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J. Biol. Chem., Vol. 279, Issue 22, 22799-22802, May 28, 2004
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From the
Department of Life Sciences, University of Tokyo, Komaba 38-1, Meguro-ku, Tokyo 1538902 and
Okazaki Institute for Integrative Bioscience, National Institutes of Natural Sciences, Higashiyama 51, Myodaiji, Okazaki 4448787, Japan
Received for publication, December 8, 2003 , and in revised form, March 1, 2004.
| ABSTRACT |
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4s 1), which was further activated by >15-fold on the addition of 40 µM microtubules. These results show that the 380-kDa recombinant fragment retains all the structures required for motor functions, i.e. the ATPase activity highly stimulated by microtubules and the robust motility. | INTRODUCTION |
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The 380-kDa fragment of the Dictyostelium dynein heavy-chain has been successfully expressed in Dictyostelium cells (13). Electron microscopic studies on the fragment have shown that it has a single ring-shaped head and a stalk protruding from it (14). This fragment binds to microtubules in an ATP-sensitive fashion and is susceptible to VO4-mediated photo-cleavage (13), an indication that the fragment may have ATPase activity. However, it remains to be shown whether this recombinant dynein fragment is an active motor with microtubule-activated ATPase activity and motility. Here, we have established an expression and purification system of the single-headed Dictyostelium 380-kDa dynein fragment fused with the N-terminal His tag and GFP1 (designated HG380) and have shown that it can drive the robust sliding of microtubules and has high microtubule-activated ATPase activity. Thus, this heavy-chain fragment is an active motor that contains all structures required for force generation.
| MATERIALS AND METHODS |
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MT.
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MT, the extrachromosomal plasmids carrying the corresponding genes were introduced into these Dictyostelium cells by electroporation as described (17). The transformed cells were grown as described (18) in an axenic medium supplemented with 10 µg/ml blasticidin S, 10 µg/ml G418, and 10 µg/ml tetracycline to inhibit expression of the recombinant proteins. When cells grew to 5 x 106/ml, the medium was replaced with one without tetracycline to start the induction of these recombinant proteins.
Protein PurificationAll procedures were carried out at 4 °C or on ice unless otherwise stated. Twenty-four hours after induction, Dictyostelium cells expressing HG380 or HG380
MT were harvested and resuspended in an equal volume of PMG buffer (100 mM PIPES-KOH, 4 mM MgCl2, 0.1 mM EGTA, 0.9 M glycerol, 10 mM imidazole, 1 mM
-melcaptoethanol, 10 µg/ml chymostatin, 10 µg/ml pepstatin, 50 µg/ml leupeptin, 500 µM phenylmethylsulfonyl fluoride, and 0.1 mM ATP, pH 7.0). The cells were homogenized by sonication and centrifuged at 24,000 x g for 20 min and then at 187,000 x g for 60 min. The high speed supernatant was mixed with Ni-nitrilotriacetic acid-agarose beads and incubated for 60 min. After precipitation and washing, the adsorbed proteins were eluted from the beads four times with PMG buffer supplemented with 250 mM imidazole. The four eluted fractions were mixed and loaded onto a PD-10 desalting column (Amersham Biosciences) to replace the solvent to a PMEG buffer (100 mM PIPES-KOH, 4 mM MgCl2, 5 mM EGTA, 0.1 mM EDTA, 0.9 M glycerol, 1 mM dithiothreitol, 10 µg/ml chymostatin, 10 µg/ml pepstatin, 50 µg/ml leupeptin, 500 µM phenylmethylsulfonyl fluoride, and 0.1 mM ATP, pH 7.0) (19). The eluted fraction from PD-10 was mixed with microtubules prepared from porcine brain (20), paclitaxel, and adenosine 5'-(
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-imido) triphosphate so that their final concentrations would be 1 mg/ml, 10 µM, and 0.5 mM, respectively. The mixture was then incubated at room temperature for 30 min. The HG380 molecules complexed with microtubules were pelleted at 46,200 x g for 20 min at 25 °C through a PMEG cushion containing 25% sucrose without ATP. The pellet was suspended in a PMEG buffer containing 10 mM MgATP and 10 µM paclitaxel. The microtubules were pelleted again at 356,000 x g for 10 min at 25 °C to release HG380. The supernatant contained HG380. Glycerol and ATP were included throughout the HG380 preparation because we observed that the protein was unstable without them. The purified proteins were stored on ice and used within 2 days.
