RNA surveillance and Translation I (1)

What is mRNA surveillance?

The recognition of improperly processed pre-mRNAs and their earmarking for degradation

Nonsense-mediated decay (NMD)

Degradation of mRNAs that stop translating (stop codon) before the last splice junction

Errors in mRNA (2)

1. Error in splicing

2. Mutations (deamination, etc)

Exon-exon junction proteins

EJC or exon joining complex

How is a premature stop codon identified?

By ribosomes being unable to displace UPF3

What happens when UPF3 remains associated?

Formation of a complex that targets the mRNA for degradation: P-bodies

3 RNAs of translation

1. The rRNA

2. The tRNA

3. The mRNA

Amount of rRNA

80% of all cellular RNA

Arrangement of pre-rRNA

Repetitive clusters

What RNA Pol makes rRNA?

Pol III

Bacterial ribosome composition

50S+30S=70S

Eukaryotic ribosome composition

60S+40S=80S

Highly conserved secondary structure of rRNA

Stem-loop structures = SIMILAR 16S and 18S small subunits

Position of the anticodon

Bottom of the tRNA

Amicoacyl-tRNA synthetases

1. Carry the AA

2. Recognize and bind to cognate tRNA

Rule ab aminoacyl tRNA synthetases

Can bind to more than 1 tRNA

Degenerate code

Different codons can code for the same AA

The tRNAi Met

+methionine = INITIATION

The tRNAi Met in bacteria

Needs to be modified by formyl (CHO) group before +methionine = initiation

The tRNA Met

+methionine = ELONGATION

Pre-initiation complex

40S subunit+ elF2 ternary complex+ elF5 (GTPase)

elF2 ternary complex

= elF2-GTP + tRNAi Met

How can protein synthesis be negatively regulated?

By phosphorylation of elF2 bc it won’t be able to bind ATP= energetically taxing

Why is the 5’ cap required for efficient translational initiation?

Bc the dimeric guanylyltransferase binds to P-CTD

2nd step of pre-initiation

elF4E binds to 5’ cap of mRNA

How is elF4 activity a limiting factor?

It’s under tight regulation bc its overexpression leads to tumour formation

3rd step (2)

elF4G…

1. Bind elF3 from preinitiation complex

2. Bind PABPC (3’ end)

How do 5’-3’ interactions favour re-intiation?

Protein:Protein interactions (elF3-elF4)

4th step of pre-initiation

elF4A-RNA helicase complex removes secondary structures in 5’ region→3’

What happens to elF4E?

It stays bound to the 5’ cap resulting in the formation of a loop

5th step of pre-initiation

AUG codon is recognized=

1. elF2-GTP hydrolysis

2. Large subunit joining

Once translation is initiated…

The ribosomal complex is irreversibly bound until termination of translation