Activation of the dynein adenosinetriphosphatase by microtubules

Mechanochemical coupling in eukaryotic flagella

Quantitative analyses of ATP hydrolysis coupled to movement of eukaryotic flagella is important for understanding the relationship between ATP hydrolysis and movement. The difference in ATPase activity between intact motile axonemes (that is the cytoskeletal core of flagella) and homogenized or immotile axonemes has been assumed to be coupled to movement. However, recent findings on rates of steps in the dynein ATPase cycle and the effect of interaction with microtubules on those steps call for reassessment of movement-coupled ATPase. From these studies, it is clear that dynein ATPase activity is not as tightly coupled to interaction with microtubules as myosin ATPase activity is coupled to interaction with actin. The method by which axonemal movement is inhibited will critically affect the interpretation of difference in ATPase activity. If the homogenization or similar methods uncouple dynein, the difference in ATPase activity is not a useful measurement. Greater understanding of the relationship between dynein kinetics and axonemal movement may be obtained by use of conditions and substrates with known effects at specific steps in the dynein mechanochemical cycle and quantitating their effects on movement.

Preparation of microtubules from rat liver and testis: cytoplasmic dynein is a major microtubule associated protein

A microtubule associated protein from brain tissue (MAP 1C), has been found to possess many properties in common with ciliary and flagellar dyneins (Paschal et al.:J. Cell Biol. 105:1273-1282, 1987). However, this protein, now designated as cytoplasmic dynein, exhibited several properties which distinguish it from axonemal forms of the enzyme. We have investigated these characteristics further in a study of cytoplasmic dyneins from non-neuronal tissues. Rat liver and testis in particular were found to contain high levels of cytoplasmic dynein. The yield of dynein from testis was over 70 micrograms/g of tissue, making this the best source of cytoplasmic dynein of all tissues so far examined. The characterization of dynein from these sources has confirmed and extended our previous observations concerning the unique properties of cytoplasmic dynein. Activation of liver and testis dynein occurred at low (less than 1 mg/ml) tubulin concentration. Polypeptides identified as subunits of brain cytoplasmic dynein (74, 59, 57, 55, and 53 kDa) were present in liver and testis preparations. In addition, polypeptides at 150 and 45 kDa were found to copurify with the non-neuronal dyneins. The liver and testis enzyme hydrolyzed pyrimidine nucleotides at rates up to 12.5 times faster than ATP, though the relative affinity of cytoplasmic dynein for CTP was much lower (Km = 1.0 mM) than that for ATP. The properties of the testis enzyme were consistent with its identification as a cytoplasmic dynein rather than a sperm axonemal precursor. These data indicate that cytoplasmic dyneins may be widespread in distribution and that they share certain biochemical properties unique from those of axonemal dyneins. These characteristics are consistent with the proposal that cytoplasmic dynein plays a universal role in retrograde organelle motility.

2-Chloro adenosine triphosphate as substrate for sea urchin axonemal movement

The 2-substituted ATP analog 2-Chloro ATP was tested for its capacity to support axonemal movement. The movement of sea urchin axonemes reactivated with 2-Cl ATP appeared very similar to that with ATP. Detailed waveform analysis indicated that bend angle and shear amplitude were not significantly different for ATP and 2-Cl ATP. Although wavelength differs at particular nucleotide concentrations, if normalized to the beat frequency, it is similar for ATP and 2-Cl ATP. The main difference in the movement with the two analogs was seen in beat frequency and sliding velocity. The Vmax for beat frequency and mean sliding velocity was lower for 2-Cl ATP. The apparent Km for beat frequency and sliding velocity was much lower for 2-Cl ATP. The ratio of these two effects, that is, (Vmax/Km) is higher for 2-Cl ATP. Thus 2-Cl ATP is a good substrate for axonemal movement. The significantly lower Km of 2-Cl ATP was also demonstrated by its ability to support oscillatory motion at concentrations below that for ATP. The observations identify the structures and conformation of substrate necessary to support axonemal movement.

The role of axonemal components in ciliary motility

1. The axoneme is the detergent-insoluble cytoskeleton of the cilium. 2. All axonemes generate movement by the same fundamental mechanism: microtubule sliding utilizing ATP hydrolysis during a mechanochemical cycling of dynein arms on the axonemal doublets. 3. Structure, fundamental biochemistry and physiology of the axoneme are conserved evolutionarily, but the phenotypes of beating movements and the responses to specific cytoplasmic signals differ greatly from organism to organism. 4. A model of asynchronous dynein arm activity--the switch point hypothesis--has been proposed to account for cyclic beating in the face of unidirectional sliding. The model suggests that the diversity of beat phenotype may be explicable by changes in the timing of switching between active and inactive states of doublet arm activity. Evidence of axonemal splitting in arrested axonemes provides new support for the hypothesis.

