BIOL 213 1st Edition Lecture 20 Outline of Last Lecture I. The fate of proteins after synthesis – compartmentalizationII. Protein sortinga. Necessary and sufficientIII. Transport through nuclear poresIV. Transport across membranesa. Mitochondria and chloroplastb. ER membranei. Soluble proteinsii. Transmembrane proteinsV. Transport by vesiclesa. Highly specificOutline of Current Lecture I. Protein modification in the ERa. Disulfide bond formationb. GlycosylationII. Exit of proteins from the ER is controlleda. Chaperones hold onto misfolded proteinsIII. Protein modification and sorting in the GolgiThese 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 goIV. Exocytosis a. Constitutive exocytosis pathwayi. Continuousb. Regulated exocytosis pathwayi. Ex: neurotransmittersV. Endocytosisa. Phagocytosis (“eating”)i. Really large molecules like bacterial cellsb. Pinocytosis (“drinking”)i. Smaller moleculesii. Indiscriminate pinocytosis1. Continuousiii. Receptor-mediated endocytosis1. Ex: cholesterolVI. Lysosomes a. Three different pathways to a lysosomeCurrent LectureI. Protein modification in the ERa. Disulfide bond formationi. The ER lumen is an oxidizing environment, meaning it will cause atoms to lose electrons/electron density1. The cytosol is a reducing environmentii. 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 hydrogeniii. This stabilizes the proteinb. Glycosylationi. This is when a 14-sugar oligosaccharide is added to a protein ii. The main one we study is the N-linked side groupiii. This is the attachment of the 14-sugar oligosaccharide to an asparagine1. Before it’s attached to the protein, it’s bonded to dolichol in the ER membranea. Dolichol is a transmembrane lipid with two phosphate groupsb. The oligosaccharide is bonded to the last of the two phosphate groups2. The oligosaccharide is then added to the polypeptide chain in the ER lumen while it’s still being synthesized by the ribosomea. The asparagine acts as the signal moleculeb. The oligosaccharide is transferred from the dolichol to the protein by oligosaccharide protein transferaseiv. Functions1. To protect the protein from degradation2. Aids in proper folding of the proteina. Because the protein’s still being synthesized when the oligosaccharide is added3. Signal for further sorting4. Part of carbohydrate layer on the cell surfacea. Because most proteins in the ER lumen are either going to be excreted or become part of the plasma membraneII. Exit of proteins from the ER is controlleda. If a protein destined to leave the ER is misfolded, a chaperone protein will bind toit and hold onto it so that it can’t leave the ERi. It will also keep misfolded proteins from clumping togetherb. It will continue to hold onto the protein until it is properly foldedc. This can sometimes be detrimental if a protein is only a little misfolded but still could functions properlyi. An example is cystic fibrosisii. The protein that is necessary for chloride transport out of the cell is mutated so that it’s slightly misfolded, but can still function properlyiii. 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 cell1. This pump is necessary because water follows the chloride so that the mucous in the lungs and throat is thin and watery2. With cystic fibrosis, the mucous is thick so that the person has a hard time breathingd. Sometimes there are too many misfolded proteins for the chaperones to keep upwithi. When this happens, they build up in the lumen and trigger the unfolded protein responseii. This signals the cell to synthesize more chaperones and other proteins involved in properly folding proteinsIII. Protein modification and sorting in the Golgia. The golgi consists of multiple stacks of flattened membrane-enclosed sacs called cisternae or cisternab. Each sac has two facesi. Cis – the face towards the ER and nucleusii. Trans – the face towards the plasma membranec. Proteins travel through the cisternae via vesicles from the cis face to the trans facei. These vesicles use the same mechanism as the one taught in the previouslectureii. The vesicles know which cisternae to go to because the proteins are modified and have signal sequences1. They also go through glycosylationd. Vesicles that bud from the trans face either fuse with the plasma membrane or with the membranes of other organellesi. Exocytosis is when the vesicles fuse with the plasma membrane and dump their contents outside the cellIV. Exocytosisa. Constitutive exocytosis pathwayi. Continuousii. Always sending stuff out of the celliii. This supplies new lipids and proteins to the plasma membrane because the vesicles fuse with itb. Regulated exocytosis pathwayi. Only in specialized secretion cellsii. 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 thereiii. It’ll sit there until a signal tells it to fuse1. This would be when the organism needs the cargo of the vesicle outside the cell2. An example is the vesicles containing neurotransmitters in the neurons3. Another example is pancreatic beta cells that excrete insulin whenblood glucose levels are high4. The SNAREs are “hidden” by other proteins so that they don’t bindtogether immediately and dump the vesicles contents like what happens in constitutive exocytosis5. When a signal is received, the “hiding” proteins move so that the SNAREs can bondV. Endocytosisa. Phagocytosis (eating)i. This is only really big stuff like other cellsii. Especially important in immune cellsiii. 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 cella. It’s still surrounded by a layer of the cell’s plasma membrane2. The membrane that’s surrounding the bacterial/damaged cell fuses with the lysosomea. The enzymes in the lysosome then break down the bacterial celliv. Often regulatedb. Pinocytosis (drinking)i. Ingestion of