Protein Trafficking Flashcards
What is the difference between free ribosomes and rough ER ribosomes?
No difference
What proteins are made by free ribosomes?
Nuclear
Peroxisomal
Mitochondrial
What is the signal sequence for peroxisomes?
C-terminal sequence SKF (ser, lys, phe)
What is the signal sequence for the mitochondrial matrix?
N-terminal sequence rich in positive charged amino acids (lys, ser, thr)
What proteins are made in the rough ER?
Secretory
Lysosomal
Integral
What are secretory proteins? Examples?
Proteins exported from the cell
Ex. Collagen, insulin
What signals a ribosome to attach to the ER?
An N-terminal signal sequence on the nascent polypeptide
How is the signal sequence removed from the secreted protein?
Cleavage by signal peptidase
How long are N-terminal secretory peptide signal sequences?
13-26 residues
The N-terminal secretory peptide signal has what on the amino part?
At least one positively charges residue
What forms the enter of the N-terminal secretory peptide signal sequence?
A highly hydrophobic stretch of about 10-15 residues
What does the residue on the amino-terminal side of the N-terminal secretory peptide signal have?
Small, neutral side chain (commonly alanine)
How does synthesis of all proteins begin?
By free ribosomes binding to mRNA and commencing synthesis of N-terminal region of polypeptide
What is the signal recognition particle (SRP)?
An RNA-protein complex that recognizes and binds to signal sequences
What happens when SRP binds?
Translation by the ribosome is temporarily arrested
What does SRP do?
Directs peptide to translocation complex
Bound SRP directs the ribosome with an incomplete peptide to go where?
To a specific SRP receptor on the cytosolic face of the ER
What serve as chaperones to make sure the peptide chain folds correctly?
The ATP-driven heat shock proteins
Once a protein has entered the ER, can it get back into the cytosol?
No
Where do glycoproteins acquire their core sugars?
In the ER
In secretory proteins, what does the N-linked moiety connect to?
Asp
In secretory proteins, what does the O-linked moiety connect to?
Ser, thr
N-linked oligosaccharides have what common core?
Pentasaccharide core
In N-linked oligosaccharides, how is the pentasaccharide core derived?
By trimming a 14 residue core oligosaccharide added to specific Ans residues
What do N-linked glycoproteins use as a carrier?
Dolichol phosphate
The core oligosaccharide is assembled stepwise on a dolichol phosphate carrier molecule embedded where?
The ER membrane
Dolichol phosphate is a ___ derivative.
Isoprenoid
Half of the core oligosaccharides are synthesized where?
On the cytosolic side of ER
What are the first 7 sugars of the core oligosaccharides?
2 GlcNAc + 5 mannose
What sugars are added to the core oligosaccharide in the ER lumen?
4 mannose + 3 glucose
How is dolichol phosphate regenerated?
Recycled to dolichol phosphate by the action of a phosphatase
What does Tunicamycin do?
Blocks the first step in oligosaccharide synthesis
What does Bacitracin do?
Blocks phosphatase that recycles dolichol phosphate
Proteins in the lumen of ER are tranported where? How?
Transported to the cis side of the golgi complex by transport vesicles
What things are additionally modified in the Golgi?
Proteins and carbohydrates
Which side of the Golgi is closest to the cell membrane?
The trans face
Mannose-6-phosphate is the signal for what?
The protein to go to the lysosome
Individuals with I-cell disease are deficient in what?
Phosphotransferase
What are symptoms of I-cell disease?
Severe psychomotor retardation
Skeletal deformities
Early death
Undigested glycosaminoglycans and glycolipids build up as ___ where?
Inclusions in lysosomes
What does KDEL stand for?
Lys, Asp, Glu, Leu
What is KDEL?
A carboxy-terminal sequence on ER proteins that is a “return” signal
How do proteins get back into the ER?
They enter transport vesicles at the Golgi and are taken back to ER
What is receptor-mediated endocytosis?
Process of import of specific proteins into a cell by protein binding to receptor in plasma membrane and inclusion into vesicles
How are essential metabolites delivered?
Via receptor-mediated endocytosis
What are most cell surface receptors?
Transmembrane glycoproteins
Where are many receptors located?
In coated pits (in the plasma membrane)
What coats the “coated pits”?
Clathrin
How is invagination of a coated pit triggered?
Receptor-ligand binding
What is the first step of endocytosis?
Invagination of the coated pit
What function does clathrin play in endocytosis?
Forms a lattice around the coated pit
Excise it from the membrane
Forms a coated vesicle
After being excised from the pit, what does the clathrin coated vesicle do?
Loses the clathrin shell, and fuses with an endosome
What are endosomes?
Larger, often irregular vesicles with acidic lumens
What happens to endocytosed proteins?
They are acidified by ATP-driven proton pumps in membranes
Causes the dissociation of protein-receptor complexes
Then often recycled (may be degraded or transported)
What is an example of an endocytosed protein in which both the receptor and the protein are recycled?
Transferrin
What does transferrin do?
Transports iron from sites of absorption and storage to sites of utilization
What does transferrin bind to?
Receptor in coated pits
How do viruses get past the cell membrane?
Membrane-enveloped viruses get through endocytic pathways
How does the diptheria toxin enter cells?
Binds to growth factor receptor - internalized by endocytosis
What are the 4 stable proteins that are poorly ubiquitinated?
Gly
Ala
Cys
Met
What are the 4 unstable proteins?
Arg
His
Ile
Leu
