Vesicular Trafficking
Post on 24-Feb-2016
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Vesicular Trafficking
Movement From the ER Through the Golgi
Getting to the Golgi
Anterograde and Retrograde Traffic
Genetic Analysis of ER-Golgi Transport
General Principles
• Cargo selection– signals (ie amino acid sequences)
• Vesicle formation (budding) from donor compartment
• Vesicle/Cargo targeting and fusion to acceptor compartment
• Most regulation is mediated by cargo- and target-compartment-specific small GTPases
• Anterograde transport– COPII apparatus– protein complex mediating cargo selection and
assembly of budding components from ER
• Retrograde transport– COPI– protein complex mediating transfer of vesicles back to
ER
Retrograde Transport Gives Golgi Compartments Dynamic Identity
Cargo Selection: Export Targeting Sequences
• Y xnDxE – tyrosine-diacidic domain• Found in cytoplasmic tail of TM proteins
Cargo Selection: Just Getting the Right Stuff
• Exclusion of ER resident proteins– not well understood– fidelity factors (genes)
• mutations in some genes cause increased excretion of ER resident proteins
– Retrieval• COPI vesicles retrieve ER proteins and return them • KDEL signal on lumenal proteins• KKxx-COOH on TM proteins (dilysine motif)
COPII Vesicular Coat Assembly & Budding• COPII coat
formation regulated by sar1 GTPase
• sar1 activates cargo receptors– TM proteins
recognizing export signals
• sar1 recruits sec23/24 coatamer proteins
• sec23/24 in turn recruit sec31/13 coat proteins
• COPII vesicles form and bud
• Once apart from ER, coat proteins are triggered to release by hydrolysis of GTP by sar1
Vesicular Delivery• Rab-GTPases• Recruits tether
and fusion proteins into vesicles
• These form a targeting complex recognized by a docking complex
• targeting– v-tethers – p115 – vSNAREs –
synaptobrevin• docking
– t-tethers – GRASP65
– tSNAREs – syntaxin 1
Tethering and Fusion Mediated by SNAREs (SNAP Receptors)
• SNAREs aggregated by Rab-GTPase (vSNAREs)• Vesicle buds• SNAP25-NSF interact with vesicle to dis-aggregate and prime
vSNAREs for interaction with tSNAREs
COPI Vesicles
Recycle ER Resident Proteins
• COPI Vesicles– Arf family of small GTPases– Recruit/activate coatomer (6 cytosolic proteins)– Recruit ER targeting SNAREs
Retrieval of ER Proteins with
KDEL Signal by COPI Vesicles
Cycle of Vesicular Transport
Cargo Sorting by the Trans-Golgi Network (TGN)• Specific coat proteins direct TGN vesicles to targets
Lysosomal/Endosomal Targeting
• Mannose-6-phosphate receptor targeting to lysosome/endosome
• MPR cytoplasmic tail (C-term) has multiple signals for multiple routing possibilities
Lysosomal Targeting of
Prohydrolase by M6P and MPRs
•Mannose-6-Phosphate Receptors •Cation-independent (CI)•Cation dependent (CD)
Cytoplasmic Tails of MPRs Allow Sorting to Lysosomes vs Other Routes
Assembly of Clathrin on Lysosomal/Endosomal Destined TGN Vesicles
Directed Translocation of Secretory Vesicles
• apical targeting in caveolin coated, lipid-raft vesicles
• basolateral targeting signals– direct protein to this
membrane• constitutive basolateral
targeting– subsequent sorting of
apical proteins• untargeted vesicles
– sorting to redistribute surface proteins
Proteolytic Processing of Pro-proteins in Secretory
Vesicles
• Cleavage of proinsulin in secretory vesicles of pancreas cells
Secretory Vesicles/Granules
• Material in secretory vesicle aggregates due to decreased pH and divalent cations
• Membrane recycling to TGN directly or via endosome reduced vesicle size
Regulated Secretion
• Fusion of the vesicle and plasma membrane is regulated by various means– receptor-ligand interaction and second messenger
signaling– depolarization of membrane– etc…
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