TAMU BIOL 213 - Protein Modification in the ER, Endocytosis, & Exocytosis (8 pages)

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Protein Modification in the ER, Endocytosis, & Exocytosis



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Protein Modification in the ER, Endocytosis, & Exocytosis

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Lecture number:
20
Pages:
8
Type:
Lecture Note
School:
Texas A&M University
Course:
Biol 213 - Molecular Cell Biol
Edition:
1
Unformatted text preview:

BIOL 213 1st Edition Lecture 20 Outline of Last Lecture I The fate of proteins after synthesis compartmentalization II Protein sorting a Necessary and sufficient III Transport through nuclear pores IV Transport across membranes a Mitochondria and chloroplast b ER membrane i Soluble proteins ii Transmembrane proteins V Transport by vesicles a Highly specific Outline of Current Lecture I Protein modification in the ER a Disulfide bond formation b Glycosylation II Exit of proteins from the ER is controlled a Chaperones hold onto misfolded proteins III Protein modification and sorting in the Golgi These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute a Glycosylation and other signal sequences on the cargo proteins tell the vesicles where to go IV Exocytosis a Constitutive exocytosis pathway i Continuous b Regulated exocytosis pathway i Ex neurotransmitters V Endocytosis a Phagocytosis eating i Really large molecules like bacterial cells b Pinocytosis drinking i Smaller molecules ii Indiscriminate pinocytosis 1 Continuous iii Receptor mediated endocytosis 1 Ex cholesterol VI Lysosomes a Three different pathways to a lysosome Current Lecture I Protein modification in the ER a Disulfide bond formation i The ER lumen is an oxidizing environment meaning it will cause atoms to lose electrons electron density 1 The cytosol is a reducing environment ii This causes disulfide bridges to form between the side chains of amino acids because a sulfur bonded to a sulfur is more oxidized than a sulfur bonded to a hydrogen iii This stabilizes the protein b Glycosylation i This is when a 14 sugar oligosaccharide is added to a protein ii The main one we study is the N linked side group iii This is the attachment of the 14 sugar oligosaccharide to an asparagine 1 Before it s attached to the protein it s bonded to dolichol in the ER membrane a Dolichol is a transmembrane lipid with two phosphate groups b The oligosaccharide is bonded to the last of the two phosphate groups 2 The oligosaccharide is then added to the polypeptide chain in the ER lumen while it s still being synthesized by the ribosome a The asparagine acts as the signal molecule b The oligosaccharide is transferred from the dolichol to the protein by oligosaccharide protein transferase iv Functions 1 To protect the protein from degradation 2 Aids in proper folding of the protein a Because the protein s still being synthesized when the oligosaccharide is added 3 Signal for further sorting 4 Part of carbohydrate layer on the cell surface a Because most proteins in the ER lumen are either going to be excreted or become part of the plasma membrane II Exit of proteins from the ER is controlled a If a protein destined to leave the ER is misfolded a chaperone protein will bind to it and hold onto it so that it can t leave the ER i It will also keep misfolded proteins from clumping together b It will continue to hold onto the protein until it is properly folded c This can sometimes be detrimental if a protein is only a little misfolded but still could functions properly i An example is cystic fibrosis ii The protein that is necessary for chloride transport out of the cell is mutated so that it s slightly misfolded but can still function properly iii However because it s misfolded it is retained in the ER by the chaperones so that it doesn t ever reach the plasma membrane and doesn t ever pump chloride out of the cell 1 This pump is necessary because water follows the chloride so that the mucous in the lungs and throat is thin and watery 2 With cystic fibrosis the mucous is thick so that the person has a hard time breathing d Sometimes there are too many misfolded proteins for the chaperones to keep up with i When this happens they build up in the lumen and trigger the unfolded protein response ii This signals the cell to synthesize more chaperones and other proteins involved in properly folding proteins III Protein modification and sorting in the Golgi a The golgi consists of multiple stacks of flattened membrane enclosed sacs called cisternae or cisterna b Each sac has two faces i Cis the face towards the ER and nucleus ii Trans the face towards the plasma membrane c Proteins travel through the cisternae via vesicles from the cis face to the trans face i These vesicles use the same mechanism as the one taught in the previous lecture ii The vesicles know which cisternae to go to because the proteins are modified and have signal sequences 1 They also go through glycosylation d Vesicles that bud from the trans face either fuse with the plasma membrane or with the membranes of other organelles i Exocytosis is when the vesicles fuse with the plasma membrane and dump their contents outside the cell IV Exocytosis a Constitutive exocytosis pathway i Continuous ii Always sending stuff out of the cell iii This supplies new lipids and proteins to the plasma membrane because the vesicles fuse with it b Regulated exocytosis pathway i Only in specialized secretion cells ii A vesicle containing the cargo will be made and sent to the plasma membrane But it won t fuse with it it will just sit there iii It ll sit there until a signal tells it to fuse 1 This would be when the organism needs the cargo of the vesicle outside the cell 2 An example is the vesicles containing neurotransmitters in the neurons 3 Another example is pancreatic beta cells that excrete insulin when blood glucose levels are high 4 The SNAREs are hidden by other proteins so that they don t bind together immediately and dump the vesicles contents like what happens in constitutive exocytosis 5 When a signal is received the hiding proteins move so that the SNAREs can bond V Endocytosis a Phagocytosis eating i This is only really big stuff like other cells ii Especially important in immune cells iii Neutrophils ingest bacterial cells and damaged cells ex damaged red blood cells 1 Sheets of plasma membrane are extended around the bacterial damaged cell and fuse on the other side so that the bacterial damaged cell is now inside the cell a It s still surrounded by a layer of the cell s plasma membrane 2 The membrane that s surrounding the bacterial damaged cell fuses with the lysosome a The enzymes in the lysosome then break down the bacterial cell iv Often regulated b Pinocytosis drinking i Ingestion of extracellular fluid along with the small molecules in it via vesicles ii This can include proteins


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