20.1 Microtubule Organization and Dynamics

1. 20.1 Microtubule Organization and Dynamics By Katelyn Ward

2. Introduction Microtubule is a polymer of globular tubulin subunits . Cells have 2 populations of microtubules: stable long-lived and unstable short-lived. Stable microtubules are found in the nonreplicating cells, which have a central bundle of microtubles in cilia and flagella, that are used as extensions of the plasma membrane to propel material across the epithelial surface. Are also found in nerve cell axons that provide support and tracks for vesicles to move through the axon cytoplasm. Unstable microtubules are found in cells that need to assemble and disassemble microtubule-base structures quickly. Like in mitosis, when the spindle disassembles and the interphase microtubule network reforms.

3. Heterodimetric Tubulin Subunits Compose the Wall of a Microtubule • Building blocks of microtubules is tublin subunits composed of Heterodimer alpha and beta tubulin. • There is a third tubulin, gamma tubulin, it is not a part of the subunit, but nucleates the polymerization of subunits to form alpha-beta microtubulin. • Each tubulin is bound to each other by binding two molecules of GTP to each subunit of tubulin. • Alpha tubulin does not hydrolyze the GTP which is irreversible bounded • Beta tubulin hydrolyzes GTP to GDP, and it is reversibly bounded.

4. Instability of Microtubules • Microtubules experience instability in growth and shortening phases. • The rate of growth is much slower than the rate of shortening. • Results have shown that in vivo the individual cytosolic and mitotic microtubules show an dynamic instability. • Since most microtubules in a cell associate by their negative ends with MTCOs and their instability is largely limited to the positive ends. • MTCOs, microtubule-organizing center: is the general term for any structure that organizes microtubules in nonmitotic (interphase) cells.

5. Numerous Proteins Regulate Microtubule Dynamics and Cross-Linkage to Other Subunits • A large number of proteins that influence the assembly and stability of microtubules and their associations with other cell structures are Microtubule-Associated Proteins, MAPs • The two types of MAPs are • Stabilizes microtubules • Destabilizes microtubules • Phosphorylated MAPs are unable to bind to microtubules, they promote disassembly by MAP kinase. • MAP kinase and Cyclin-dependent kinase (CDK) both phosphorylate MAPS.

6. Numerous Proteins Regulate Microtubule Dynamics and Cross-Linkage to Other Subunits • Depending on function the two groups of MAPs are stabilizing microtubules and destabilizing microtubules. • The two domains of Stabilizing MAPs • A basic microtubule-binding domain • Has several repeats of a conserved, positively charged 4-residue amino acid that is binds to the negatively charged C-terminal part of tubulin • Mainly functions to stabilize by neutralizing the charge between the tubulin subunits and stabilizing the polymer • Found in the different types of the cells of the nervous system • A acidic projection domain • Are stabilized by the reversible phosphorylation of the domain • This done by MAP kniase and CDK (Cyclin-dependent kinase) • Found in most cell types

7. Numerous Proteins Regulate Microtubule Dynamics and Cross-Linkage to Other Subunits • Destabilizing MAPs are found in most cell types • The two types of Destabilizing MAPs • Karatin • Severs intact cytosolic microtubules by an ATP-dependant process • Op18 (stathmin) • Increases the frequency of rapid disassembly of microtubules in the mitotic spindles

9. Colchicine and Other Drugs Disrupt Microtubule Dynamics • Colchicine • From plants has been used with the synthetic relative, colcenid as an mitotic inhibitor • This causes the cell to become locked in metaphase by the cytosolic microtubules to deploymerize and leave the MTOC. • To reverse this effect the cells just have to be washed with a colcemid-free solution and the colcemid diffuses from the cell and mitosis is resumed as normal. • Taxol. • At low concentrations, taxol binds to the microtubule and stabilizes the by inhibiting their shortenings. • These microtubule inhibitors have been used in the primary treatment of many diseases. • Like grout, joint and skin diseases, and ovarian cancer

10. MTOCs Orient Most Microtubules and Determine Cell Polarity • During cell replication microtubules are arranged as: • Centrosomes • Is where the microtubules radiate out from the centrosome as spokes • Centroils • Are a type of microtubule protein found with in the centrosome. • They are located in the center of the MTOC but does not make contact with the other spindle fibers.

11. The γ-Tubulin Complex Nucleates Polymerization of Tubulin Subunits • γ-Tubulin ring complex is named for its ring like structure, which has been viewed by the electron microscope, that is composed of 8 polypeptides and measures 25nm in diameter. • γ-Tubulin complex major function is to directly nucleate microtubule assembly at subcritical concentration, so with out the γ-Tubulin complex polymerization would not take place. • It is also a major component of pericentriolar material and is able to nucleate the polymerization of tubulin in subunits to form microtubules in vitro

12. Cytoplasmic Organelles and Vesicles are Organized by Microtubules • There are four major organelles and vesicles that are organized by microtubules are • Endoplasmic reticulum (ER) • Golgi • Endosomes • Mitochondria • When a depolymerizing drug is used on the cells; • the ER loses it network organization, • the Golgi vesicles break lose and are lose in the cytoplasm, • but after the cell is washed to remove the drug the ER reforms its connection to the microtubules and the Golgi move to form the large mass of vesicles by following the microtubule tracts.

13. Tubulin belongs to the ancient family GTPases Microtubules exhibit both structural and functional polarity Dimeric alpha and beta tubulin interact end-to-end to form protofiliaments Microtubule-associated proteins (MAPs) organize and affect their stability. Various drugs affect their stability and are useful in the treatment of some diseases. γ-Tubulin Complex is a major component of pericentriolar material and is able to nucleate the polymerization of tubulin in subunits to form microtubules in vitro Overview

14. The End