NE102 Lecture Notes 2

2 the secretory pathway is central to generating

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2. The secretory pathway is central to generating membrane-embedded proteins  (aka, transmembrane proteins) Most transmembrane proteins span the phospholipid bilayer via a-helical stretches of  hydrophobic amino acids QUESTON: How are transmembrane proteins inserted into the ER membrane? Insertion of transmembrane proteins is triggered by stop-transfer sequences Multi-pass transmembrane proteins contain alternating signal sequences & stop-transfer  sequences Golgi = Central “protein sorting center”
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Vesicular Transport 19:00 Protein sorting via secretory pathway takes place by  vesicular transport . Entails four steps: 1. Sorting of cargo into a vesicle bud Packaging of proper proteins into vesicle bud. 2. Vesicle budding Orientation of the membrane leaflets is maintained. Orientation of the transmembrane protein is maintained.  3. Vesicle transport to target membrane. 4. Vesicle docking & fusion with the target membrane.  OVERRIDING QUESTION: What molecular machinery executes & regulates vesicular  transport? Q1: How do cell execute vesicle budding? Vesicle budding is driven by the assembly of  vesicle coat proteins Steps: a.i.1. Coat proteins attach to the cytosolic side of membranes. Clathrin = coat protein b/w Golgi & the plasma membrane COP = coat protein b/w ER & Golgi a.i.2. While attaching they interact with one another to assemble  into a “lattice” structure.  a.i.3. The lattice structure distorts the membrane to initiate vesicle  budding. a.i.4. The protein dynamin then drives release of a coated vesicle. 
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Vesicular Transport 19:00 Aside: the vesicle coat is disassembled before fusion.  How do the coat proteins attach to the membrane? Closer look at Clathrin Multi-subunit protein that assembles into a triskelion structure  Assemble together from tail to tail to form a lattice structure – form a concave structure. Q2: How do cells sort the correct protein cargo into budding vesicles? Cells have  cargo receptors  that recruit cargo into budding vesicles. Transmembrane proteins that stay in the vesicle How are trafficking proteins sorted into budding vesicles? Trafficking proteins interact with vesicle coat proteins via adaptor proteins (connected to  coat protein) Adaptor proteins are in the cytosol.  FINAL Q’s: How are vesicles targeted to the correct membrane? How do the  membranes fuse? Q3: How are vesicles targeted to the correct membrane? Cell membranes are associated with specific Rab proteins, which recruit tethering  proteins.  Protein-protein interactions between tethering proteins result in vesicle docking at its  target membrane. 
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Vesicular Transport cont’d.
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