Protein Targeting Flashcards
How do proteins get made and then targeted in Eukaryotes?
mRNA made in nucleus, translated in the cytosol, and if given an amino acid targeting sequence -are taken to their targets.
What is so special about nuclear protein transport?
bidirectional
How do Amino Acid sequences enter the nucleus?
The soluble protein importin acts as a chaperone to AAs that have an NLS, nuclear localization sequence.
How do Amino Acid sequences exit the nucleus?
The soluble protein exportin acts as a chaperone to AAs that have an NES, nuclear exit sequence.
What are the steps for a AA to pass through the nuclear membrane?
importin/exportin docks at the nucleoporin, Ran protein needs GTP to move the protein through the nuclear pore, after reaching the other side…importin/exportin grabs another AA sequence
Reactive Oxygen Species
molecules with unpaired electrons that can cause a domino effect of instability on a cell.
Describe how peroxisomal proteins are targeted
They are synthesized by free ribosomes, transported fully folded, tagged with persoxisome targeting signals (PTS 1, PTS2), they meet receptor proteins (Pex5p, Pex7p), bind to a docking/RING combination named Importomer
3 differences between proteins entering ER vs Nucleus or Peroxisome?
cotranslational, arrives unfolded, unidirectional
Describe how mRNA gets to the translocon of the ER
- mRNA meets a ribosome in the cytosol
- when the ER signal sequence is translated the SRP binds and changes shape in order to…
- bind to the SRP receptor that docks it to the translocon.
Describe what happens after docking at the translocon of the ER?
- GTP in both the SRP and its receptor change to GDP, causing them to leave the ER
- Protein folds as it enters the ER
- The signal sequence is removed by signal peptidase
Transmembrane domains
sequence of 30 predominantly hydrophobic amino acids that attach to the translocon and then the membrane
signal anchor sequence
Like an ER signal sequence but binds to the interior translocon channel and isn’t cleaved.
Describe membrane protein type 2
It has the N terminus in the cytosol and C in the lumen
4 posttranslational modifications once inserted into lumen
- signal sequence cleaved off
- N-oligosaccharyl transferase (OT) enzyme complex is attached
- Glycophosphatidylinositol (GPI) is added by a transamidase complex
- Disulfide bonds are created by protein disulfide isomerases (PDIs) to assist in protein folding
What does the attachment of the OT complex do?
attaches a core oligosaccharide to some polypeptides which is synthesized by dolichol phosphate. It is then flipped into the lumen.
What is a sequon?
What the OT recognizes. Has N.
What does GPI do?
anchors certain polypeptides to the ER membrane.
Chaperonins
heat-shock proteins that help fold nascent polypeptide chains
What is BIP?
an ER chaperonin that uses ATP hydrolysis to detach and reattach folding polypeptides
What is reverse translocation?
a misfolded polypeptide is pushed back into the cytosol
How do cystolic proteins become targeted to the nucleus?
mitochondrial signal sequences/ transit peptides - chloroplasts
What energy is needed for proteins to enter mitochondria or chloroplasts?
ATP
What does Hsp70 do?
clings to proteins before entering mito or chloro to keep proteins unfolded
TOM, TIM
translocase of the outer/inner membrane - recognizes signal sequence in mito
Where are all proteins synthesized?
cytosol
Explain hydrophobicity scales
+ hydrophobic - hydrophilic
What does protein disulfide isomerase do?
rearranges disulfide bonds
Whats special about amino acid 48?
If it gets ubiquinated it gets degraded.
ERAD
ER assisted degradation
