Protein sorting and transport: ER resident proteins, secretion, targeting to lysosomes, endocytosis Flashcards
How does the ssemby and disassembly of a coated vesicle (e.g. a clathrin coated vesicle) occur ?
Clathrin bind adaptor proteins in the cytosol, which bind cargo receptors on the membrane = coat assembly and cargo selection
Cargo molecules inside the compartment (e.g. ER) bind the cargo receptor = bud formation + vesicle formation
Dissociation of clathrin + adaptor proteins = uncoating (leaving a naked transport vesicle
What happens after vesicle uncoating ?.
After uncoating, vesicles dock and fuse with the target membrane
What is required for vesicle fusion specificity ?
t-SNARE/v-SNARE coupling, along w/ other factors e.g. tethers, small GTPases)
What is the difference between heterotypic and homotypic fusion ?
Heterotypic fusion occurs between compartments that differ in composition. For example, a vesicle derived from one compartment fuses heterotypically with a different compartment.
Homotypic fusion occurs between compartments that are biochemically identical. For example, endosomes fuse with one another, as do lysosomes, the ER and the mitochondria.
What are the steps of vesicle secretion ?
- Vesicle docking :
- Rab-GTP on the vesicle interacts w/ a rab effecot ron the membrane - Assembly of SNARE complexes :
- a v-SNARE e.g. VAMP interacts w/ t-SNARE e.g. syntaxin and SNAP-25 to form a trans-SNARE complex - Membrane fusion :
- cis-SNARE complex forms
- the GTPase NSF and its accessory factor alpha-SNAP break apart the cis-SNARE complex (allows recycling of v-SNAREs) - Disassembly of SNARE complexes (ATP –> ADP + Pi)
What is the KDEL receptor ?
What is its retrieval signal ?
How does protein retrieval occur ?
KDEL receptor = endoplasmic reticulum protein retention receptor
Lys-Asp-Glu-Leu (KDEL) at C-terminus KDEL is a retrieval signal (usually for soluble ER resident proteins) :
- the KDEL receptor, a soluble ER protein w/ KDEL e.g. BiP, and a secreted protein can all be packaged into a COPII besicle to travel for ER to cis-Golgi
- when the cis-Golgi is reached, KDEL receptor is inserted in the membrane , the secreted protein is free in the cytoplasm, and BiP binds back to KDEL receptor
- KDEL receptor, bound to BiP, is packaged in a COPI vesicle to travel back from cis-Golgi to ER
What is the retrieval signal for an membrane protein ?
How does membrane protein retrieval occur ?
Lys-Lys-X-X (-KKXX) at C-terminus of a membrane protein is a retrieval signal (usually for ER transmembrane proteins)
- the KDEL receptor, a soluble ER protein w/ KDEL e.g. BiP, a secreted protein and an ER membrane protein can all be packaged into a COPII besicle to travel for ER to cis-Golgi
- when the cis-Golgi is reached, KDEL receptor and the ER membrane protein are inserted in the membrane , the secreted protein is free in the cytoplasm, and BiP binds back to KDEL receptor
- KDEL receptor, bound to BiP, and the ER membrane protein are packaged in a COPI vesicle to travel back from cis-Golgi to ER
Another example of protein sorting is secretion of exocytosis.
What are the 2 types of secretions possible ?
Constitutive secretion : secreted protein + p membrane protein in a vesicle
Regulated secretion : just hormone molecules in a vesicle
What happens to a protein as it travels through the Golgi stack ?
It undergoes a series of modifications :
- phosphorylation of oligosaccharides on lysosomal proteins in the cis-Golgi network
- removal of manose in the cis-cisterna (Golgi manosidase I)
- removal of manose + addition of GlcLAc (N-acetylglucosamine) in the medial cisterna (Golgi manosidase II + GlcNAc transferase I)
- addition of galactose and NANA (N-acetylneuraminic acid or sialic acid) in the tras-cisterna
- sulfation of tyrosines and carbohydrates in the trans-Golgi network
What is the structure of the Golgi apparatus ?
cis-Golgi + trans-Golgi on each side
- cis-cisterna, medial-cisterna + trans-cisterna = Golgi-stack
Protein comme from the ER, travel for cis- to trans-Golgi, and are then directed to lysosomes, p membrane or secretory vesicles
What is O-linked glycosylation ?
- Sugars added to -OH groups of serine or threonine
* Catalyzed by a series of glycosyl transferase enzymes in the Golgi
What are the 2 models for the organization of the Golgi and the transport of proteins from one cisterna to the next ?
A. Vesicular transport model
B. Cisternal maturation model
What are the characteristics of the lysosome ?
- Contains proteases and lipases for degrading proteins and lipids
- Can be considered a “concentrated bag of enzymes”
- Fuses directly with the late endosome and is then reformed
- The lumen is acidic
What is an examoke of protein sorting for soluble lysosomal resident proteins ?
- manose 6-phosphate is added in the cis-Golgi
- travels through Golgi, uncovering of M6P signal
- M6P binds to its receptor, is transported to the late endosome
- M6P dissociates at acidic pH
- the pposphate is removed
- M6P receptor is recycled
What is mucolipidosis ?
Mucolipidosis (Type II or Type I-cell disease) is a group of inherited metabolic disorders that affect the body’s ability to carry out the normal turnover of various materials within cells. When originally named, the mucolipidoses derived their name from the similarity in presentation to both mucopolysaccharidoses and sphingolipidoses.
