Dynein

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Dynein is one of the motor proteins (also called molecular motors) in biological cells that is able to convert the chemical energy contained in ATP into the mechanical energy of movement. Dyneins' function is to transport various cellular cargos along cytoskeletal microtubules towards the central region of the cell, where the minus end of the microtubule is usually located. Thus they are called are minus-end directed motors, while kinesins, motor proteins that move toward the microtubules' plus end, are called plus-end directed motors. Dynein is involved in retrograde axoplasmic transport, bringing materials from neurons' synapses to their cell bodies.

Dyneins can be divided into two groups: cytoplasmic dyneins and axonemal dyneins, which are also called ciliary or flagellar dyneins. Image:Cytoplasmic dynein.PNG

1 Function
2 Structure
- 2.1 Cytoplasmic dynein
- 2.2 Ciliary dynein
3 History
4 References
5 External links

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Function

Axonemal dynein causes sliding of microtubules in the axonemes of cilia and flagella and is found only in cells that have those structures. Cytoplasmic dynein, found in all animal cells and possibly plant cells as well, performs functions necessary for cell survival such as organelle transport and centrosome assembly (Karp, 2005).

Cytoplasmic dynein probably moves processively along the microtubule; that is, one or the other of its stalks is always attached to the microtubule so that the dynein can "walk" a considerable distance along a tubule without detaching.

Cytoplasmic dynein probably helps to position the Golgi complex and other organelles in the cell (Karp, 2005). It also helps transport cargo needed for cell functioning such as vesicles made by the endoplasmic reticulum, endosomes, and lysosomes (Karp, 2005). Dynein is also probably involved in the movement of chromosomes and positioning the mitotic spindles for cell division (Karp, 2005). In addition to carrying organelles and other products along microtubules, dynein is also capable of carrying lengths of microtubule, as it probably does in neurons (Karp, 2005). It also carries the HIV virus to the nuclei of cells that have been infected.

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Structure

Each molecule of the dynein motor is a complex protein assembly composed of many smaller polypeptide subunits. Cytoplasmic and axonemal dynein contain some of the same components, but they also contain some unique subunits.

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Cytoplasmic dynein

Cytoplasmic dynein, which has a molecular mass of about 1500 kilodaltons (kDa), contains approximtely twelve polypeptide subunits: two identical "heavy chains," 520 KDA in mass, which contain the ATPase activity and are thus responsible for generating movement along the microtubule; two 74 kDa intermediate chains which are believed to anchor the dynein to its cargo; four 53-59 kDa intermediate chains and several light chains which are less well-understood.

The force-generating ATPase activity of each dynein heavy chain is located in its large doughnut-shaped "head", that is related to other AAA proteins, while two projecting extending from the head connect it to other cytoplasmic structures. One projection, the coiled-coil stalk, binds to the surface of a microtubule. where repeated cycles of detachment and reattachment to successive sites along the length cause the dynein to move along the microtubule. The other projection the extended tail (also called "stem"), binds to the intermediate and light chain subunits that function to attach the dynein to its cargo. The alternating activity of the paired heavy chains in the complete cytoplasmic dynein motor enables a single dynein molecule to transport its cargo by "walking" a considerable distance along a microtubule without becoming completely detached.

In eukaryotes, cytoplasmic dynein needs to be activated by binding of dynactin, another multisubunit protein that is essential for mitosis. Dynactin may regulate the activity of dynein, and possiblty facilitates the attachment of dynein to its cargo.

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Ciliary dynein

Ciliary dynein come in multiple forms that contain either one, two or three non-identical heavy chains (depending upon organism and location in the cilium). Each heavy chain has a globular motor domain with a doughnut-shaped structure believed to resemble that of other AAA proteins as well as a coiled coil stalk that binds to one microtubule and an extended tail (or "stem") that attaches to a neighboring microtubule of the same axoneme. Each dynein molecule thus forms a cross-bridge between two adjacent microtubules of the ciliary axoneme. During the "power stroke", which causes movement, the AAA ATPase motor domain undergoes a conformational change that causes the microtubule-binding stalk to pivot relative to the cargo-binding tail with the result that one tubule slides relative to the other (Karp, 2005). This sliding produces the bending movement needed for cilia to beat and propel the cell or other particles. Groups of dynein molecules responsible for movement in opposite directions are probably activated and inactivated in a coordinated fashion so that the cilia or flagella can move back and forth.

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History

The protein responsible for movement of cilia and flagella was first discovered and named dynein in 1963 (Karp, 2005). 20 years later, cytoplasmic dynein, which had been suspected to exist since the discovery of flagellar dynein, was isolated and identified (Karp, 2005).

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References

Karp G. Cell and Molecular Biology: Concepts and Experiments, Fourth ed, pp. 346-358. John Wiley and Sons, Hoboken, NJ. 2005.
Schroer, Trina A. DYNACTIN Annual Review of Cell and Developmental Biology 2004 20, 759-779 Template:Entrez Pubmed

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