Flashcards in Lecture 19 Deck (38)
What happens to a protein being translated once it is about 70 amino acids in length?
Since it is being translated in the cytosol and needs to be further processed in the rough ER, a cytosol ribonucleo protein blocks elongation of the protein. A signal recognition particle (SRP) then takes the protein being translated and associates it with an SRP receptor in the rough ER. Once bound to the receptor, elongation continues in the rough ER
What is the only codon with two tRNAs encoding it?
Methionine: One as an initiator (Met-tRNAF [prokaryotes] & Met-tRNAI [eukaryotes]) and one as an internal codon (Met-tRNAM)
How many Aminoacyl-tRNA synthetases are there?
One for each amino acid; 20
How is translation initiated in prokaryotes?
The 16S-rRNA from the 30S ribosomal subunit recognizes the Shine-Dalgarno initiation sequence (box). The large 50S ribosomal subunit then joins and allows translation to begin
How is translation initiated in eukaryotes?
1) Cap-dependent scanning (90-95%) - 5' of eukaryotic mRNA is capped. EIF4 is an initiation factor that binds to the cap. 40S ribosomal subunit binds to the initiation factor and moves along the 5' region of the mRNA until it encounters an AUG. The 60S ribosomal subunit then binds and begins translation. This means that you can only have a monocistronic message, since the first AUG will always be the first start codon to be recognized
2) Internal ribosome entry (5-10%) - Initiation sequence upstream of AUG codon allows direct binding of 40S ribosomal subunit to begin translation (monocistronic again)
Difference between initiation in prokaryotes and eukaryotes?
In prokaryotes there can be multiple Shine-Dalgarno sequences upstream of several AUG codons, allowing the mRNA to be polycistronic. In eukaryotes, there is only one 5' cap or one internal ribosomal entry site upstream of the AUG codon, allowing for mRNA to be only monocistronic
What does streptomycin do?
Streptomycin messes up base pairing between the 30S ribosomal subunit and the Shine Dalgarno sequence, disallowing translation to begin; Also causes miscoding during elongation
What are the steps to elongation of translation?
1) Amino acid to tRNA binding
2) Peptidyl transfer
What is puromycin?
Puromycin is an inhibitor because it imitates aminoacyl-tRNA, however does not have the same function. Because of this, the amino acid sequence must terminate translation
What is Diphtheria?
An upper respiratory tract illness caused by Corynebacterium diphtheriae. It is characterized by sore throat, low fever, and an adherent membrane (a pseudomembrane) on the tonsils, pharynx, and/or nasal cavity.
How is Diphtheria cured?
An antitoxin developed from horse serum. Some people may be sensitive to the serum and therefore must undergo a skin test. If the patient is not sensitive to the antitoxin, you can administer it along with antibiotics to help
What causes Diphtheria?
Diphtheria toxin is a protein that is cleaved into two fragments, A and B. Fragment B causes the toxin to be internalized. Fragment A poisons translation. One molecule is sufficient to kill a cell. Specifically, the toxin ADP ribosylates elongation factor 2 (eEF2) causing the transfer of ADP-ribose from NAD to a modified histidine residue (called diphthamide) and inactivation of eEF2.
What does erythromycin do?
Erythromycin inhibits EF2. This causes translocation inhibition
Where does ricin come from?
Castor beans are used to make castor oil, which is a laxative. However, in the process of making castor oil, the castor beans separate into two components that are not miscible: ricin and castor oil.
What does ricin do?
1) The A chain of ricin depurinates 28S rRNA at a specific A residue, which inactives that ribosome
2) The B chain binds a cell surface receptor for uptake
How are stop codons recognized?
There are no tRNAs that pair up with stop codons. Instead, they are recognized by release structures
How much energy does charging an amino acid cost?
How much energy does unwinding and scanning of mRNA cost?
How much energy does Met-tRNA binding cost?
How much energy does AA-tRNA binding cost?
How much energy does translocation cost?
How much energy does termination cost?
Unknown number of GTP
How many high energy bonds must be cleaved to polymerize an amino acid?
What are types of flexible translational control?
1) Specific - one or few mRNAs (micro RNAs)
2) General - all mRNAs (eIF2 phosphorylation)
3) Selective - classes of mRNAs (eIF4E activity)
How is the translation of the protein ferritin modulated?
Iron response element (IRE) binds IRE-binding protein (IRE-BP), except when Fe2+ levels are high. Therefore, Ferritin needs to be synthesized when Fe2+ in cells is high in concentration because it is able to spherically pack excess iron
What is hereditary hyperferritinemia - cataract syndrome (HHCS)?
IRE mutations in ferritin mRNA cannot bind IRE-BP. Therefore, ferritin synthesis is de-repressed causing it to be released into the serum. Cataracts forms as a result. This is an example of specific translational control
What happens in the down-regulation of the supply of initiator Met-tRNAi via eIF2 kinases?
eIF2 is the factor that transports initiator tRNA to the 40S ribosomal subunit for initiation. It does so through a complex with GTP that goes to the ribosome and deposits the initiator tRNA. If eIF2 is phosphorylated, it is inhibited from assisting in initiation because eIF2B becomes trapped. eIF2B is a protein that replaces GDP with GTP for eIF2 to complete its function of transporting tRNA
What are the four types of eIF2 kinases that inhibit translation?
1) HRI - reticulocytes minus heme
2) PKR - interferon plus virus-infection (dsRNA)
3) PERK - ER stress
4) GCN2 - amino acid starvation
They are all induced by some kind of stress
What causes vanishing white matter (VWM)?
A mutation in eIF2B, which reduces its functionality
What is vanishing white matter (VWM)?
An autosomal recessive neurological deterioration exacerbated by head trauma. It can cause ataxia (Uncoordinated movement is due to a muscle control problem that causes an inability to coordinate movements) and ovarian failure
What is eIF4E?
All eukaryotic cellular mRNAs are blocked at their 5-prime ends with the 7-methyl-guanosine cap structure. This structure is involved in several cellular processes including enhanced translational efficiency, splicing, mRNA stability, and RNA nuclear export. eIF4E is a eukaryotic translation initiation factor involved in directing ribosomes to the cap structure of mRNAs. It requires eIF4G to create a complex that will bind the cap to the mRNA
How is the function of eIF4E increased?
Phosphorylation of 4E-BP or eIF4E
What does Tetracycline do?
Targets 30S ribosomal subunit to inhibit binding of AA-tRNA to A-site
What does Chloramphenicol do?
Targets 50S ribosomal subunit to inhibit peptidyl transferase
What does Erythromycin do?
Targets 50S ribosomal subunit to inhibit translocation
What does Puromycin do?
Targets 50S and 60S ribosomal subunits to cause premature release of nascent polypeptide
What does Diphtheria toxin do?
Targets 60S ribosomal subunit to inhibit binding of AA-tRNA to A-site