Sorting from the Golgi Flashcards
Secretory Granules
- involved in regulated secretion, occurs only in some cells and triggered by specific signal.
- these granules contain cargo such as hormones, neurotransmitters, and digestive enzymes.
- in neurons and endocrine cells, a specific type of secretory granule called dense core vesicles can be found - which contain large neuropeptides rather than low weight neurotransmitters.
What is the role of the trans-golgi network?
- Acts as a sorting station for outbound cargo from the Golgi.
- Directs proteins to plasma membrane or endosomes.
- Interface between secretory and endocytic pathways.
What are the two ways that cargo can be transported to the TGN?
- Direct: TGN -> Target.
- Indirect: TGN -> Intermediates (e.g., vesicles/tubules) -> Target.
What are the two types of Secretion Pathways?
- Constitutive Secretion
- occurs in all cells
- continuous delivery of lipids and proteins to the plasma membrane - Regulated Secretion
- found in specialized cells (neurons, endocrine cells)
- cargo stored in vesicles until triggered by a signal (e.g. Ca2+ influx.)
(packing for regulated pathway, cargo aggregates at TGN, budding -> acidification -> maturation -> signal triggered fusion).
What is the difference between apical and basolateral targeting? (polarized epithelial cells)
- Sorting occurs at the TGN using signals recognized by carriers.
- Basolateral signals:
- simpler
- tyrosine motifs/dileucine motifs
- requires clathrin and AP-1B - Apical signals
- more varied
- o-glycosylation
- gpi anchors
- lipid rafts
what is the function of tubular transport intermediates?
- transport cargo to apical surface
- use kinesins to move along microtubules
- cargo often associate with lipid rafts
Where do clathrin-coated vesicles go to?
- cell surface (basolateral)
- endosomes/lysosomes
- require adaptor proteins (APs, GGA)
What clathrin adaptors are on the golgi?
- AP-1a and AP-1b: sorts to basolateral surface.
- AP-3 sorts to endosomes.
- AP-4 (golgi localized)
- GGA: TGN to endosomes/lysosomes.
What is the structure of APs?
- 4 subunits
- bind cytoplasmic motifs (tyrosine, dileucine)
- bind clathrin via ear domain
what is the structure and function of GGAs? (golgi-localized, gamma-ear containing ARF-binding proteins)?
structure:
- VHS domain binds DXXLL motifs (M6PR, Sortilin)
- GAT binds ARF1
- Hinge & ear: bind clathrin and accessory proteins
function:
- facilitate clathrin-coated vesicle formation from golgi.
- sort cargo to endosomes and lysosomes.
Describe the process of M6P-dependent lysosomal targeting
- Hydrolase processing
- synthesized in ER
- modified in golgi with M6P tag
-> step 1 (cis golgi): GlcNAc phosphotransferase adds UDP-GlcNAc.
-> step 2 (medial golgi): GlcNAcase removes GlcNAc → exposes M6P. - M6PR
- Recognizes M6P-tagged hydrolases.
- Uses clathrin coats for forward trafficking, retromer for recycling.
- Receptors have tyrosine/dileucine motifs for adaptor recognition (APs, GGA).
What causes IDC?
- Caused by GlcNAc phosphotransferase mutation.
- M6P tagging fails → hydrolases secreted instead of sent to lysosomes.
- Severe lysosomal storage disorder.
What are the symptoms of ICD?
Mental retardation
Skeletal deformities (dysostosis multiplex)
Coarse facial features
Hepatosplenomegaly
Corneal clouding
Dwarfism
What is sortilin?
- receptor that binds cargo without M6P tag
- binds proteins like prosaposin
- has DXXLL motif on its C-terminus -> recognized by GGA and AP-1
- Facilitates trafficking of select proteins to lysosomes.
Impact of Truncated GGA (missing hinge & ear)
- Cannot bind clathrin → vesicle formation disrupted.
- Cargo trafficking impaired despite cargo still binding.
- Lysosomal enzyme delivery affected
