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Lecture 5

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University of Toronto Scarborough
Biological Sciences
Bebhinn Treanor

Lecture 5 PROTEIN TARGETING IN INDIVIDUAL CELLS Polypeptide in the liquid portion of cytoplasm: made on a free ribosome and has no Protein Targeting: Getting proteins to where they need to be used localizational signal o Where the polypeptide will have its translation completed is related to where it functions Cytosol: liquid part of the cytoplasm Proteins location: based on where it was made Starting of translation of all nuclear encoded proteins: free ribosomes in the cytoplasm PROTEINS WITH SIGNAL PEPTIDES THAT COMPLETE TRANSLATION ON RIBOSOMES o NOT where the production of all polypeptides finish ATTACHED TO THE ER o Production of some polypeptides: ribosomes that are free in the cytoplasm 1. mRNA leaves the nucleus and combines with a large and small subunit of the ribosome and o Synthesis of other polypeptides: completed on ribosomes that are attached to the translation is initiated and elongation occurs for the addition of several amino acids. membrane of the endoplasmic reticulum (ER) o First amino acid synthesized through translation: signal peptide Rough ER: ER with ribosomes (rough appearance) 2. Signal peptides emerges from the ribosomes: once enough cycles of elongation occurs Once translation begins: amino acids of the polypeptide begin to emerge from the ribosome 3. signal recognition particle: protein which attracts the signal peptide o DECSION MAKING?!?!?! About where to continue the translation process 4. Role of the signal recognition particle: blocks further translation until the entire ribosome 2 fates for the location where translation is completed: moves to the ER, and binds with a receptor protein (signal peptide receptorSRP) that is 1. Free ribosomes: 1 category of proteins are completed on free ribosomes embedded in the membrane of ER 2. Ribosomes attached to the ER: the other categories of proteins have their protein 5. Translation resumes: only when the ribosome has been attached to the ER synthesis completed on ribosomes attached to the membrane of ER o Positioning of ribosome: such a way that newly synthesized protein is pushed into Decision to move a ribosome with a growing polypeptide to the ER: info is encoded in the the lumen of ER mRNA as the set of codons that dictate a string of specific amino acids tha serves as the signal o Lumen: interior space of ER peptide 6. In the lumenSignal Peptidase: enzyme which usually removes signal sequence Polypeptides that have a signal peptide: polypeptides that will have their translation completed o Note: methionine at the end would be chewed off. Thus, that is why proteins start on a ribosome attached to the membrane of the rough ER off as their first amino acid as methionine. BUT, not all mature and functioning Signal peptide: a few amino acids in a row near the amino-terminus of the growing polypeptide proteins have methionine as first amino acid Attachment of a signal peptide to a growing polypeptide: to the membrane of ER Summary: Ribosomes that make a polypeptide with a signal polypeptide move to the rough ER, attach to the ER and then continue synthesis of the polypeptide. As the polypeptide is being made, it is being fed into the lumen of ER PROTEINS WITHOUT A SIGNAL PEPTIDE COMPLETE TRANSLATION ON FREE RIBOSOMES EXAMPLES: PROTEINS MADE ON RIBOSOMES WHICH ARE ATTACHED TO THE ER The type of proteins that do NOT have signal peptides, thus are synthesized to completion on Membrane proteins free ribosomes are: 1. Double membrane organelles: nucleus, chloroplasts or mitochondria Proteins that function in the ER 2. Proteins of the cytosol: Ex) microtubules, microfilaments, intermediate filaments Proteins that function in the Gogli apparatus Proteins that function in a lysosome 3. Proteins of the peroxisome o The proteins: have translation begun and completed on free ribosomes (in cytosol) Proteins that are secreted from the cell o Once these polypeptides move from the free ribosomes: move to the proper organelle Significance of the presence or absence of localization signal: determines whether it is controlled post translational Cell makes certain proteins that are destined for the lumen of the endomembrane: via signal Polypeptides that re completed on free ribosomes that need to get into a double membrane sequences organelle: have a few amino acids that serve as localizational signals to get them recognized a Once in the luman of ERfinal place of proteins is based on: whether or not the aimino acids belongings in that organelle of the polypeptide encode any other address labels. o Large Nuclear proteins: have nuclear localizational peptides that allws them to get through o Components of address label: carbohydrates that are added to specific amino acids the nuclear pores, and into the nucleus o Glycosylation: addition of carbohydrates o Mitochondrial proteins: have mitochondrial localizational peptides that gets them into the Make sure protein ends up at final destination: based on the type of carbohydrate added and mitochondria where it is attached to the protein o Plastids (such as chloroplasts): have a plastid localizational signals that gets them into the A) Some proteins remain at work in the ER: get a particular carbohydrate tag which plastid indicates they can remain in the lumen of ER o Peroxisomal proteins: have peroxisomal localizational signal that gets them into the B) Some proteins are destined for a membrane: get a particular type of carbohydrate peroxisome shipping tag and then move away from an area of rough ER to an area of smooth 1 | P a g e
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