BIOC 212 Lecture Notes - Lecture 13: Protein Folding, Ammonia, Phenylalanine
29 May 2018
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2- Intracellular Trafficking
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Secretory Pathway
• Transport through the secretory pathway goes through a series of vesicles that
bud off from membrane and diffuse into other membrane of next organelle
o Vesicular transport pathways go forward, backward and to the lysosome
Coated Vesicle Transport
• Major transport pathway is through coated vesicles
• Lipid vesicles that are coated with proteins which control how and where the
vesicles form + where vesicle should go
• COP-II vesicles transport from ER to Golgi (anterograde)
o Forward transport
• COP-I vesicles transport from Golgi back to ER (retrograde)
o Transport backwards
• COP-I and COP-II coats have related mechanisms but different proteins involved
• Clathrin-coated vesicles (CCV) transport from Golgi and PM to endosomes
o Golgi to PM, and from PM to endosomes
o Late segments of secretory pathway
o Work differently from COP-I & COP-II
Vesicle Formation
• General principles of vesicle formation are the same for all vesicle types
• Initiation
o Cytosolic adaptor proteins interact with membrane to start vesicle formation
o Collect transmembrane cargo, or cargo receptors
§ Cargo = proteins that will be inside the vesicle
§ Specificity since want to select proteins for transport
• Coat formation
o Protein framework is formed on cytosolic adaptors to shape the vesicle bud
from the membrane
§ Coat is what gives vesicle its shape
• Fission
o Bud is pinched off to separate the vesicle from membrane
o Now vesicle not attached to membrane anymore
• Uncoating
o Coat is removed to allow vesicle targeting and fusion
o When reach target organelle, need to fuse the vesicle membrane with the
target membrane, and the coat gets in the way
§ Cannot finish targeting until remove the coat
Ras GTPase Family
• One of the major mechanisms to control vesicle formation and other pathways in
the cell is through family of protein called Ras GTPase
• Monomeric GTPase “switches”
o Family of proteins that bind & hydrolyze GTP
o GTP-bound state provides binding site for various effectors
§ ON form, can interact with proteins called effectors, that do different
things
o When are in OFF state, after GTP hydrolysis , no longer interact with
effectors
• Ras: signal transduction
• Sar1 and Arf: initiate COP-I & COP-II formation
• Rab: vesicle targeting to the final destination membrane
• Cycle is regulated by other proteins
• GTPase Activating Proteins (GAP) stimulate:
o GTP hydrolysis
o Turn protein from on to off
• Guanine Exchange Factors (GEF) cause:
o Release of GDP
o Binding of GTP
o Turns Ras protein on
COP-II Vesicle Initiation
• COP-II vesicles form at specific ER exit sites
o These sites are separate from where chaperones are
o Proteins with exit signals are collected at the exit sites (cargo receptors)
§ Proteins to be transported bind cargo receptors
o Misfolded proteins are kept away (calnexin)
§ Calnexin will bind the glucose and anchor them in place
• Initiation relies on Sar1 from the Ras GTPase family
• Transmembrane GEF at exit site induces GTP binding by Sar1
o There is a GEF specific for Sar1; TM protein anchored at the exit site
o Removes GDP and adds GTP to turn Sar1 ON
§ Turns it on only at the ER exit site
• Sar1-GTP exposes amphipathic helix and partially inserts into membrane, to
initiate vesicle formation
o Helix hydrophobic one side and polar on the other
o Sar1, which is normally soluble (in cytoplasm), will attach the membrane by
inserting the hydrophobic part of the helix into the membrane
Document Summary
Secretory pathway: transport through the secretory pathway goes through a series of vesicles that bud off from membrane and diffuse into other membrane of next organelle, vesicular transport pathways go forward, backward and to the lysosome. Vesicle formation: general principles of vesicle formation are the same for all vesicle types. Initiation: cytosolic adaptor proteins interact with membrane to start vesicle formation, collect transmembrane cargo, or cargo receptors. Cargo = proteins that will be inside the vesicle. Specificity since want to select proteins for transport: coat formation, protein framework is formed on cytosolic adaptors to shape the vesicle bud from the membrane. Cannot finish targeting until remove the coat. Cop-ii vesicle initiation: cop-ii vesicles form at specific er exit sites, these sites are separate from where chaperones are, proteins with exit signals are collected at the exit sites (cargo receptors) Proteins to be transported bind cargo receptors: misfolded proteins are kept away (calnexin)
