Glass-adsorbed intact sea urchin outer arm dynein and its beta/IC1 subunit supports movement of microtubules, yet does not form a rigor complex upon depletion of ATP (16). We show here that rigor is a feature of the isolated intact outer arm, and that this property subfractionates with its alpha heavy chain. Intact dynein mediates the formation of ATP-sensitive microtubule bundles, as does the purified alpha heavy chain, indicating that both particles are capable of binding to microtubules in an ATP-sensitive manner. In contrast, the beta/IC1 subunit does not bundle microtubules. Bundles formed with intact dynein are composed of ribbon-like sheets of parallel microtubules that are separated by 54 nm (center-to-center) and display the same longitudinal repeat (24 nm) and cross-sectional geometry of dynein arms as do outer doublets in situ. Bundles formed by the alpha heavy chain are composed of microtubules with a center-to-center spacing of 43 nm and display infrequent, fine crossbridges. In contrast to the bridges formed by the intact arm, the links formed by the alpha subunit are irregularly spaced, suggesting that binding of the alpha heavy chain to the microtubules is not cooperative. Cosedimentation studies showed that: (a) some of the intact dynein binds in an ATP-dependent manner and some binds in an ATP-independent manner; (b) the beta/IC1 subunit does not cosediment with microtubules under any conditions; and (c) the alpha heavy chain cosediments with microtubules in the absence or presence of MgATP2-. These results suggest that the structural binding observed in the intact arm also is a property of its alpha heavy chain. We conclude that whereas force-generation is a function of the beta/IC1 subunit, both structural and ATP-sensitive (rigor) binding of the arm to the microtubule are mediated by the alpha subunit.

We used in vitro translocation and cosedimentation assays to study the microtubule binding properties of sea urchin sperm outer arm dynein and its beta/IC1 subunit. Microtubules glided on glass-absorbed sea urchin dynein for a period of time directly proportional to the initial MgATP2- concentration and then detached when 70-95% of the MgATP2- was hydrolyzed. Detachment resulted from MgATP2- depletion, because (a) perfusion with fresh buffer containing MgATP2- reconstituted binding and gliding, (b) microtubules glided many minutes with an ATP-regenerating system at ATP concentrations which alone supported gliding for only 1-2 min, and (c) microtubules detached upon total hydrolysis of ATP by an ATP-removal system. The products of ATP hydrolysis antagonized binding and gliding; as little as a threefold excess of ADP/Pi over ATP resulted in complete loss of microtubule binding and translocation by the beta/IC1 subunit. In contrast to the situation with sea urchin dynein, microtubules ceased gliding but remained bound to glass-absorbed Tetrahymena outer arm dynein when MgATP2- was exhausted. Cosedimentation assays showed that Tetrahymena outer arm dynein sedimented with microtubules in an ATP-sensitive manner, as previously reported (Porter, M.E., and K. A. Johnson. J. Biol. Chem. 258: 6575-6581). However, the beta/IC1 subunit of sea urchin dynein did not cosediment with microtubules in the absence of ATP. Thus, this subunit, while capable of generating motility, lacks both structural and rigor-type microtubule binding.

Dyneins are multimeric ATPases, which make up the inner and outer arms that bridge the outer doublet microtubules of eukaryotic cilia and flagella. They are responsible for the generation of sliding between outer doublets, which in turn is the basis for the formation and propagation of bending waves in both cilia and flagella. Outer arm dyneins are composed of two to three ATPases of Mr > 400,000, referred to as the α, β, and, where appropriate, γ heavy chains. Trout sperm is a new source of vertebrate dynein. Sperm can be repeatedly obtained in large quantities from the same trout (up to 4 × 10 spermatozoa per ejaculate), their axonemes can be readily isolated, and the dynein can be extracted efficiently and without significant proteolytic degradation. The advantages of trout sperm have permitted the detailed characterization of trout outer arm dynein to progress rapidly, so that it is now one of the best characterized of all dyneins. This chapter presents an overview of trout physiology and spermatogenesis for those not well acquainted with teleost physiology and anatomy and describes the methods for purification and characterization of Salmo gairdneri outer arm dynein.

Extraction of isolated axonemes from trout (Salmo gairdneri) sperm with 0.6 M NaCl removed 97% of the outer arms, approximately 12% of the protein, and approximately 50% of the MgATPase activity. Fractionation of this high salt extract by sucrose density gradient centrifugation yielded a single peak of ATPase activity with an apparent sedimentation coefficient of 19 S. Electrophoretic analysis showed that this 19 S particle was composed of two heavy chains (termed alpha and beta; Mr 430,000 and 415,000, respectively), five intermediate molecular weight chains (IC1-IC5; Mr 85,000, 73,000, 65,000, 63,000, and 57,000), and six light chains (LC1-LC6; Mr 22,000-6,000). A similar complex was obtained following further purification by DEAE-Sephacel column chromatography. Quantitative densitometry of Coomassie Blue-stained gels indicated that the heavy and intermediate chains were present in equimolar amounts. Electron microscopic examination of the 19 S particles revealed that it consisted of two globular heads joined together by a Y-shaped stem. The 19 S particle had a specific MgATPase activity of 1.1 +/- 0.3 mumol of phosphate released/min/mg and exhibited an apparent Km for MgATP2- of 40 +/- 16 microM. MnATP2- and CaATP2- were hydrolyzed at rates 100 and 80% that of MgATP2-, respectively. The Mg-ATPase activity was inhibited by vanadate, but not by ouabain or oligomycin, and exhibited a high activity between pH 7.0 and 10.0 with a maximum at pH 9.0-9.5. ATP was the preferred nucleotide, although GTP and CTP (but not ITP) did interact with the dynein to a minor extent. Based on its origin, sedimentation coefficient, polypeptide composition, and enzymatic properties, we conclude that this two-headed 19 S particle represents the entire trout sperm axonemal outer arm dynein. This dynein is probably exemplary of the outer arm dyneins of other vertebrates.

The inner and outer arms of the flagellar axoneme generate the forces needed for flagellar movement; these arms contain ATPases called dyneins. To date, there has been no method for studying the mechanochemical transducing activity of isolated dyneins. Recently, it was found that the brain microtubule-associated protein (MAP) 1C is a microtubule-activated ATPase with the structural and force-producing properties of dynein. MAP 1C translocates microtubules in an in vitro gliding assay, suggesting that such an assay could also be used with axonemal dyneins. Here, we demonstrate that outer-arm dynein isolated from sea urchin (Strongylocentrotus purpuratus) sperm and adsorbed to a glass coverslip can translocate calf-brain microtubules along the surface of the coverslip. Our results conclusively demonstrate that outer-arm dynein by itself is capable of generating shearing forces. The ability to examine the force-generating properties of flagellar dynein in vitro should greatly facilitate studies of the mechanism of action of this important mechanochemical transducer.