Protein Trafficking Flashcards
Is there a difference between free and RER ribosomes?
Nope. None.
Where is the signal on a protein for destination located on the protein?
Signal may be a run of 3 to <30 amino acids at N- or C-terminal or internal
Some may be post translational modifications
Some signals may be 3D domains on protein
What kinds of proteins are synthesized in free ribosomes?
Proteins destined for peroxisomes, mitochondria (except ones made in mitochondria) and the nucleus
What sort of signals are used for proteins to enter the nucleus?
Internal nuclear localization sequences
What sort of signal sequences are used for proteins to enter peroxisomes?
C-terminal sequence SKF
What sort of signal sequences are used for proteins to be imported into the mitochondrial matrix?
N-terminal sequence rich in positive charged amino acids and serine, threonine
What are the 3 major classes of proteins produced in the RER?
Secretory proteins
Lysosomal proteins
Integral membrane proteins
How is a signal sequence removed from a secreted protein?
By signal peptidase
What are the common signal sequences of N-terminal secretory peptides?
Range in length from 13-26
Amino terminal contains at least one positively charged residue
A hydrophobic 10-15 residue stretch forms the center of the signal
Residue of the signal peptidase cleavage site has small, neutral side chain (alanine common)
Where does synthesis of all proteins begin?
By free ribosomes binding to mRNA and commencing synthesis of N-terminal region of polypeptide
What is signal recognition particle (SRP)?
An RNA protein compex that recognizes and binds to signal sequences
What doe SRP when bound to a signal sequence?
It temporarily arrests translation by the ribosome
Can a protein return to the cytosol once it enters the endoplasmic reticulum?
No, entry into ER is irreversible
What chaperones assist correct peptide chain folding?
ATP driven heat shock proteins
What do N-linked glycoproteins use as a phosphate carrier and how?
They use dolichol embedded in the ER membrane
What is dolichol phosphate?
An isoprenoid derivative
A lipid
Localized to membranes
What happens to dolichol phosphate as oligosaccharides are synthesized?
On the cytosolic side sugar precursors are added to the phosphate group forming dolichol pyrophosphate intermediate. Once the oligosaccharide is translocated across the ER membrane into the lumen the incomplete core-dolichol pyrophosphate is released as the oligosaccharide is transferred to an Asn residue
Where are oligosaccharides synthesized?
Half on cytosol side of ER, half in the ER lumen
How is dolichol phosphate regenerated?
Using a phosphatase
What are some drugs that affect the addition of sugars?
Tunicamycin - blocks first step in oligosaccharide surface
Bacitracin - blocks phophatase that recycles dolichol phosphate
How is bacitracin a useful antibiotic?
The bacterial enzyme used to recycle an isprenoid pyrophosphate in cell wall synthesis is very sensitive to it.
How are proteins transferred to the Golgi apparatus from the ER?
Via transport vesicles (transfer vesicles) to the cis side of the Golgi complex
What happens in the Golgi apparatus to proteins and carbohydrates?
They are additionally modified. Proteins are then sorted and directed to their destination by transfer vesicles leaving the trans face of the Golgi
How is lysosomes targeting conducted?
Phospho-N-acetyl-glucosamine added to mannose by phosphotransferase > phosphotransferase recognizes 3D motif > phosphodiesterase removes N-AC-Gln leaving mannose-6-phosphate > mannose-6-phosphate binds to receptor in Golgi membrane > vesicles targeted to lysosome
What is the autosomal recessive disease where individuals are deficient in phosphotransferase?
I-cell disease (mucolipidosis II)
What happens in I-cell disease?
Eight acid hydrolases are not targeted to lysosomes but secreted instead which results in glycosaminoglycans and glycolipids build up as inclusions in lysosomes
What are the symptoms of I-cell disease?
Severe psychomotor retardation and skeletal deformities
How are heat shock proteins returned to the ER after traveling to the Golgi?
They have KDEL (lys-asp-glu-leu) as their return signal which has them be repackaged in vesicles in the Golgi and returned to the ER.
What disease results from disorders in receptor mediated uptake?
Some forms of familial hypercholesterolemia
What kind of regions of the plasma membrane are many receptors located in?
Coated pits with a thick coat of the protein clathrin at the cytosolic side of the indentation
How are endosomes different than other vesicles?
Endosomes are often larger, often irregular vesicles with acidic lumens
How does endocytosis of receptors begin?
Invagination of the coated pit triggered by receptor-ligand binding > clathrin then forms a lattice around the coated pit forming a coated vesicle > vesicle loses clathrin shell and fuses with endosome
What is the function of transferrin?
Transports iron from sites of absorption and storage to sites of utilization
Two Fe3+ ions bind/apotransferrin molecule to make transferrin, the two together bind to the coated pits
How is transferrin processed?
Transferrin internalized into endosome then the endosome is acidified which allows Fe3+ to dissociate from apotransferrin which remains bound to the receptor
When the receptor is recycled how is Fe3+ recaptured?
By ferritin in the cytosol
How does the semliki virus (related to yellow fever) enter cells?
It enter susceptible cells by binding to receptors in coated pits and is endocytosed
How does diphtheria toxin enter cells?
By binding to growth factor receptors and being internalized by the endocytosis
How are proteins turned over?
Ubiquity in tags proteins for destruction in eukaryotic cells.
