TAMU BIOL 213 - Cytoskeleton (9 pages)

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Lecture number:
Lecture Note
Texas A&M University
Biol 213 - Molecular Cell Biol
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BIOL 213 1st Edition Lecture 21 Outline of Last 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 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 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 1 Continuous iii Receptor mediated endocytosis 1 Ex cholesterol VI Lysosomes a Three different pathways to a lysosome Outline of Current Lecture I Cytoskeleton II Intermediate filaments a Structure b Main functions c Two main different kinds i Cytoplasmic and nuclear lamins III Microtubules a Structure i Made of tubulin b Formed from centrosomes c Dynamic instability d Intracellular transport i Motor proteins dynein and kenisin e Mitosis IV Actin filaments a Structure b Growth occurs at both ends c Interaction with myosin to cause muscle contraction Current Lecture I II Cytoskeleton a This is like the cell s skeleton and muscles b It supports the cytosol and organelles c It allows the cell to move d There are three main types of protein filaments i Intermediate filaments 1 Proved mechanical strength ii Microtubules 1 Create cell polarity 2 Intracellular transport iii Actin filaments 1 Cell motility 2 Contraction Intermediate filaments a These are only found in eukaryotes b They are virtually indestructible c Structure i One helical region in a monomer 1 The helix is in the middle 2 The NH2 of one terminal amino acid caps one end while the COOH of the other terminal amino acid caps the other end ii One coiled coil dimer 2 monomers interacting so that the helical region twists together 1 The ends match so that the two NH2 s are at one end while the COOH s are at the other iii Staggered tetramer 2 coiled coil dimer staggered iv Two tetramers packed together end to end 1 This has a springy structure v 8 tetramers side by side vi These 8 tetramers are twisted into a rope like structure d Main function i To protect cells that are subject to mechanical stress such as 1 Skin and muscle cells 2 Nerve cell axons ii They are able to do this because they span the length of the cell and are attached to the desmosomes that connect the cells III 1 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 apart e There are different kinds of intermediate filaments based on in which cell they are i Cytoplasmic 1 Keratins a These are found in epithelial cells such as skin and hair i 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 stress b These are the most diverse c These are why your skin stretches 2 Vimentin and vimentin related a These are found in connective tissue muscle cells and glial cells 3 Neurofilaments a These are found in nerve cells i The axons these need a lot of structural support and protection because they re so long and fragile ii Nuclear lamins 1 These are found on the inside of the nuclear membrane and provide the nucleus with structural support the nuclear lamina a They are broken down and rebuilt for cellular replication Microtubules a Structure i Tubulin heterodimer tubulin tubulin GTP bonded to the tubulin ii Protofilament chain of tubulin heterodimers 1 Plus end end with tubulin 2 Minus end end with tubulin iii Microtubule 13 parallel protofilaments arranged to form a hollow tube b They are formed from centrosomes i Centrosomes are 1 A pair of centrioles surrounded by 2 The centrosome matrix which has 3 tubulin ring complexes which are 4 The nucleating sites starting points of 5 Microtubule synthesis ii Microtubules grow from the end 1 The subunits are added to the tubulin ring complexes so that the end is facing the centrosome and 2 The end is facing away from the centrosome 3 The of the next subunit is added to the end of the previous subunit c They have dynamic instability i They are constantly growing and shrinking ii Assembly and disassembly occurs only at the ends 1 An heterodimer that is bound to GTP is added to the end of a growing microtubule 2 The GTP is hydrolyzed after the addition 3 If the addition of heterodimers is faster than the rate of GTP hydrolysis a GTP cap will form at the end of the growing microtubule a 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 GDP b This increased stability means the heterodimers will not frequently dissociate from the microtubule c Presence of GTP microtubule will grow 4 Sometimes the heterodimers at the end of the growing microtubule are hydrolyzed before the addition of another heterodimer a This decreases the stability causing the microtubule to disassemble iii Assembly and disassembly is balanced if it is unhindered 1 The cell becomes polarized when microtubules on one side of the cell attach to capping proteins 2 These hold onto the microtubules and keep them from disassembling by stabilizing the end a An example of when this is necessary is embryo development b It can tell the cell which end will become the head and which will become the tail d Intracellular transport i Motor proteins can walk along the microtubule via ATP hydrolysis ii There are two main kinds of motor protein 1 Dynein a This protein always moves towards the end towards the centrosome IV 2 Kinesin a This protein always moves towards the end towards the plasma membrane iii There is a difference in the proteins conformations which allows them to only move in one direction iv Important for long distance transport 1 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 cell e Mitosis i Mitotic spindles position the chromosomes and pull them apart during mitosis ii The ability of microtubules to assemble and disassemble constantly is essential for this iii

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