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Lecture

Intro To The Cytoskeleton & Microtubules - Lecture 1

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Department
Biology
Course
Biology 2382B
Professor
Jessica Kelly
Semester
Fall

Description
Intro To The Cytoskeleton & Microtubules What is The Cytoskeleton? Cytoskeleton – an intricate network of protein filaments that extend throughout the cytoplasm Three Main Types:  Microtubules – 25 nm diameter  Microfilaments – 7-9 nm diameter  Intermediate filaments – 10nm diameter The functions of these cytoskeletal components often require energy… Roles of The Cytoskeleton  Organelle/protein Trafficking  Cilia & Flagella Structure/Function  Mitosis & Cytokinesis  Muscle Contraction  Cell Adhesion  Cell Migration  Extravasation Microtubules: Structure A Polymer of a and b tubulin … • Monomers 55kDa each • Have polarity (+/- ends) Hollow tubes  25 nm diameter and up to 100s of mm long (enough to cross cell) 1 • ab dimer is the subunit (8nm in length) polymerize end to end to form the protofilaments • Dimer & protofilamet both have polarity  ends are different (i.e. one ends in a the other ends in   +/- side  refers to the polarity and also the speed at which these sides will grow or polymerize at (+ end = - end = a • 13 protofilaments  form a 25nm (diameter) hollow tube  8nm length of dimer  allows prediction of how things are moving based on the distance they move and the known size of the dimers Not positive on how these tubes form but suspected that the protofilaments form a sheet that rolls up on itself to form a tube but this is not certain… Dimeric Tubulin Subunit • Dimer is VERY stable  remain as dimer once formed whether in a microtubule or as depolymerized as free dimers (i.e. extreme conditions required to break dimer interaction) • a binds permanently to GTP   can hydrolyze GTP (i.e. can be bound to GTP or GDP)  both subunits have a binding pocket for GTP • As polymer (protofilaments) grows… ’s GTP is hydrolyzed • Polymerization energy is provided by this  subunit GTP hydrolysis… 2 Arrangement of MT Protofilaments Singlet MT most common & unstable (i.e. consistently grow/shrink) microtubule in the cell & consists of 13 protofilaments in a hollow tube Doublet MT  addition of another 10 protofilaments (second tubule)…similarly a Triplet MT can be formed with 20 protofilaments (2 new tubules)  these structures are much more stable MT’s organize the interior of the cell… Two Types: Cytoplasmic – MT’s that composes structure within the cytoplasm Axonemal (i.e. cilia) – MT’s that refer only to cilia and flagella (nothing to do with axons) Microtubule Organization Centrosome - the major microtubule-organizing center (MTOC) in animal cells (not found in plants) • Centrosomes contain centrioles • Centrioles  made up of triplet oicrotubules and are part of the MTOC, cylindrical structures oriented 90 from one another & surrounded by PCM Pericentriolar Matrix/Material (PCM)  an amorphous mass of protein which makes up the part of the animal centrosome that surrounds the two centrioles • PCM contains proteins responsible for microtubule nucleation and anchoring including γ-tubulin, pericentrin and ninein 3 Centriole Details Made of Triplet microtubules  usually have an A, B, and C tubule Can divide and give rise to a mother and a daughter (mitosis) Mother centriole  has more proteins on it with unknown roles but gives it a structural difference from the daughter centriole (can be identified The MTOC functions to nucleate (start) the assembly of microtubules (a dynamic process)… • Centrosome is the MTOC of animal cells (non-mitotic cells) • Upon mitosis  the microtubules are reorganized and the centrosome gives rise to the spindle poles (i.e. replication of centrosome/centrioles) Spindle poles - the MTOC’s of mitosis  they are not centrosomes  consist of different proteins and thus have different properties… 4 Minus (-) ends of microtubules are associated with MTOC (note dendrites are different)… -Tubulin Nucleates Polymerization  -tubulin ring complex (TuRC) – multi-protein complex that provides nucleating site for MT’s • tubulin forms a TuRC which is involved in starting or nucleating the polymerization and growth of the microtubule  this occurs in the centrosome/PCM… Unsure of exactly how it works… diagram is an artistic depiction… Microtubule Formation MTs assembly and disassembly is important to their function… Subunit critical concentration (Cc) and temperature important… Polymerization is temperature dependent…Lower the temperature  C incceases… (i.e. microtubules disassemble when chilled to 4°C = stop polymerization completely) 5 • Polymerization is based on the concentration of dimers C c critical concentration - the concentration of dimers required for polymerization/formation of microtubules • Above the C cnew dimers being made are added to the growing microtubules • Dimer concentration no longer changes  new ones being made that would increase it are simply added to the growing microtubule • Change in temperature is not used by cells to regulate microtubule growth • Cells maintain at constant temperature Cells regulate MT growth by increasing or decreasing the concentration of dimers in the cell (protein synthesis)… • Growth will occur at + end as it has a lower C  growth is faster c Polarity of Tubulin Polymerization 6 • Microtubule assembly and disasse
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