Dyneins are a family ofcytoskeletalmotor proteins (though they are actually protein complexes) that move alongmicrofilaments incells. They convert the chemical energy stored inATP tomechanical work. Dyneintransports various cellular cargos, provides forces and displacements important inmitosis, and drives the beat of eukaryoticcilia andflagella. All of these functions rely on dynein's ability to move towards the minus-end of the microtubules, known asretrograde transport; thus, they are called "minus-end directed motors". In contrast, mostkinesin motor proteins move toward the microtubules' plus-end, in what is calledanterograde transport.
| Dynein heavy chain, N-terminal region 1 | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| Identifiers | |||||||||
| Symbol | DHC_N1 | ||||||||
| Pfam | PF08385 | ||||||||
| InterPro | IPR013594 | ||||||||
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| Dynein heavy chain, N-terminal region 2 | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| Identifiers | |||||||||
| Symbol | DHC_N2 | ||||||||
| Pfam | PF08393 | ||||||||
| InterPro | IPR013602 | ||||||||
| |||||||||
| Dynein heavy chain and region D6 of dynein motor | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| Identifiers | |||||||||
| Symbol | Dynein_heavy | ||||||||
| Pfam | PF03028 | ||||||||
| InterPro | IPR004273 | ||||||||
| |||||||||
| Dynein light intermediate chain (DLIC) | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| Identifiers | |||||||||||
| Symbol | DLIC | ||||||||||
| Pfam | PF05783 | ||||||||||
| Pfam clan | CL0023 | ||||||||||
| |||||||||||
| Dynein light chain type 1 | |||||||||
|---|---|---|---|---|---|---|---|---|---|
 structure of the human pin/lc8 dimer with a bound peptide | |||||||||
| Identifiers | |||||||||
| Symbol | Dynein_light | ||||||||
| Pfam | PF01221 | ||||||||
| InterPro | IPR001372 | ||||||||
| PROSITE | PDOC00953 | ||||||||
| SCOP2 | 1bkq /SCOPe /SUPFAM | ||||||||
| |||||||||
| Roadblock | |||||||||
|---|---|---|---|---|---|---|---|---|---|
 Structure of Roadblock/LC7 protein - RCSB PDB 1y4o | |||||||||
| Identifiers | |||||||||
| Symbol | Robl1, Robl2 | ||||||||
| Pfam | PF03259 | ||||||||
| InterPro | IPR016561 | ||||||||
| SCOP2 | 1y4o /SCOPe /SUPFAM | ||||||||
| |||||||||
Dyneins can be divided into two groups:cytoplasmic dyneins andaxonemal dyneins, which are also called ciliary or flagellar dyneins.
Axonemal dynein causes sliding of microtubules in theaxonemes ofcilia andflagella 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 asorganelle transport andcentrosome assembly.[1] Cytoplasmic dynein movesprocessively 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 helps to position theGolgi complex and other organelles in the cell.[1] It also helps transport cargo needed for cell function such asvesicles made by theendoplasmic reticulum,endosomes, andlysosomes (Karp, 2005). Dynein is involved in the movement ofchromosomes and positioning themitotic spindles for cell division.[2][3] Dynein carries organelles, vesicles and possibly microtubule fragments along theaxons ofneurons toward the cell body in a process called retrogradeaxonal transport.[1] Additionally, dynein motor is also responsible for the transport of degradative endosomes retrogradely in the dendrites.[4]
Cytoplasmic dynein positions the spindle at the site ofcytokinesis by anchoring to the cell cortex and pulling on astral microtubules emanating fromcentrosome. While a postdoctoral student at MIT, Tomomi Kiyomitsu discovered how dynein has a role as a motor protein in aligning the chromosomes in the middle of the cell during the metaphase of mitosis. Dynein pulls the microtubules and chromosomes to one end of the cell. When the end of the microtubules become close to the cell membrane, they release a chemical signal that punts the dynein to the other side of the cell. It does this repeatedly so the chromosomes end up in the center of the cell, which is necessary in mitosis.[5][6][7][8] 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.[9][10]
Dynein andkinesin can both be exploited by viruses to mediate the viral replication process. Many viruses use the microtubule transport system to transport nucleic acid/protein cores to intracellular replication sites after invasion host the cell membrane.[11] Not much is known about virus' motor-specific binding sites, but it is known that some viruses contain proline-rich sequences (that diverge between viruses) which, when removed, reducesdynactin binding, axon transport (in culture), and neuroinvasion in vivo.[12] This suggests that proline-rich sequences may be a major binding site that co-opts dynein.
