Dynein structure

Oiwa, K. and Sakakibara, H. (2005) Recent progress in dynein structure and mechanism, Curr, Opin. Cell Biol. 17, 98-103. 

One of the key achievements of the STEM program was elucidation of the dynein structure. This motor protein for microtubules consists of three head groups connected by thin threads to a baseplate and resembles a bouquet of flowers (Johnson and Wall, 1983). This approach was then applied to other systems (Witman et ai, 1983 Marchese-Ragona et ai, 1988 Hastie etai, 1988), which have all been shown to be variations on the same theme. 

Burgess S., Walker M.L.,SakakibaraH., Knight P.J., and OiwaK. 2003. Dynein structure and power stroke. Nature 421 715-718. 

FIGURE 17.5 The structure of an axoneme. Note the manner in which two microtubules are joined in the nine outer pairs. The smaller-diameter tubule of each pair, which is a true cylinder, is called the A-tubule and isjoined to the center sheath of the axoneme by a spoke structure. Each outer pair of tubules isjoined to adjacent pairs by a nexin bridge. The A-tubule of each outer pair possesses an outer dynein arm and an inner dynein arm. The larger-diameter tubule is known as the B-tubule. 

Some specialized eukaryotic cells have cilia that show a whiplike motion. Sperm cells move with one flagella, which is much longer than a cilium but has a nearly identical internal structure called axoneme. It is composed of nine doublet MTs that form a ring around a pair of single MTs. Numerous proteins bind to the MTs. Ciliary dynein motors generate the force by which MTs slide along each other to cause the bending of the axoneme necessary for motion. 

An isolated flagellum will continue to bend actively, indicating that this function is linked to its intrinsic structure. Treatment of cilia from the protozoan Tetra-hymena with the proteolytic enzyme trypsin selectively dissolves the nexin links and radial spokes but leaves unaffected the microtubules and dynein arms. If such a preparation is treated with a small amount of ATP, the loosened microtubule doublets slide against each other and through longitudinal overlap, extend for a distance that is up to nine times the original length of the cilium (Warner and Mitchell, 1981). 

Even though dynein, kinesin, and myosin serve similar ATPase-dependent chemomechanical functions and have structural similarities, they do not appear to be related to each other in molecular terms. Their similarity lies in the overall shape of the molecule, which is composed of a pair of globular heads that bind microtubules and a fan-shaped tail piece (not present in myosin) that is suspected to carry the attachment site for membranous vesicles and other cytoplasmic components transported by MT. The cytoplasmic and axonemal dyneins are similar in structure (Hirokawa et al., 1989 Holzbaur and Vallee, 1994). Current studies on mutant phenotypes are likely to lead to a better understanding of the cellular roles of molecular motor proteins and their mechanisms of action (Endow and Titus, 1992). 

Holzbaur, E.L.F. Vallee, R.B. (1994). Dyneins Molecular structure and cellular function. Ann. Rev. Cell Biol. 10,339-372. 

With regard to microtubular ultrastructure, micro filaments (5-7 run in diameter) are composed of filamentous actin. The tubule-like structures are formed by a, P-tubulin heterodimers. The wall is composed of 13 parallel protofilaments. Various microtubule-associated proteins and motor proteins (kinesin and dynein) are bound to the wall. The microtubule is a polar structure, i.e., plus and minus ends. 

Microtubules in the long axons of nerve cells function as "rails" for the "fast transport" of proteins and other materials from the cell body down the axons. In fact, microtubules appear to be present throughout the cytoplasm of virtually all eukaryotic cells (Fig. 7-32) and also in spirochetes.311 Motion in microtubular systems depends upon motor proteins such as kinesin, which moves bound materials toward what is known as the "negative" end of the microtubule,312 dyneins which move toward the positive end.310 These motor proteins are driven by the Gibbs energy of hydrolysis of ATP or GTP and in this respect, as well as in some structural details (Chapter 19), resemble the muscle protein myosin. Dynein is present in the arms of the microtubules of cilia (Fig. 1-8) whose motion results from the sliding of the microtubules driven by the action of this protein (Chapter 19). 

Figure 19-23 (A) Diagram of a cross-sectional view of the outer portion of a lamellibranch gill cilium. This has the 9+2 axoneme structure as shown in Fig. 1-8 and in (B). The viewing direction is from base to tip. From M. A. Sleigh.329 (B, C) Thin-section electron micrographs of transverse (B) and longitudinal (C) sections of wild-type Chlamydomonas axonemes. In transverse section labels A and B mark A and B subtubules of microtubule doublets oa, ia, outer and inner dynein arms, respectively sp, spokes cpp, central pair projections bk, beaks. From Smith and Sale.329a...

Fig. 7. General structures of a number of different motors (A) myosin II interacting with actin, (B) kinesin carrying a cargo and interacting with a microtubule, and (C) cytoplasmic dynein, with its associated cargo-laden dynactin complex, interacting with a microtubule.

Microtubules are the intracellular tracks for two classes of motor proteins kinesins and dyneins. During the past few years, the motor domain structures of several kinesins from different organisms have been determined by X-ray crystallography. Compared with kinesins, dyneins are much larger proteins and attempts to crystallize them have failed so far. Structural information about these proteins comes mosdy from electron microscopy. In this chapter, we mainly focus on the crystal structures of kinesin motor domains. 

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