Eukaryotic Translation Flashcards
ribosome structure
total 80S
large 60S subunit - 28S:5.8S + 5S + 50 proteins
small 40S subunits - 18S + 33 proteins
ribotoxins ricin and a-sarcin
act on conserved step-loop in 28S rRNA#bind aa-tRNA and inhibit protein synthesis
diptheria toxin
catalytic on EF-2
toxin transfers ADP ribone from NAD to imidazole ring so inhibits translocation and translation
initiator and its properties
tRNAi^(met)
not formylated
to insert internal methionines
pre-bound to 40S so binding is not codon-directed
RBS
only in prokaryotes
so may skip first AUG which is useful if leader sequence - goes to diff locations like 1st sequence to mitochondria and 2nd elsewhere
initiation site
Kozak sequence
initiation of translation
cap binding complex eIF-4F
(eIF-4E, eIF-4A, eIF-4G
1) eIF-4E recognise and bind 5- cap on mRNA
eIF-4A (ATP-dependent RNA helicase) unwinds 2ndary structure near 5’, needed for scanning of 40S along mRNA
eIF-4G (scaffold subunit) links initiation complex, interacts with poly-A binding protein so mRNA forms circle (cleavage by protease results in inhibition of initiation)
2) Met bound to P site not A site, scans for start codon which goes to A site
3) polysomes (cluster of ribosomes held by mRNA) are observed circular
poly-A tail stimulates rate of translation because can bind keep going round circle
poly-A binds PAB1 (poly A binding protein) which interacts with eIF4G and eIF4E on cap so polyA is close to the cap (and translate go round in circle)
rapid recycling of ribosomal subunits in initiation
increases efficiency
dissociate into subunits after translation so find m7G cap and initiate again
IRES
internal ribosome entry sites
alternative method of initiation when 5’ cap is missing
complicated tertiary structure
binds 40S close to AUG codon
diagram lecture 7 page 3
regulation of cell cycle during translation
G2/M division phase 75% protein synthesis shut down
eIF-4E dephosphorylated so shuts down cap associated translation
IRES is unaffected because don’t need cap so relative translation rate increases
apoptosis - eIF-4G cleaved by caspase 3 so all translation stops
picornavirus protein synthesis
shuts off 90% of host translation
uses IRES itself so cell cycle independent and then max competition with host because both only using IRES
termination
release factor mimics the AA acceptor stem of tRNA (CCA terminus) and has the same L shape
Gly Gly Gln binds water instead of AA so water hydrolyses ester bond in peptidyl tRNA and releases polypeptide
translation control mechanisms unique to eukaryotes
reversible binding of repressor protein to response element in 5’ UTR
differential stability of mRNA
reversible binding of repressor protein to response element in 5’ UTR
when excess Fe (iron), ferritin binds and prevents toxic levels
if Fe is limiting ferretin competes for it with iron-requiring enzymes
transferring receptor also regulated (Fe uptake into cells)
iron response elements - iron starvation means transferrin open and no ferritin, excess means ferritin is made and receptor shuts
differential stability of mRNA
poly-A tail shortening (<30 As) means binding protein is lost so 3’ end can’t associate with cap so decapping and degradation
cleavage at endonuclease cleavage site in 3’ UTR also leads to decapping and degradation
