosomal entry site (IRES)
have binding site for 43s complex does not need for cap recognition
Elongation and termination
iniator trna at start codon to right is unoccupied aminoacyl site, depending on codon correct amino acid is brought and catalysed by eEF1 requires GTP
2 amino acid bond formed by peptydyle transferase activity.
iniator tRNA is released and causes translocation
carrys on until aminoacyl site occupies stop codon
release factor recogises this and allows formation of last peptide bond and chain is released
What is a polysome
A polysome is a structure with multiple riboasomes attached to a single mRNA
