lec 11 Flashcards
(21 cards)
in order to be exported from the nucleus
- mature mRNA needs to show proof of proper 5’ capping and 3’ poly-A tail and splicing
- cap binding complex at 5’ end
- poly-A binding proteins
- exon junction complexes
> Proteins left by spliceosome alter splicing
improperly processed mRNAs and other debris stay in the nucleus and are degraded by exonucleases
selective degradation of RNA
- prevention of export of incomplete/unprocessed RNA from the nucleus
- unexported RNA = degraded in the nucleus - prevention of translation of damaged/unwanted RNA in the cytosol
- untranslated RNA is degraded in the cytosol to avoid translating broken/defective mRNAs
nonsense-mediated decay
cause premature termination codons (PTCs)
- destroy protein structure and func
ensures mRNA transcripts with PTCs = degraded in the nucleus
- EJC = deposited at each splice junction
stop codons should always be downstream of EJCs
- splicing occurs in ORF
if stop codon = upstream of EJC
- will be detected by molecular machinery that will quickly degrade defective mRNA
req the help of ribosome
- only occurs in the cytosol
regulation of translation
preparing for translation: mRNA = circularized
control of translation at 5’ and 3’ UTRs
microRNA
preparing for translation
- export ready mRNA = transported through nuclear pore into the cytosol
- cap binding complex recruits eukaryotic initiation factors (eIFs) to 5’ cap
- CBC = released - eIFs bind to poly-A binding proteins
- joins 5’ and 3’ UTRS
> creates circular mRNA
= ready to be translated once eIFs recruit ribosomal subunits
5’ and 3’ UTRs regulate translation
- proteins that affect translation bind to UTRs
- cytosolic mRNA = circular
if translational inhibitor protein binds to 3’ UTR
- can inhibit translation
translation initiation at 5’ UTR
normal 5’ cap allows proper assembly of the translation initiation complex
- eIFs, initiator tRNA, ribosomal subunits, mRNA small subunit binds eIFs at 5’ cap
- after initiator tRNA = recruited
> complex slides along until it recognizes AUG
= large ribosomal subunit binds and translation starts
leaky scanning and kozak consensus seq
leaky scanning
- ribosome misses initial AUG and starts translating at a downstream AUG
kozak consensus seq
- bases around AUG that are important for translation initiation
- can result in shorter (less amino acids) at the N-terminus
ways to generate different proteins from the same gene
- leaky scanning
- alternative splicing
- alternative poly-A sites
miRNAs regulation
small noncoding RNA transcripts that can regulate translation
- via RNA interference
> interfere with the ability of the transcript to be translated
small pieces of mRNA that bind to complementary regions of mRNA in 3’ UTR and inhibit translation
first evolved as a defense against viruses and transposons (parasitic RNA)
RNA interference
miRNAs can inhibit the mRNA translation mechanism
- bind to mRNA
> block the binding to initiation factors/ribosomes
>recruit RNA-digesting enzymes (nucleases) to degrade RNA
> recruit protein-digesting enzymes (proteases) to digest nascent protein
co/post-translational regulation
- protein folding
- energy dynamics of protein conformation
> final form of any polypeptide chain is one that minimizes free energy
= driven by polar aqueous and nonpolar membrane phases
> aq phase
= polar side chains face out, but in (hydrophobic) membranes are hidden
importance of proper protein folding
- ex of improper protein misfolding
- beta globin mutation
- causes sickle cell anemia
> Missense mutation causes nonpolar amino acids to be exposed
= leads to aggregation - alzheimer’s
- Huntington’s disease
molecular chaperones help guide
folding of polypeptides into the most energetically favorable form during/after synthesis
molecular chaperone ex
- in cytosol
- heat shock proteins (HSP)
> HSP70: co translational
> HSP60: post translational - in ER
> Calnexin and Calreticulin: co-translational
HSP70 mechanism
binds to hydrophobic moieties as peptide is being translated in the cytosol during synthesis
HSP60 mechanism
aids in peptide folding after peptides have been fully translated in the cytosol
Calnexin and Calreticulin
ER chaperone proteins that retain incompletely folded proteins in the ER to give them a chance to properly fold
improperly folded proteins = exported from the ER, ubiquitylated, and degraded
misfolded protein
ubiquitylated
- sent to the proteasome for degradation
membrane insertion
have start- and stop- transfer seq
- get recognized by protein translocator
- can be internal or sometimes at N-terminus
> is cleaved
= must be at least a subsequent stop- transfer to cause protein to be embedded in the membrane
no stop- transfer seq
- protein in the ER lumen as soluble protein, not membrane embedded
folding and membrane insertion
in multi-pass transmembrane proteins
- polypeptide chain passes back and forth repeatedly across the lipid bilayer
- must have many start- and stop- transfer seq
