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TAMU BIOL 213 - Cytoskeleton
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BIOL 213 1st Edition Lecture 21 Outline of Last 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 Golgia. 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 pinocytosisThese 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.1. Continuousiii. Receptor-mediated endocytosis1. Ex: cholesterolVI. Lysosomes a. Three different pathways to a lysosomeOutline of Current Lecture I. CytoskeletonII. Intermediate filamentsa. Structureb. Main functionsc. Two main different kindsi. Cytoplasmic and nuclear laminsIII. Microtubulesa. Structurei. Made of tubulinb. Formed from centrosomesc. Dynamic instabilityd. Intracellular transporti. Motor proteins: dynein and kenisine. Mitosis IV. Actin filamentsa. Structureb. Growth occurs at both endsc. Interaction with myosin to cause muscle contractionCurrent LectureI. Cytoskeleton a. This is like the cell’s skeleton and musclesb. It supports the cytosol and organelles c. It allows the cell to moved. There are three main types of protein filaments:i. Intermediate filaments1. Proved mechanical strengthii. Microtubules1. Create cell polarity2. Intracellular transportiii. Actin filaments1. Cell motility2. Contraction II. Intermediate filamentsa. These are only found in eukaryotesb. They are virtually indestructible c. Structurei. One α-helical region in a monomer1. The α-helix is in the middle2. The NH2 of one terminal amino acid caps one end, while the COOHof the other terminal amino acid caps the other endii. One coiled-coil dimer = 2 monomers interacting so that the α-helical region twists together1. The ends match so that the two NH2’s are at one end while the COOH’s are at the otheriii. Staggered tetramer = 2 coiled-coil dimer staggerediv. Two tetramers packed together end-to-end1. This has a springy structurev. 8 tetramers side-by-sidevi. These 8 tetramers are twisted into a rope-like structured. Main functioni. To protect cells that are subject to mechanical stress such as1. Skin and muscle cells2. Nerve cell axonsii. They are able to do this because they span the length of the cell and are attached to the desmosomes that connect the cells1. This provides structural support so that when the cell is stretched, the intermediate filament and desmosome connection keeps the cells together instead of them breaking aparte. There are different kinds of intermediate filaments based on in which cell they arei. Cytoplasmic1. Keratinsa. These are found in epithelial cells such as skin and hairi. Epidermolysis bullosa simplex is a disease in which keratin filaments don’t form properly so that the epithelial cells don’t stay connected when subjected to mechanical stressb. These are the most diversec. These are why your skin stretches2. Vimentin and vimentin-related a. These are found in connective tissue, muscle cells and glial cells3. Neurofilamentsa. These are found in nerve cellsi. The axons – these need a lot of structural support and protection because they’re so long and fragileii. Nuclear lamins1. These are found on the inside of the nuclear membrane and provide the nucleus with structural support – the nuclear laminaa. They are broken down and rebuilt for cellular replicationIII. Microtubulesa. Structurei. Tubulin heterodimer = α tubulin + β tubulin + GTP (bonded to the β tubulin)ii. Protofilament = chain of α/β tubulin heterodimers1. Plus end = end with β tubulin2. Minus end = end with α tubuliniii. Microtubule = 13 parallel protofilaments arranged to form a hollow tube b. They are formed from centrosomesi. Centrosomes are1. A pair of centrioles surrounded by2. The centrosome matrix, which has3. γ-tubulin ring complexes, which are4. The nucleating sites (starting points) of5. Microtubule synthesisii. Microtubules grow from the + end1. The α/β subunits are added to the γ-tubulin ring complexes so that the α/- end is facing the centrosome and2. The β/+ end is facing away from the centrosome3. The α of the next subunit is added to the β end of the previous subunitc. They have dynamic instabilityi. They are constantly growing and shrinkingii. Assembly and disassembly occurs only at the + ends1. An α/β heterodimer that is bound to GTP is added to the + end of a growing microtubule2. The GTP is hydrolyzed after the addition3. If the addition of heterodimers is faster than the rate of GTP hydrolysis, a GTP cap will form at the end of the growing microtubulea. This allows the microtubule to keep growing because the heterodimers bonded to GTP bind more tightly to their neighbors and are therefore more stable than when they are bonded to GDPb. This increased stability means the heterodimers will not (frequently) dissociate from the microtubulec. Presence of GTP = microtubule will grow4. Sometimes, the α/β heterodimers at the end of the growing microtubule are hydrolyzed before the addition of another heterodimera. This decreases the stability, causing the microtubule to disassembleiii. Assembly and disassembly is balanced if it is unhindered1. The cell becomes polarized when microtubules on one side of the cell attach to capping proteins2. These hold onto the microtubules and keep them from disassembling by stabilizing the + enda. An example of when this is necessary is embryo developmentb. It can tell the cell which end will become the head and which will become the taild. Intracellular transporti. Motor proteins can “walk” along the microtubule via ATP hydrolysisii. There are two main kinds of motor protein1. Dyneina. This protein always moves towards the (-) end / towards the centrosome2. Kinesina. This protein always moves towards the (+) end/ towards the plasma membraneiii. There is a difference in the proteins conformations which allows them to only move in one directioniv. Important for long distance transport1. It’s not very fast, but it’s more efficient than waiting for a vesicle to eventually wind up in the right place of the celle.


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TAMU BIOL 213 - Cytoskeleton

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