Dynein

Dynein is a motor protein (also called molecular motor or motor molecule) in cells which converts the chemical energy contained in ATP into the mechanical energy of movement. Dynein transports various cellular cargo by "walking" along cytoskeletal microtubules towards the minus-end of the microtubule, which is usually oriented towards the cell center. Thus, they are called "minus-end directed motors," while kinesins, motor proteins that move toward the microtubules' plus end, are called plus-end directed motors.

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

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). Dynein carries organelles and microtubule fragments along the axons of neurons in a process called axoplasmic transport (Karp, 2005). It also carries HIV to the nuclei of cells that have been infected.

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.

Cytoplasmic dynein
Cytoplasmic dynein, which has a molecular mass of about 1500 kilodaltons (kDa), contains approximately 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", which is related to other AAA proteins, while two projections from the head connect it to other cytoplasmic structures. One projection, the coiled-coil stalk, binds to and "walks" along the surface of the microtubule via a repeated cycle of detachment and reattachment. The other projection, the extended tail (also called "stem"), binds to the intermediate and light chain subunits which 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 must be activated by binding of dynactin, another multisubunit protein that is essential for mitosis. Dynactin may regulate the activity of dynein, and possibly facilitates the attachment of dynein to its cargo.

Axonemal dynein


Axonemal dynein come in multiple forms that contain either one, two or three non-identical heavy chains (depending upon the 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, a coiled coil "stalk" that binds to the 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 microtubule 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. The radial spoke has been proposed as the (or one of the) structures that synchronizes this movement.

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).