Vesicular Trafficking Movement From the ER Through the Golgi
Feb 24, 2016
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…