Characterization of the microtubule-activated ATPase of brain cytoplasmic dynein (MAP 1C)

We recently found that the brain cytosolic microtubule-associated protein 1C (MAP 1C) is a microtubule-activated ATPase, capable of translocating microtubules in vitro in the direction corresponding to retrograde transport. (Paschal, B. M., H. S. Shpetner, and R. B. Vallee. 1987b. J. Cell Biol. 105:1273-1282; Paschal, B. M., and R. B. Vallee. 1987. Nature [Lond.]. 330:181-183.). Biochemical analysis of this protein (op. cit.) as well as scanning transmission electron microscopy revealed that MAP 1C is a brain cytoplasmic form of the ciliary and flagellar ATPase dynein (Vallee, R. B., J. S. Wall, B. M. Paschal, and H. S. Shpetner. 1988. Nature [Lond.]. 332:561-563). We have now characterized the ATPase activity of the brain enzyme in detail. We found that microtubule activation required polymeric tubulin and saturated with increasing tubulin concentration. The maximum activity at saturating tubulin (Vmax) varied from 186 to 239 nmol/min per mg. At low ionic strength, the Km for microtubules was 0.16 mg/ml tubulin, substantially lower than that previously reported for axonemal dynein. The microtubule-stimulated activity was extremely sensitive to changes in ionic strength and sulfhydryl oxidation state, both of which primarily affected the microtubule concentrations required for half-maximal activation. In a number of respects the brain dynein was enzymatically similar to both axonemal and egg dyneins. Thus, the ATPase required divalent cations, calcium stimulating activity less effectively than magnesium. The MgATPase was inhibited by metavandate (Ki = 5-10 microM for the microtubule-stimulated activity), 1 mM NEM, and 1 mM EHNA. In contrast to other dyneins, the brain enzyme hydrolyzed CTP, TTP, and GTP at higher rates than ATP. Thus, the enzymological properties of the brain cytoplasmic dynein are clearly related to those of other dyneins, though the brain enzyme is unique in its substrate specificity and in its high sensitivity to stimulation by microtubules.

Regulation of axonemal Mg2+-ATPase from Paramecium cilia: effects of Ca2+ and cyclic nucleotides

Ciliary activity is regulated by Ca2+ and cyclic nucleotides, but the molecular mechanisms of the regulation are unknown. We have tested the ability of Ca2+ and cyclic nucleotides to alter ciliary Mg2+-ATPase or to stimulate phosphorylation of axonemal dynein. Mg2+-ATPase activity in cilia and axonemes from Paramecium was stimulated 2-fold by micromolar Ca2+, but this Ca2+ sensitivity was lost upon solubilization of the dyneins from the axoneme. The Ca2+-sensitive component of ciliary Mg2+-ATPase activity was inhibited by the dynein inhibitors vanadate and Zn2+, but was insensitive to the calmodulin antagonists calmidazolium and melittin. Dynein activity in the high-salt extract from axonemes was also insensitive to calmidazolium. Calmodulin did not sediment with 22 S or 12 S dyneins on sucrose gradients containing Ca2+, but it did sediment in the region from 19 S to 14 S. Mg2+-ATPase activity in ciliary fractions was unaltered in the presence of cAMP or cGMP. However, polypeptides associated with the 22 S and 12 S dyneins, as well as proteins of 19 S, 15 S, and 8 S, were substrates for endogenous ciliary kinases. High molecular weight polypeptides that sedimented at 22 S and 19 S were phosphorylated in a cyclic nucleotide-stimulated manner.

An ATPase with properties expected for the organelle motor of the giant amoeba, Reticulomyxa

The rapid, vectorial, microtubule-associated transport of organelles is believed to be mediated by specific mechanochemical transducers. Recent studies of various metazoan cells have allowed the identification of novel microtubule-dependent translocator molecules capable of promoting microtubule gliding across glass surfaces and translocation of inert beads along microtubules. These translocators could be involved in force generation for directional organelle movements in vivo. Here we report the identification of a microtubule-binding protein with characteristics expected for an organelle translocator in the giant freshwater amoeba Reticulomyxa. This factor has an apparent relative molecular mass (Mr) of 440,000 (440K) and sediments at 20-22S in sucrose-density gradients. It binds to microtubules under conditions of ATP depletion, possesses an ATPase activity and is sensitive to ultraviolet-induced, vanadate-dependent cleavage. Although its pharmacological properties differ from those of axonemal dynein, it can be considered to be a variant of cytoplasmic dynein. The Reticulomyxa high-molecular-weight protein (HMWP) promotes rapid, bidirectional movement of latex beads along Reticulomyxa microtubules in vitro at an average speed of 3.6 micron s-1. This protein, therefore, is a likely candidate for a microtubule-dependent motor.