Each molecule of the dynein motor is a complex protein assembly composed of many smallerpolypeptide 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 theATPase 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 otherAAA 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 themicrotubule 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,[14] and the MTBD exists in a high affinity state.[15] Much about the AAA domains remains unknown,[16] butAAA1 is well established as the primary site of ATP hydrolysis in dynein.[17] When ATP binds to AAA1, it initiates a conformational change of the AAA domain ring into the "closed" configuration, movement of the buttress,[14] and a conformational change in the linker.[18][19] The linker becomes bent and shifts from AAA5 to AAA2 while remaining bound to AAA1.[14][19] One attachedalpha-helix from the stalk is pulled by the buttress, sliding the helix half a heptad repeat relative to its coilled-coil partner,[15][20] and kinking the stalk.[14] As a result, the MTBD of dynein enters a low-affinity state, allowing the motor to move to new binding sites.[21][22] Following hydrolysis of ATP, the stalk rotates, moving dynein further along the MT.[18] Upon the release of the phosphate, the MTBD returns to a high affinity state and rebinds the MT, triggering the power stroke.[23] The linker returns to a straight conformation and swings back to AAA5 from AAA2[24][25] and creates a lever-action,[26] producing the greatest displacement of dynein achieved by the power stroke[18] The cycle concludes with the release of ADP, which returns the AAA domain ring back to the "open" configuration.[22]
Yeast dynein can walk along microtubules without detaching, however in metazoans, cytoplasmic dynein must be activated by the binding ofdynactin, another multisubunit protein that is essential formitosis, and a cargo adaptor.[27] 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 includeBicD2,Hook3,FIP3 and Spindly.[27] The light intermediate chain, which is a member of theRas superfamily, mediates the attachment of several cargo adaptors to the dynein motor.[28] The other tail subunits may also help facilitate this interaction as evidenced in a low resolution structure of dynein-dynactin-BicD2.[29]
One major form of motor regulation within cells for dynein is dynactin. It may be required for almost all cytoplasmic dynein functions.[30] 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.[31][32] There is also some evidence suggesting that it may regulate kinesin-2.[33] The dynactin complex is composed of more than 20 subunits,[29] of which p150(Glued) is the largest.[34] There is no definitive evidence that dynactin by itself affects the velocity of the motor. It does, however, affect the processivity of the motor.[35] 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.[36]
Axonemal dyneins come in multiple forms that contain either one, two or three non-identical heavy chains (depending upon the organism and location in thecilium). Each heavy chain has a globular motor domain with a doughnut-shaped structure believed to resemble that of otherAAA proteins, a coiled coil "stalk" that binds to the microtubule, and an extended tail (or "stem") that attaches to a neighboring microtubule of the sameaxoneme. 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. Theradial 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).[37]
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 ofhomologous chromosomes to opposite poles of the cell occurs during the first division ofmeiosis. Proper segregation is essential for producinghaploid meiotic products with a normal complement of chromosomes. The formation ofchiasmata (crossover recombination events) appears to generally facilitate proper segregation. However, in the fission yeastSchizosaccharomyces pombe, when chiasmata are absent, dynein promotes segregation.[38] Dhc1, the motor subunit of dynein, is required for chromosomal segregation in both the presence and absence of chiasmata.[38] The dynein light chain Dlc1 protein is also required for segregation, specifically when chiasmata are absent.