chapter 15 notes
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This 2 page Class Notes was uploaded by Brenna Eisenberg on Wednesday October 5, 2016. The Class Notes belongs to 327 at Syracuse University taught by erdman in Fall 2016. Since its upload, it has received 3 views. For similar materials see Cell Biology in Biology at Syracuse University.
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Date Created: 10/05/16
Intracellular compartments and protein transport Chapter 15 Wednesday, October 5, 2019:52 PM • Membrane enclosed organelles ○ Nucleus- surrounded by nuclear envelope that is perforated by the nuclear pores. Outer nuclear membrane is continuouswith the membrane of the endoplasmic reticulum ○ Endoplasmic reticulum (ER)- site of synthesis of new membranes Rough ER has ribosomes attached to the outside. The ribosomes synthesize proteins that are sent into the ER Smooth ER- synthesizes steroid hormones, sequesterscalcium from cytosol ○ Golgi apparatus- modifies proteins and lipids from ER ○ Lysosomes- degrade particles and worn out organelles Particles pass through endosomes that organize the ingested molecules and cycle some back to the plasma membrane ○ Peroxisomes- carry out oxidative reactions, break down lipids and toxic molecules ○ Mitochondria-site of oxidative phosphorylation ○ Chloroplasts- site of photosynthesis • Organelles surrounded by cytosol that is enclosed by the plasma membrane • Organelles held in place by attachment to cytoskeleton by microtubules, moved by cytoskeletal filaments • Endomembranesystem- ER, golgi, peroxisomes, lysosomes, endosomes. ○ Communicate extensively with small vesicles that bud off • Before cell division, the proteins that are being produced must be delivered to the correct organelle • Mechanisms of protein transport: Proteins going from cytosol to nucleus go through the nuclear pores that are selective gates ○ Proteins going from cytosol to ER, mitochondria or chloroplasts get across the membrane by membrane translocators. Protein usually has to unfold to get through ○ Proteins moving from 1 compartment to another are carried in transport vesicles that pinch off from 1 compartment and fuse to another • Signaling sequence: sorting signal sends the protein to the correct organelle. Without the signal, they remain in the cytosol. Usually removed after it has been sorted. • Inner nuclear membrane- contains proteins that are binding sites for chromosomes or are an anchor for the nuclear lamina • Nuclear lamina- meshwork of protein filaments. Lines the inner part of the membrane and is structural support for the nuclear envelope. • Outer nuclear membrane- similar to the membrane of the ER that it is continuous with • Nuclear pore- composed of ~30 different proteins. Has a tangled meshwork that prevents large molecules from passing. Small molecules that are water soluble can pass freely. Large molecules need the correct sorting signal to get in ○ Nuclear localization signal is recognized by nuclear import receptors that interact with the fibrils of the pore. They then open a local passageway. ○ Import of nuclear proteins requires energy from GTP hydrolysis • Proteins must unfold to enter mitochondria and chloroplasts ○ Chloroplasts have 3rd membrane system ○ Chaperone proteins pull protein across and help fold when it is inside • Peroxisomes have enzymes that produce H2O2. Also synthesize some phospholipids ○ Their proteins are mainly imported from the cytosol ○ Membrane has a protein translocator ○ Proteins do not need to unfold • Proteins travel from cytosol to ER, then to other destinations • Once inside the ER, they then travel via vesicles ○ Cytosol to ER: water soluble proteins translocated across ER and into the lumen, as well as prospective transmembrane proteins that are partially translocated across membrane and are embedded in it. Water soluble proteinsare then destined for secretion at the cell surface or for the lumen of an organelle ○ Most of the proteins entering the ER are not completely synthesized and are still being synthesized by the ribosome of the rough ER. ○ Membrane bound ribosomes make proteins that are to be transported to ER ○ Free ribosomes make all other proteins encoded by DNA • Signal recognition particles (SRP) bind to ribosome and ER signal sequence • SRP receptors recognize the SRP • Vesicular transport-allows communication between the inside of the cell and its surroundings ○ Secretory pathway- outward. Starts with synthesis of proteins ○ Endocytic pathway- responsible for ingestion and degradation of extracellular molecules ○ Vesicles must fuse to the appropriatemembrane ○ Coated vesicles- bud from membranes and have a distinctive protein coat on the cytosolic surface. They then shed the coat and interact with the membrane it will fuse with Ex- clathrin coated vesicles. ○ Adaptins- secure clathrin coat to the vesicle membrane and also select the molecules which are to be transported. These molecules will carry transport signals. Adaptins trap the cargo receptors that bind to the cargo molecules • Vesicle docking: Chapter 15 Page 1 • Vesicle docking: ○ Identificationof the vesicles is done by Rab proteins that are on the surface. There are tethering proteins on the surface of the target membrane that recognize the Rab proteins. ○ SNAREs on the vesicle interact with complimentary SNAREs on the target membrane, which docks the vesicle in place. Also have a role in membrane fusion-- catalyze the fusion process • Exocytosis: ER (made and modified→ golgi (modified and sort→d)plasma membrane • Disulfide bonds are catalyzed in the ER. Glycosylation of proteins also occurs here. • Some proteins are kept in the ER by an ER retention signal • Unfolded protein response- occurs when there is a large buildup of misfolded proteins. The cell will produce more ER • Golgis have two faces: entry/cis face, and exit/trans face (points toward the plasma membrane). ○ Additional modification of oligosaccharide chains • Constitutive exocytosis pathway- vesicles bud from trans face of golgi and fuse with the membrane. Supplies plasma membrane with new lipids and proteins • Regulated exocytosis pathway- for secretory cells • Endocytosis: ○ Pinocytosis- "cellular drinking" ingestion of fluid and molecules ○ Phagocytosis- "eating" ingestion of large particles. Done mainly by phagocytic cells such as macrophages • Endosomes sort endocytosed molecules • Autophagy- degrades old parts of the cell Chapter 15 Page 2
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