Where does most polypeptide synthesis take place?
in the cytosol with transcripts leaving the nucleus and associating with free ribosomes
what is cotranslational import?
transfer of a growing polypeptide chain across the ER membrane as polypeptide synthesis proceeds
How does cotranslational import occur?
ribosomes become attached to the ER membranes early in translation, and polypeptide chains are transferred across the ER membrane as synthesis takes place
What is postranslational import?
uptake by organelles of completed polypeptide chains after they have been synthesized, mediated by specific targetting signals within the polypeptide
Where do the polypeptide chains go during postranslational import?
cytosol, mitochondira, cholorplast, peroxisomes or nuclear interior
What did the signal hypothesis model describe?
proposed that some sort of intrinstic molecular signal distinguishes polypeptides from the many polypeptides destined to be released into the cytosol
for polypeptides destined for the ER, the first segment of the polypeptide to by synthesized (n-terminus) contains an ER signal sequence that directs the ribosome mRNA polypeptide complex to the surface of the rough ER
What did Milstein et al determine about protein targeting and sorting?
found evidence that a 20 amino acid sequence at the N-terminus of the light chain of immunoglobulin G is a signal sequence
What are preproteins?
proteins containing sequences at the N-terminus to target the ER
What is the Signal Recognition Particle? (SRP)
a particle that reconizes and binds to the ER signal sequence of the newly forming polypeptide and then binds to the ER membrane
What does the SRP contain?
both protein and RNA
Describe the first 3 steps that occur
1. SRP binds to the signal sequence and blocks further translation
2. SRP binds the ribosome to the ER membrane with a translocon protein, the SRP binds SRP receptor with the help of GTP allowing ribosome to bind to ribosome receptor. This transfers the signal sequence to the pore protein
3. The central channels open as the signal sequence is inserted
describe the last 3 steps that occur
4. GTP is hydrolyzed and the SRP is released
5. the polypeptide elongates and passes into the ER lumen where signal peptidase cleaves the signal peptide
6. polypeptide is released into the lumen, ribosome detaches from the ER membrane and the subunits dissociate and release the mRNA
After Cotranslational Import in the ER lumen:
What are BiP proteins?
member of the Hsp 70 family of chaperones; present in the ER lumen, where it facilitates protein folding by reversibly binding to the hydrophobic regions of polypeptide chains
How do BiPs assist with protein folding in the ER lumen?
BiP binds to the hydrophobic regions of polypeptide chains and prevents aggregation of polypeptides with similar regions
releases the polypeptide chains, accompanied by ATP hydrolysis, giving the polypeptides a short opportunity to fold
the hydrophobid region is buried in the interior of the molecule
How do disulfide bonds form during folding of proteins?
facilitated by protein disulfide isomerase, enzyme present in the ER lumen that catalyzes the formation and breakage of disulfide bonds between cytesine residues
When proteins repeatedly fail to fold what helps control this?
unfolded protein response - use sensor molecules in the ER membrane to detect misfolded proteins, activate pathways that shut down the snthesis of most proteins while enhancing the production of those required for protein folding and degradation
ER-associated degradation (ERAD) - recognizes misfolded or unassembled proteins and exports or "retranslocates" then back across the ER membrane to the cytosol where they are degraded by proteasomes
What happens after proteins are folded in the ER lumen?
glycoproteins are sent to the Golgi complex for further glycosylation and processing of carbohydrate side chains
soluble proteins move from the Golgi to secretory vesicles for secretion from the cell
the proteins not destined for secretion have specific side chains that target them to destinations within the endomembrane system
proteins who return to the ER have a KDEL sequence
Describe the two main mechanisms postulated for the insertion of integral membrane proteins.
1. polypeptide with an internal stop-transfer sequence and a terminal ER signal sequence - allows an SRP to bind the ribosome-mRNA complex to the ER membrane, elongation of the polypeptide continues until the hydrophobic transmembrane domain is synthesized
2. polypeptide with onle a single, internal stop-transfer sequence - STP binds and targets to the ER membrane, hydrophobic region functions as a membrane anchor