Rotation and translocation of microtubules in vitro induced by dyneins from Tetrahymena cilia

Dynein, the force-generating enzyme that powers the movement of cilia and flagella, has been characterized biochemically, but no simple system has been available for examining its motile properties. Here we describe a quantitative in vitro motility assay in which dynein adsorbed onto a glass surface induces linear translocation of purified bovine microtubules. Using this assay, we show that both 22S and 14S dyneins from Tetrahymena cilia induce movement but have distinct motile properties. A unique property of 14S dynein, which has not been described for other motility proteins, is its ability to generate torque that causes microtubules to rotate during forward translocation. In the axoneme, 14S dynein-induced torque may induce rotation of central-pair microtubules and may play an important role in generating three-dimensional ciliary beating patterns.

Kinetics of vanadate dissociation: estimation of the rate by inhibitor inactivation

Vanadate (+5) is a potent inhibitor of a variety of ATPases including dynein ATPase. We describe a method useful for estimating the functional dissociation rate of vanadate from the active site which does not rely on classical physical separation techniques. The method involves spectrophotometrically monitoring the enzymatic activity as the inhibitor dissociates from the enzyme and is inactivated by norepinephrine. Norepinephrine effectively reverses vanadate inhibition by reducing vanadate (+5) to oxovanadium (+4). This reduction by norepinephrine is sufficiently fast for these purposes--addition of vanadate after norepinephrine shows no inhibition of ATPase activity. The mathematical estimation procedure is generally useful for estimation of dissociation rates of other reversible inhibitors which can be quickly inactivated after dissociation from the enzyme. The rate of dissociation of vanadate from dynein with ATP and 2-N3ATP as substrates using this method was estimated to be in the ranges 0.0023-0.0042 and 0.0057-0.0075 s-1, respectively. These rates permit estimation of the rates of vanadate association with dynein by using the reported dissociation constant for vanadate. The results are consistent with the very fast and potent inhibition of dynein ATPase activity observed.

MAP 1C is a microtubule-activated ATPase which translocates microtubules in vitro and has dynein-like properties

We observe that one of the high molecular mass microtubule-associated proteins (MAPs) from brain exhibits nucleotide-dependent binding to microtubules. We identify the protein as MAP IC, which was previously described in this laboratory as a minor component of standard microtubule preparations (Bloom, G.S., T. Schoenfeld, and R.B. Vallee, 1984, J. Cell Biol., 98:320-330). We find that MAP 1C is enriched in microtubules prepared in the absence of nucleotide. Kinesin is also found in these preparations, but can be specifically extracted with GTP. A fraction highly enriched in MAP 1C can be prepared by subsequent extraction of the microtubules with ATP. Two activities cofractionate with MAP 1C upon further purification, a microtubule-activated ATPase activity and a microtubule-translocating activity. These activities indicate a role for the protein in cytoplasmic motility. MAP 1C coelectrophoreses with the beta heavy chain of Chlamydomonas flagellar dynein, and has a sedimentation coefficient of 20S. Exposure to ultraviolet light in the presence of vanadate and ATP results in the production of two large fragments of MAP 1C. These characteristics suggest that MAP 1C may be a cytoplasmic analogue of axonemal dynein.

Bovine brain kinesin is a microtubule-activated ATPase

Recently, a protein called kinesin was described, which is capable of inducing movement of inert particles along microtubules. To purify this protein from bovine brain, we used the ability of kinesin to bind to taxol-stabilized microtubules in the presence of inorganic tripolyphosphate. The brain kinesin preparation contained one major polypeptide of 135 kDa and four minor polypeptides of 45-70 kDa. The minor polypeptides were eluted from a gel-permeation chromatography column at the same position as the major component. All the polypeptides of the preparation were capable of binding to the microtubules under identical conditions. The kinesin molecule is most probably a complex of these polypeptides. Brain kinesin had a very low ATPase activity (0.06-0.08 mumol X min-1 X mg-1 in 3 mM Mg2+ at pH 6.7). ATPase activity was strongly stimulated by microtubules (Vmax = 4.6 mumol per min per mg of kinesin). Microtubule-activated kinesin ATPase had a Km for ATP between 10 and 12 X 10(-6) M and a Kapp for microtubules (i.e., polymerized tubulin concentration required for a half-maximal activation) of 12-14 X 10(-6) M. Kinesin had a significant ATPase activity even without microtubules if 2 mM Ca2+ was substituted for Mg2+ (Vmax = 1.6 mumol X min-1 X mg-1; Km = 800 X 10(-6) M). Kinesin is therefore a mechanochemical ATPase that is activated by microtubules.