Cytoplasmic dynein has two heavy chains with globular "heads" that "walk" along the microtubule, to which they are bound by the "stalks". Dynactin (not shown) may help attach the light chains to the cargo. Interactions between the "stalks" and the microtubule must repeatedly form and break (see main text for details)
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 cytoskeletalmicrotubules towards the minus-end of the microtubule, which is usually oriented towards the cell center. Thus, they are called "minus-end directed motors." This form of transport is known as retrograde transport. In contrast, kinesins, which are motor proteins that move toward the microtubules' plus end, are called plus-end directed motors.
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. Cytoplasmic dynein 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 microtubule without detaching.
Cytoplasmic dynein positions the spindle at the site of cytokinesis by anchoring to the cell cortex and pulling on astral microtubules emanating from centrosome. Budding yeast have been a powerful model organism to study this process and has shown that dynein is targeted to plus ends of astral microtubules and delivered to the cell cortex via an offloading mechanism.
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, which has a molecular mass of about 1.5 megadaltons (MDa), is a dimer of dimers, containing 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; two 53–59 kDa light intermediate chains; and several light chains..
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, binds to the light intermediate, intermediate and light chain subunits which attach 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.
Yeast dynein can walk along microtubules without detaching, however in metazoans, cytoplasmic dynein must be activated by the binding of dynactin, another multisubunit protein that is essential for mitosis, and a cargo adaptor. The tri-complex, which includes dynein, dynactin and a cargo adaptor, is ultra-processive and can walk long distances without detaching in order to reach the cargo's intracellular destination. Cargo adaptors identified thus far include BicD2, Hook3, FIP3and Spindly. The light intermediate chain, which is a member of the Ras superfamily, mediates the attachment of several cargo adaptors to the dynein motor. The other tail subunits may also help facilitate this interaction as evidenced in a low resolution structure of dynein-dynactin-BicD2.
A cross-section of an axoneme, with axonemal dynein arms
Axonemal dyneins 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.
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).
Segregation of homologous chromosomes to opposite poles of the cell occurs during the first division of meiosis. Proper segregation is essential for producing haploid meiotic products with a normal complement of chromosomes. The formation of chiasmata (crossover recombination events) appears to generally facilitate proper segregation. However, in the fission yeast Schizosaccharomyces pombe, when chiasmata are absent, dynein promotes segregation. Dhc1, the motor subunit of dynein, is required for chromosomal segregation in both the presence and absence of chiasmata. The dynein light chain Dlc1 protein is also required for segregation, specifically when chiasmata are absent.
^Samora, CP; Mogessie, B; Conway, L; Ross, JL; Straube, A; McAinsh, AD (Aug 7, 2011). "MAP4 and CLASP1 operate as a safety mechanism to maintain a stable spindle position in mitosis.". Nature Cell Biology13 (9): 1040–50. doi:10.1038/ncb2297. PMID21822276.