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Lecture 4

Lecture 4 - Protein Synthesis & Transport (The ER).docx

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Biology 2382B
Jessica Kelly

Lecture 4: Protein synthesis and transport (The Endoplasmic Reticulum) (Chp 13: p. 533-539, 549-554) Protein Sorting/Targeting  Newly made peptides must be directed to the correct destination  Typical mammalian cell = 10,000 proteins  Must be localized correctly to function where they are supposed to Targeting - Direct proteins to the right destinations (organelles)  During or after synthesis Sorting - Direct proteins to the secretory pathway (ER, Golgi, lysosomes) Note: Though slightly different meaning sometimes used interchangeably… General Principles of Protein Synthesis & Sorting Many proteins are synthesized just by cytosolic ribosomes…  Those which remain in the cytosol  Those which are targeted to intracellular organelles such as (ER), mitochondria, chloroplasts, peroxisomes, and nucleus (they have a specific signal sequence) Other proteins are synthesized by ribosomes attached to ER (the rough ER)…  Those which reside in ER, secreted proteins, and proteins which are targeted to PM, Golgi complex, and lysosomes 2 major protein-sorting pathways are known  Non-secretory and Secretory Where a protein will function in the cell will determine where it will be synthesize (i.e. cytosol vs. ER rough)… 1 • Targeting sequence  many found at the N-terminus (i.e. beginning of the protein) will direct the protein to the ER where its synthesis will be competed • Secreted proteins  cannot cross the hydrophobic cell membrane or organelle membranes • Usually these targeted proteins must enter organelles through pores or channels and often this occurs through ATP consumption (i.e. active transport) ER Structure  Uninterrupted membranous tubules & vesicles separated from cytoplasm  cisterna which are stacked  RER has ribosomes on the tubules (cisterna) giving it the name “rough”  ER extend from nuclear membrane 2 Secretory Proteins Enter The ER • Before cell is lysed it is pulsed  addition of radioactive amino acids to to cell’s environment  cell will uptake these and use them for protein synthesis • Protein translation is allowed to continue for a period of time where the radioactive amino acids are used in this synthesis • Disruption during synthesis  results in microsomes being created Microsomes - small vesicles of RER membrane with ribosomes attached that are actively translation proteins Bringing the previous techniques together we can see how microscopy allowed visualization of organelles, homogenization and centrifugation helped purify organelles such as microsomes and ER, and SDS-PAGE allows for newly translated proteins to be identified  thus we can determine what kind of proteins are being produced in the ER… 3 ER Functions Secreted & membrane proteins are sorted through RER…  Sugars/carbohydrates are added to the polypeptide  Disulfide bonds are formed  Proteins are folded by “chaperones” Translocation & Translation Occur Simultaneously This was determined by cell-free translation experiments… • Synthetic mRNA was introduced into cell free system where it begins being translated • Completed proteins cannot be incorporated into the microsomes in solution • If microsomes and mRNA are added simultaneously into the cell free system the mature proteins will be found within the microsomes lumen Translocation and translation occur simultaneously… 4 RER: What
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