Protein Synthesis and Secretion
Terms
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- Polypeptides are synthesized by _____?
- ribosomes
- Where are polypeptides bound for secretion synthesized?
- ribosomes ass. w/ RER
- What is transcription?
- DNA -> mRNA
- What is a codon?
- A sequence of 3 consecutive DNA bases: one amino acid
- What is translation?
- mRNA -> protein
- What does translation require?
- adaptor molecules (tRNA, ribosomes [ribosomal protein, rRNA])
- What are the subunits of the ribosome?
- Large (60s) and small (40s)
- What are they composed of
- rRNA and protein
- How many steps are involved in translation?
- 3
- To which end of the previous A.A. is the new A.A. attached to?
- carboxyl end
- During the elongation phase, an __________ molecule binds with the codon at the ___ site of the ribosome?
- amino-acyl tRNA; A
- What does this pairing result in?
- GTP hydolysis of the elongation factor
- What ensures that the correct A.A. is being added to the chain (a translational proofreading)?
- A delay in the time from when the amino-acyl tRNA binds and the amino acid is added to the chain allows for assurance of the correct anticodon.
- What inactivates elongation factors?
- phosphorylation by kinases
- Therefore, protein synthesis is indirectly regulated by ______
-
the kinases that inactivate elongation factors
1.insulin
2.IGF
3.angiotensin II
4.glutamate
5.AMP
6.intracellular free A.A.'s - What type of cells commonly have many free polysomes?
- erythroblasts; synth. lots of protein for internal use
- What type of cells have many ER-bound polysomes?
- cells that synthesize proteins for export
- What is the Signal Hypotesis?
- mRNA's of proteins destined for secretion have a sequence (approx. 20 A.A.'s) that directs the mRNA/ribosome complex to be translocated to the ER.
- What enzyme cleaves the signal sequence?
- signal peptidase
- What molecule recognizes the signal ER sequence?
- Signal-recognition particle (SRP)
- Why does translation pause after SRP binding to the signal sequence?
- To allow time for travel to ER membrane
- What is the complex that allows the polypeptide chain to be injected to the lumen of the ER?
- Sec61
- Once in the ER, what protein modifications can take place?
-
1. Signal Sequence cleaved by signal peptidase
2. Disulfide bridge formation - cysteines bond. Maintains s(x) integrity of folded proteins
3.N-Linked Glycosylation - How does N-linked glycosylation occur?
- a precursor oligosaccharide is attached to Asn on the polypeptide chain.
- What catalyzes this reaction?
- Oligosaccharyl transferase
- What is the purpose of this transfer?
- Marks the state of protein folding. Chaperones will bind to incompletely folded proteins, prevents bad folding.
- What happens if the protein in the ER folds improperly?
- Translocated back into cytosol (via Sec61) for proteosomal proteolysis (death!)
- What post-translational modifications happen at the Golgi?
-
1.Removal of precursor oligosaccharide
2.Addition of sugars
3.Proteolytic processing of pro-proteins
4.Addition of glycosaminoglycan (GAG) to proteoglycans - Where does protein sorting take place?
- Trans golgi network
- What are the two modes of secretion for proteins intended to be exported?
-
Constitutive - not a response to signal; move straight to membrane
Regulated - restrained until a signal arrives (hormone, etc.) - What do lysosomal proteins bind to before leaving the golgi?
- mannose-6-phosphate receptors in trans-golgi network