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 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. In the apo-state of dynein, the motor is nucleotide free, the AAA domain ring exists in an open conformation, and the MTBD exists in a high affinity state. Much about the AAA domains remains unknown, but
AAA1 is well established as the primary site of ATP hydrolysis in dynein. When ATP binds to AAA1, it initiates a conformational change of the AAA domain ring into the "closed" configuration, movement of the buttress, The linker becomes bent and shifts from AAA5 to AAA2 while remaining bound to AAA1. and kinking the stalk. Following hydrolysis of ATP, the stalk rotates, moving dynein further along the MT. Upon the release of the phosphate, the MTBD returns to a high affinity state and rebinds the MT, triggering the power stroke. The linker returns to a straight conformation and swings back to AAA5 from AAA2 and creates a lever-action, producing the greatest displacement of dynein achieved by the power stroke 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,
FIP3 and Spindly. The other tail subunits may also help facilitate this interaction as evidenced in a low resolution structure of dynein-dynactin-BicD2. One major form of motor regulation within cells for dynein is dynactin. It may be required for almost all cytoplasmic dynein functions. Currently, it is the best studied dynein partner. Dynactin is a protein that aids in intracellular transport throughout the cell by linking to cytoplasmic dynein. Dynactin can function as a scaffold for other proteins to bind to. It also functions as a recruiting factor that localizes dynein to where it should be. There is also some evidence suggesting that it may regulate kinesin-2. The dynactin complex is composed of more than 20 subunits, There is no definitive evidence that dynactin by itself affects the velocity of the motor. It does, however, affect the processivity of the motor. The binding regulation is likely allosteric: experiments have shown that the enhancements provided in the processivity of the dynein motor do not depend on the p150 subunit binding domain to the microtubules.
Axonemal dynein 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 regulation of axonemal dynein activity is critical for flagellar beat frequency and cilia waveform. Modes of axonemal dynein regulation include phosphorylation, redox, and calcium. Mechanical forces on the axoneme also affect axonemal dynein function. The heavy chains of inner and outer arms of axonemal dynein are phosphorylated/dephosphorylated to control the rate of microtubule sliding.
Thioredoxins associated with the other axonemal dynein arms are oxidized/reduced to regulate where dynein binds in the axoneme.
Centerin and components of the outer axonemal dynein arms detect fluctuations in calcium concentration. Calcium fluctuations play an important role in altering cilia waveform and flagellar beat frequency (King, 2012). ==History==