Lecture 11: Golgi and Lysosomes-Function and Trafficking Flashcards
Golgi complex
- Ordered series of flatten membrane stacks called cisternae
- Usually consist of 4-6 cisterna
- Each of the Golgi cisterna have unique protein (and lipid) composition
- Stacks are often connected by tubules to form a continuous organelle
- Located in the perinuclear region (around the nucleus) close to the centrisome
- Localization is maintained by microtubules
Model of the Golgi Complex
Which stains can be used to indicate the different golgi compartments?
Trans face- Nucleoside diphosphate
Cis face- Osmium
TGN- Acid phosphatase
___ proteins cause coating with COPII in the golgi
Sec proteins
The ____ complex begins to build a coat
Sec23/Sec24
The ___ complex finishes the coat
Sec13/31
Coat proteins also help sequester ____
cargo receptors
In what 3 ways are cargo selected for inclusion in vesicles?
- Export signals
- Retention
- Bulk flow
Export Signals
Proteins to be transported have a export signal that allows selective packaging into transport vesicles
Retention
- Proteins have a mechanism to be excluded form transport vesicles
- Chaperone binding
- Kin Recognition
Bulk flow
– Some proteins get packaged simply because they are in the right place at the right time
_____ also participates in decision about entry into COPII transport vesicles and ER exit
- The ER quality control apparatus
- Most of the time this is a good thing to prevent inappropriately folded protein or subunits of unassembled complexes from reaching the cell surface
- Large amounts (up 90%) of the T-cell receptor, acetylcholine receptor are degraded and never reach the cell surface
In what case is the quality control machinery is too good?
– Cystic Fibrosis Transmembrane Regulator (CFTR)
ER-derived transport vesicles fuse to form ____
vesicular tubular clusters (VTC)
VTC’s form by ____ membrane fusion
homotypic
What happens when ER resident proteins get transported out of the ER?
They are actively retrieved!
– Soluble residents
• KDEL sequence and receptor
– Membrane bound residents
• KKXX motif
How does the KDEL receptor know when to bind and when to release KDEL-containing proteins?
- The interaction between the KDEL tetrapeptide and the KDEL receptor is pH sensitive.
- The relatively acidic pH of the Golgi compartment permits the association of the KDEL tetrapeptide with the KDEL receptor
- The more neutral pH of the ER allows release (and prevents binding)
The KDEL receptor binds to escaped KDEL protein in the ____
cis Golgi
ER-Golgi anterograde and retrograde traffic
_____ function to anchor and localize transport gesicles
Golgins
How do golgins do anchor and localize vesicles?
Through a rab interaction domain and a cytoskeletal interactor domain
What are the two models of intra-golgi transport?
- Vesicle transport model
- Cisternal maturation model
Vesicle transport model
Cisternal Maturation Model
Biochemical functions of the Golgi
• Glycosylation
– Modification of existing Nlinked sugars
– O-linked glycosylation
– Production of proteoglycans
- Protein sorting
- Sulfation
– Specific tyrosine residues in proteins
Biochemical function of the cis golgi network
- phopsphorylization of oligosaccharides on lysosomal proteins
biochemical functions of cis cisterna
- removal of Man
Biochemical function of medial cisterna
- removal of Man
- addition of GlcNAc
Biochemical functions of trans cisterna
- addition of Gal
- addition of NANA
Biochemical functions of trans golgi network
- sulfation of tyrosines and carbohydrates
What is the purpose of protein glycosylation?
• Aid in protein folding
– ER quality control
• Surface coating
– Since animal cells do not have a cell wall, the extended surface coating by sugars provides come degree of protection from the extracellular environment
• Specific interactions
– Some sugars provide interaction surfaces for cell-cell recognition
• Extracellular Matrix
What’s the signal for glycosylation?
–Asn-X-Ser/Thr–
Two main classes of N-linked glycans?
- Complex oligosaccharides (Gal and NANA)
- High-Mannose oligosaccharide (Man)
Order of sugar modifications?
- Glucoses removed by glucoxidases in ER
- Some Mannoses removed by mannosidases in both the ER and Golgi (to become High-mannose when has 6-Man)
- GlcNAc added by N-acetylglucosamine transferase I and Man further removed
- Step 3 repeats until 3 Man are connected each to a GlcNAc, Gal, and NANA
Sugar modification diagram
In Proteoglycan production (O-linked glycan) sugars are added….
in the golgi and as repeating disaccharides
O-linked glycans are very ____ while N-linked are very____
O= straight
N= highly branched
Lysosomes
• Primary degradative organelle
– Degrades proteins, lipids and other cellular constituents taken in by endocytosis, phagocytosis, and autophagy
• Other functions
– Stores osmolytes
– Stores nutrients as well as waste products
• Heterogenous in size and function
What’s the hallmark of autophagy?
double membrane
Stages of autophagy?
• Most lysosomal proteins (including the hydrolases) leave _____ and are diverted to the lysosome
the secretory pathway
How are lysosomal proteins segregated from other secretory proteins?
They contain a specific modification on their oligosaccharides - a phosphate group on the 6-position of a terminal mannose (M6P)
How is M6P added?
- Lysosomal proteins contain a “signal patch” that is seen by the enzyme N-acetylglucosamine phosphotransferase (GlcNAc)
- This enzyme binds both the high mannosecontaining lysosomal protein to be modified and UDP-GlcNAc and catalyzes the transfer of GlcNAc phosphate to a terminal mannose residue
- The GlcNAc sugar is removed by a glycosidase leaving the mannose-6-phosphate
Inclusion-Cell (I-cell) disease is caused by a mutation in _____
GlcNAc phosphotransferase
Where does the Mannose-6 phosphate receptor (M6PR) reside?
Trans-golgi network
M6PR binding is ___ sensitive
pH, dissociates in acidic conditions of early endosome
