Cytoskeleton

The cytoskeleton is made up of 3 major elements; microfilaments ("actin") filaments, intermediate filaments, and microtubules.

Microfilaments: are the smallest component of the cytoskeleton ranging in size of about 7 nm in diameter. They are made up of actin and have a plus/barbed fast growing end, and a minus/pointed slow growing end. The addition of actin monomers occurs at the + end and is an ATP dependent process. Polymerization into filamentous f-action occurs when the concentration of globular g-actin is above a critical concentration.

Actin associated proteins perform many functions in arranging and stabilizing microfilaments including crosslinking (by filamen), bundling (the crosslinking of actin filaments into a parallel array as by α actinin and fimbrin), capping, severing and movement of structures along the fiber.

Muscle contraction depends on the ATP driven sliding of highly organized arrays of actin thin filaments against arrays of myosin thick filaments.

The zonula adherens/belt desmosome is an achorage junction that has a beltlike distribution and is associated with actin filaments.

The drug, phalloidin, from the fungus Amanita phaloides (death cap), prevents the depolymerization of actin filaments. cytochalessins bind to the fast growing end preventing further addition of G-actin

Intermediate Filaments: are intermediate in size compared to microfilaments and microtubules ranging with a diameter of about 10 nm. There are different types of intermediate filaments depending on the tissue where they are found.

  • cytokeratins are found in epithelia cells.

  • vementin in connective tissue.

  • desmin in skeletal muscle

  • neurofilaments in axons and dendrites of nerves

  • nucleur lamins in nuclear lamina

The macula adherens/spot desmosome is an achorage junction which has a spot like distribution and is associated with intermediate filaments.

Microtubules: are formed from protein subunits of tubulin. The tubulin subunit is itself a heterodimer formed from two closely related proteins called α-tubulin (exposed at the + end) and β-tubulin (exposed at the -end) tightly bound by noncovalent bonds. A microtubule is a stiff, hollow cylindrical structure built from 13 parallel protofilaments (long linear strings of subunits joined end to end that associate with one another laterally) They are larger than either microfilaments and intermediate filaments with a diameter of 25 nm.  Addition of tubulin is a GTP dependent process unlike with the ATP dependent process of actin.

Microtubules have one end attached to a single microtubule organizing center (MTOC) called a centrosome which is made up of 2 centrioles near the nucleus.

The motor proteins kinesin and dynein move on microtubules and are involved with organelle transport. kinesins typically move from the cell body or centrosome toward the periphery of the cell towards the plus end of the tubule (anterograde) whereas dyneins typically move from the periphery toward the centrosome or minus end of the tubule (retrograde)

Microtubules and the protein dynein also make up the motility structures, cilia and flagella. flagella are found on sperm and enable these cells to move. Cilia tend to be shorter than flagella. Both flagella and cilia have a distinctive 9 (doublet) + 2 arrangement of microtubules. Microtubules are also necessary in mitosis.

A number of chemical agents also inhibits microtubule dynamics such as colchicine which binds to tubulin dimers preventing their assembly into microtubules.

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