Previously Bio308 Hypotheses for molecular basis of bipolar disorder Suggest problem lies in protein targeting How are proteins targeted and delivered? Sorting places proteins in membrane and in lumen of organelles PM (and other) proteins use Sec or SRP mediated translocation to become inserted into the ER (and only the ER) After insertion non-ER proteins are sorted and delivered sorting lumenal vs membrane proteins –how?

ER ER proteins Where can a protein end up in the ER?How does it get there? Lumenal proteins Single transmembrane span proteins Multipass transmembrane proteins What category do our neurotransmitter and neurotransmitter receptor fall in?

How do you get soluble lumenal proteins vs ‘Type III’ What’s different? How does it happen?

Getting out of the ER ER Golgi enzyme involved in glycosylation Lysosomal acid hydrolase Neurotransmitter receptor Now what?

Vesicular traffic Secretory pathway: also method for delivering new PM proteins ER to Golgi to trans-Golgi network  then constitutive or regulated exocytosis

Constitutive and Regulated Exocytosis Constitutive= constant, sometimes called ‘bulk flow’ Regulated= needs additional signal to initiate fusion of vesicle with PM Constitutive does not mean ‘un-regulated’

Stages of vesicle traffic 3 Stages: Budding, targeting/docking and fusion Donor Target

Consequences of unregulated vesicular traffic Mixing of organelle contents (  won’t function correctly) Mislocalization of proteins (  won’t function correctly) Inappropriate levels of secretion (too hi or too lo) A Dead Cell

Vesicular traffic control How does a vesicle ‘know’ what components it should contain? How does it ‘know’ which membrane it should go to? How does it fuse when it gets there? Our neurotransmitter receptor need to go ‘through’ 5 cellular compartments before it gets to the post synaptic membrane

Content selection What goes inside which vesicle? Lumenal protein: Transmembrane proteins: Combination of cytosolic and lumenal proteins determine specific vesicle content

Budding Fig

Coat Components Clathrin COPI COPII Identity determined by what the vesicle contains and it’s coat.

Budding II ER vesicle budding Drin, G, and B. Antonny (2005) News and Views: Helices sculpt membrane. Nature vol: 437 Amino Acid Key Highly hydrophobic + charged - charged Other Hydroxylated Sar1p N-terminal helix Sar1p-GTP form exposes helix that anchors protein to ER surface by ‘floating’ with hydrophobic a.a. interacting with membrane core

Budding III Drin, G, and B. Antonny (2005) News and Views: Helices sculpt membrane. Nature vol: 437 ER vesicle budding Floating many Sar1p in top leaflet makes it ‘bigger’ than the bottom one. Results --> bulge that can more easily interact with coat proteins.

Fission ER vesicle budding….fission Ring of parallel helices at neck might aid fission. New data for ER; had seen a protein (epsin) help deform PM for clathrin coated vesicles. May suggest that using a helix to deform membrane is common mechanism for budding/fission

Targeting/Docking: What happens after budding? How do vesicles dock with specific target membrane?

The SNARE hypothesis Fig V-SNARE T-SNARE Role of p115 Role of Rab proteins retrograde

Synaptic vesicle fusion VAMP Syntaxin SNAP 25 Synaptotagmin Rab3a

Next: Moving in the other direction: endocytosis Types: Phagocytosis– specialized cells Pinocytosis– all cells Connection– perhaps the # of our receptor’s on PM is controlled by endocytosis Pinocytosis ‘problem’ rate of pinocytosis internalizes 100% of PM per hour ? (How can this be?)