2. Microtubules are tubules made of dimers of alpha and beta tubulin units. Spindle fibers in dividing cells and eukaryotic cilia and flagella are examples of where microtubules can be used. (look!)
- In animal cells, microtubules grow out from the centrosome, an organizing center next to the nucleus. (fig. 18-6b, Lodish), (fig. 18-6c, Lodish) The centrosome contains a pair of centrioles and nucleating sites from which microtubules grow. (look!) The "minus" end, thought to be a ring of alpha units is attached to the organizing center, the centriole and the "plus" end, thought to be a ring of beta units, grows outward.
- The network of microtubules connecting cellular organelles is a path for Kinesin and Dynein to transport materials within cells. (fig. 18-27, Lodish) (Make sure you know which direction each of these molecules moves - towards nucleus or outside of cell, towards + or - end of microtubules.) These molecules hydrolyze ATP to provide the energy for movement. (fig. 18-19, Lodish), (fig. 18-22a, Lodish)
- Spindle fibers involved in chromosome movement at mitosis are microtubules. The microtubules shorten at both end - the shortening is caused by Kinesin-like proteins (KLPs). Unlike other Kinesin proteins, the KLPs do not 'walk" along microtubules but cause depolymerization. The Kinesin-like protein KLP10A causes "reeling-in" at the minus or centrosome end of the microtubules and KLP59C causes a "Pac-Man"-like depolymerization at the chromosome end. (Look here!) (Nature 427:300-301, 2004) Kinesin-13, discussed in Lodish is another protein the depolymerizes microtubules.
- Eukaryotic cilia and flagella consist of a "9+2" arrangement of microtubules connected by Dynein, which uses ATP to create a wave like bending movement. (fig. 18-29a, Lodish), (fig. 18-29b, Lodish)
- A report in Nature (Vol. 416, March 7, 2002) shows how Dynein and microtubules cooperate to pull apart the nuclear membrane in preparation for mitosis.
3. Microfilaments are filaments consisting of a double strand of globular Actin units. Each Actin unit is very similar in size to a tubulin unit. (fig. 17-5a, Lodish), (fig. 17-5bc, Lodish)
- Adaptor proteins link actin filaments to membranes. (fig. 17-18a, Lodish), (fig. 17-19b, Lodish)
- In eukaryotic cells, myosin interacts with Actin microfilaments to cause movement or contraction (fig. 17-23, Lodish) . Myosin I has just one motor domain but Myosin II has two motor domains, or heads.
- Another form of myosin, called Myosin V, walks along microfilaments, just as a person would walk across a river on stepping stones, carrying cargo with them. (Look at this link!) Myosin V is very similar to Myosin II, having two motor domains.
- In eukaryotic cells, cytokinesis occurs by use of a contractile ring of overlapping microfilaments. The overlapping microfilaments slide with respect to each other, powered by Myosin II molecules forming "bridges" between the filaments. (fig. 17-34, Lodish)
- A news feature in Nature (Vol. 436, August 25, 2005) shows that an actin ring is involved in wound healing in fruitfly embryos
4. Finally, let's talk about drugs. Colchicine prevents polymerization of Tubulin into microtubules and Taxol prevents disassembly of microtubules into individual Tubulin units. Since the spindle fibers which move chromosomes are comprised of microtubules which assemble then disassemble as the chromosomes move, these drugs are mitotic inhibitors and are used in cancer treatment to prevent cancer cells from rapidly dividing.
A well known group of drugs called cytochalasins prevents actin polymerization from the individual globular subunits. Often, a contractile process can be characterized as involving either microtubules or microfilaments by its sensitivity to either colchicine or cytochalasins, thus making these drugs valuable research tools.
All text and images, not attributed to others, including course examinations and sample questions, are Copyright, 2007, Thomas J. Herbert and may not be used for any commercial purpose without the express written permission of Thomas J. Herbert.
