Friday, November 16, 2007

Cell Movements I: Subcellular Freeways

Reading:

Pages 572 - 574, 579 - 594 (Ch. 17 through "Actin and Tubulin Polymerize by Similar Mechanisms", excluding"Intermediate Filaments" sections).

Objectives:

It has become increasingly clear that cells are highly organized.

    • Early biochemists: a "bag of enzymes"
    • Later biochemists: many enzymes are in organelles, but the organelles float around freely
    • Current wisdom: organelles are arranged in specific ways

It is also clear that cells and intracellular structures are extremely dynamic.

The system that allows cells to be organized and dynamic is the cytoskeleton.

    • Consists of a network of interconnected tubules and filaments in the cytosol.
    • Gives cells shape and mechanical strength.
    • Serves as a scaffold for the movement of organelles.

The two major classes of cytoskeletal elements are microtubules and microfilaments. (A third class is intermediate filaments, but they are more specialized and variable in function.)

Microtubules:

    • 25 nm in diameter (about 1/4 the diameter of a clathrin-coated vesicle)
    • made up of polymerized tubulin

Microfilaments:

    • 7 nm in diameter
    • made up of polymerized actin

Microtubule assembly is initiated at the microtubule-organizing center (MTOC), a.k.a. centrosome.
Microtubules (and actin filaments) have a polarity: the two ends are different.

    • One end is arbitratily called plus ("+"), the other minus ("-").
    • The minus end of every microtubule is anchored in the MTOC. Thus, microtubules can only grow at their plus ends.

Microtubules are actually composed of two kinds of tubulin: alpha-tubulin and beta-tubulin.

    • The unit of microtubule assembly is a dimer of alpha- and beta-tubulin.
    • Tubulin dimers assemble end-to-end to form protofilaments.
    • About 13 protofilaments line up to form a hollow cylindrical microtubule.

Tubulin dimers associate reversibly with the plus-end of a microtubule.

    • Microtubules show two kinds of behaviors: growth, and catastrophic shrinkage.
    • Microtubule growth is mediated by the addition of GTP-containing tubulin dimers to the plus end.
    • After incorporation of the tubulin dimer into the microtubule, the GTP is hydrolyzed. As a result, microtubules are composed mainly of GDP-bound tubulin, with a GTP-tubulin "cap".
    • GDP-bound tubulin dimers are stable in the interior of a microtubule, but will dissociate rapidly from the end.
    • If addition of GTP-tubulin slows, the cap may disappear, and GDP-tubulin will rapidly dissociate, causing catastrophic shrinkage of the microtubule.
    • This dynamic instability is maintained by the input of energy, in the form of GTP hydrolysis.
      • Energy is always needed to keep a system away from equilibrium.
      • The advantage to the cell is increased flexibility: the microtubule network can be remodeled rapidly.
    • In some cases, stable microtubule structures are maintained by addition of "capping proteins" to the plus end.
    • Specific drugs interfere with microtubule function:
      • Colchicine and nocodazole inhibit microtubule polymerization. Because of dynamic instability, the microtubules soon disappear.
      • Taxol stabilizes microtubules in the polymerized form.

One function of microtubules is to help define cell shape. A classic example is cilia and flagella: in these structures, the microtubules truly act as a skeleton, like the bones in your fingers.

A second function of microtubules is to serve as tracks for moving organelles. Examples:

    • Golgi elements move inward toward the MTOC to form a ribbon next to the nucleus.
    • Organelles such as mitochondria are transported long distances along axons.
    • The reticular structure of the ER is generated by pulling on the membrane to form tubules.

These movements are mediated by microtubule-dependent motor proteins:

    • Kinesin moves along microtubules in the plus direction
    • Dynein moves along microtubules in the minus direction
    • ATP hydrolysis powers these force-generating motor proteins

Organelle movements and positioning make the cell more efficient.

    • Example: mitochondria are placed where the requirement for ATP is highest

Microfilaments are simpler in many ways than microtubules:

    • They are composed of only one species of actin.
    • Microfilaments are only two actin molecules thick (although they can be cross-linked into bundles).
    • Both ends of microfilaments can undergo subunit addition and loss.
    • Microfilaments do not all radiate from a central location. Instead, they form fibers that run underneath the plasma membrane ("cortex") of the cell.

Microtubules and microfilaments have partially overlapping functions.

    • Like microtubules, microfilaments can determine cell shape; eg., microvilli
    • Organelles can be transported along microfilaments by means of myosin motors

However, each types of cytoskeletal element also has unique functions:

    • Microtubules uniquely mediate chromosome movements during mitosis
    • Microfilaments uniquely function in stress transmission between cells in a tissue


In general, cells use microtubules and microfilaments for many different functions, depending upon the cell type.

Text Problems:

Questions 17-2, 17-16, 17-19

bsglick@midway.uchicago.edu