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L6 - For Thursday Oct 16 Chapter 13 use the lecture as...

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All lectures are being podcast and available at https://media4.ucsc.edu/webcast/ Exam: Tuesday October 20th: one week from today. You will need to bring a Parscore pink, whole page scantron sheet to exams. These are available at the bookstore. You will need to bring a #2 pencil The problem sets are your study guide. The old exam shows you what kind of problems there will be. For Thursday Oct 16: Chapter 13: use the lecture as guide; I have skipped a few sections What I don’t talk about, you are not responsible for.
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4) Generalized depolarization of the muscle cell membrane activates V-gated Ca++ channels in specialized regions of the transverse tubules 5) Ca++-gated Ca++release channels in the sarcoplasmic reticulum open and release stored Ca++. This sudden release of Ca+ + intracellularly caused the myofibrils to contract
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Proteins are directed to their correct locations within the cell by protein trafficking Much of the lumenal space in cells are topologically equivalent and may have evolved from vesicles pinching off the plasma membrane Arrows denote outbound and inbound traffic routes
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Purification of smooth and rough microsomes
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The Signal Hypothesis - Proteins are targeted to the ER by a signal peptide that is cleaved off after translocation into the ER. Four discoveries 1) Signal peptide: Experiment was to perform in vitro translation reaction with and without microsomes. Only w/ microsomes from RER is the protein the right size. W/out the protein is BIGGER--therefore the RER contains an enzyme that cleaves off a bit, the signal peptide. Recombinant DNA experiment confirm that the signal is necessary and sufficient
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SRP binds to the signal peptide, brings the ribosome to the ER and organizes the nascent polypeptide on the tranlocator
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7) Translocator is made up of 3 subunits. Pore is blocked when ribosome is not bound. Lateral opening is very important for transmembrane proteins (see later)
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For soluble proteins, the entire protein is translocated across the membrane of the ER and the signal peptide is cleaved off. 8) Signal sequence is cleaved by a signal peptidase. The translocon releases the cleaved signal into the lipid bilayer, therefore it can open laterally. Ribosomes are there but not pictured.
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Internal signal sequence serves as a “start-transfer” sequence and causes proteins to be inserted into the membrane in different orientations Charges on amino acids near the transmembrane segment determine the orientation of the protein. This process is poorly understood but a side with negative charges will go into the ER and a side with positive charge will be retained in the cytoplasm. - - - Negative charge is distal side of start- transfer
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For multipass transmembrane proteins with a signal peptide,the signal peptide is read and begins translocation (start) and then combinations of start transfer and stop transfer signals determine how are inserted into the lipid bilayer negative charges will go into the ER
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Once a protein gets into the ER, things happen to it…..
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1) Some proteins receive a glycophosphatidylinositol anchor in the ER
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