lecture 9 Flashcards
prokaryotic ribosome is
70S (large subunit (50S) small subunit (30S))
eukaryotic ribosome is
80S (large subunit (60S) small subunit (40S))
A,P and E sites
tRNA comes in and slots into A (aminoacylated tRNA binding site) site
everything shifts due to the elongation factor and the peptide chain is handed over to the new tRNA with the new amino acid on the P site (polypeptide chain site)
and old tRNA moves to the E (exit) site
m7G cap and poly A are key for translation
Cap is not next to the start codon
cap binding to the small subunit of the ribosome is the start of translation
small subunit scans till the first AUG then it starts translation
sequence in front of AUG is the
kozak consensus - CC(A/G)CC
is improves translation initiation
once the small subunit has found the start codon the
large subunit comes in
eIF4F complex comes in which contains
eIF4E, G and A
eiF4F complex comes in and binds to the cap
activates mRNA
the poly A tail is bound by PABP (poly A binding protein) which comes round and binds to the
eIf4F complex
mRNA is circularised, supposed to help the ribosome come back to the cap
eIF4G is what links with the PABP
eIF4A is a ATPase and RNA helicase which opens up structure before the AUG to allows the small subunit to scan along so not blocking
small subunit is bound to eIF2 which is a GTPase which forms the preinitiation complex with tRNA
then preinitiation complex binds with activated mRNA
small subunit starts scanning along for AUG
then once recognised hydrolysis of eIF2 bound GTP and phosphate release (48S initiation complex)
eIF2 then leaves and then 60S subunit comes in with eIF5B (GTPase)
rest of initiation factors leave and we have a full ribosome
hydrolysis of eIF5B GTP and release of eI5FB
elongation
next tRNA comes in with eEF1A coupled with GTP
and tRNA docks in the A site
then theres peptide bond formation between
the two amino acids
eEF2 comes along and helps
translocation on the mRNA, allowing next tRNA to come in
termination
when it gets to stop codon eRF1 comes in and sits in the A site
peptide is release from tRNA and eRF1 is released and with eRF3 and other factors the 2 ribosome subunits are pulled apart and theyre are recycled
regulation of translation:
eIF2B
involved in bringing the initiation tRNA to the small subunit
eIF2 in the subunit and eIF2B in one of the proteins
eIF2B level governs the level of active eIF2-GTP and thus overall initiation rate
eIF2B down regulated in response to stresses such as viral infection, amino acid deprivation. therefore translation is shut down
eIF2B is also activated by insulin
inactivation of eIF2 by phosphorylation
phosphorylation is of eIF2B –> general translation blockade
we still need to produce certain proteins during stress
we get stress specific mRNAs
iron requlation
iron regulates the expression of iron storage/transport proteins
low iron
storage proteins -don’t need them so theyre repressed
IRP 1/2 binds to region in 5’ UTR leads to translational repression
transport proteins - upregulated
IRP 1/2 binds to region in the 3’ UTR causes mRNA stabilisation –> more RNA in the cell therefore more transport proteins
IRP =
iron responsive proteins
UTR - untranslated region
5’ UTR - between start codon and m7G cap
£’ UTR - between stop codon and poly A
high iron
need more storage proteins to keep iron levels down in the cell
iron binds to IRP1
IRP2 is degraded when its not bound
so no binding to the 5’ UTR and no repression of the storage proteins
and no binding to 3’ end so degradation of the transport proteins
IRP1is a bifunctional proteins as it is also
c-aconitase
when theres low iron levels it becomes IRP1 so it can bind to RNA to down regulate storage proteins and upregulate transport proteins
in the presence of iron is becomes c-aconitase and converts citrate to isocitrate in the mitochondria
