NE102 Lecture Notes 2

Through the nuclear pore complex importin interacts

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Through the nuclear pore complex Importin interacts with and “walk along” the cytoplasmic filaments to import proteins SORTING VIA THE “SECRETORY PATHWAY” 3 fundamental steps: (1) Import of proteins into the rough endoplasmic reticulum (RER) RER is right next to the nucleus (2) Transport from the ER to the Golgi apparatus
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Protein Sorting pt. 2 19:00 (3) Transport from the Golgi to protein’s functional location/compartment The secretory pathway was named as such based on its discovery in the 1960’s THEIR QUESTION: How are proteins secreted across the plasma membrane? Used pancreatic cells = secrete disgestive enzymes into small intestine KEY: Most newly synthesized proteins are SECRETED by the cells Method = “Pulse-chase experiment” (3 steps) a.i.1. Pulse-labeled newly synthesized proteins with radioactive  amino acids – enables visualization/detection of proteins a.i.1.a. Newly synthesize protein in Rough ER a.i.2. Chased with non-radioactive amino acids – Discrete protein  population labeled (i.e., those synthesized during the pulse  period) a.i.2.a. Protein moved on to Golgi apparatus a.i.3. Tracked the location of the radioactive protein through time a.i.3.a. Protein in secretory vesicles & released The secretory pathway was used to sort proteins to several locations/compartments RER to Golgi (cis to stack to trans) to Other locations QUESTION: How are proteins destine for the secretory pathway identified & imported  into the rough ER? Proteins destine for the secretory pathway contain a “signal sequence” (1)  (2) The signal sequence of most proteins is typically located at the N-terminus of  proteins N-terminal signal sequences target proteins for “co-translation” import into rough ER REMINDER: The N-terminus is synthesized first during translation QUESTION: How does the signal sequence take the mRNA to the channel at the ER?
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Protein Sorting pt. 2 19:00 Co-translational targeting of proteins is mediated by SRT (signal recognition particle) *&  the SRP receptor SRP attaches to the signal sequence and binds specifically to the SRP receptor via  protein-protein interactions at the ER.  Elongation of the polypeptide drives its transfer through the translocon QUESTION: How are transmembrane proteins inserted into the ER membrane? Most transmembrane proteins span the phospholipid bilayer via a-helical stretches of  hydrophobic amino acids. Insertion of transmembrane proteins is triggered by stop-transfer sequences SIDENOTE: 1. The Rough ER is referred to as such because it appears “rough” due to  attached ribosomes.
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