Protein ConcentrationsProtein concentrations of HG380 were routinely determined by the Bradford method (21) by using bovine serum albumin as a standard. These concentrations were calibrated by using the absorption of GFP as follows. First, we determined the concentration of purified GST·GFP expressed in Escherichia coli by using the molecular extinction coefficient of GFP (55,900 at 488 nm; Ref. 22). Then we determined the concentration of HG380 by comparing the fluorescence intensity of GST·GFP at 510 nm (excited at 488 nm) to that of HG380. We did not directly use the molar extinction coefficient of GFP to determine the concentration of HG380 because the GFP absorbance of purified HG380 at 488 nm was not high enough for accurate measurements. The results showed that the Bradford method gave almost the same concentrations as those obtained by the absorption of GFP at 488 nm (within an error of 20%). The concentration of tubulin was also determined by the Bradford method as described (23).
Measurements of Microtubule-activated ATPase ActivityJust before ATPase measurements, the amount of ATP in the purified HG380 was depleted to 0.1 mM by a NAP-5 desalting column (Amersham Biosciences). The ATPase assay was performed using the EnzChek phosphate assay kit (Molecular Probes, Eugene, OR). The assay conditions were as follows: assay buffer, 10 mM PIPES-KOH, 50 mM potassium acetate, 4 mM MgSO4, 1 mM EGTA, 10 µM paclitaxel, and 1 mM dithiothreitol, pH 7.0; HG380, 5 µg/ml (12.5 nM); microtubules, 0
40 µM; temperature, 25 °C. The reaction was followed by continuously monitoring the absorbance at 360 nm. For each set of measurements of the microtubule-activated ATPase activity, we checked the background phosphate-release rate of microtubules alone. The average background rate of steady-state increase of absorption at 360 nm in the presence of microtubules without HG380 was
0.2 nM/s/µM microtubule. Thus, the free phosphate ions present in tubulin preparation or released from the intrinsic GTPase activity of microtubules rarely contributed to the ATPase activities of HG380 even at the highest microtubule concentration used here (40 µM).
The observed specific activity kobs is a sum of the basal ATPase activity kB and the maximally activated ATPase activity kCAT as shown in Equation 1, where [MT] is the microtubule concentration and KMT is the microtubule concentration at the half saturation of microtubule-activated ATPase activity.
![]() | (Eq. 1) |
In Vitro Motility AssaysAssays were performed at 25 °C in the ATPase buffer without dithiothreitol. This buffer was used for every step of the motility assay unless otherwise stated. The assay chamber was coated sequentially with streptavidin (1 mg/ml), biotinyl protein G (1 mg/ml), and anti-GFP monoclonal antibody (Qbiogene, Carlsbad, CA) diluted to 100 µg/ml by the assay buffer containing 1 mg/ml bovine serum albumin. The chamber surface was finally blocked with 10 mg/ml bovine serum albumin. The purified HG380 diluted to 50 µg/ml was introduced into the chamber twice at 5-min intervals. After 5 min, 30 µg/ml of paclitaxel-stabilized microtubules was introduced into the chamber. After another 5 min, the assay chamber was washed, and then the assay buffer containing 1 mM ATP was introduced. The control IgG2a (Zymed Laboratories Inc., South San Francisco, CA), which belongs to the same subclass as that of the anti-GFP antibody but does not cross-react with GFP, was diluted to 100 µg/ml and used for control experiments (Fig. 3b). Microtubules were observed using dark-field illumination with a x40 objective lens and recorded with video tape recorder. The recordings were digitized and analyzed. All microtubules longer than 5 µm in randomly selected microscopic fields were chosen for measurements. The sliding velocity of microtubules that translocated in a continuous manner for at least 4 s was measured. Some of the microtubules translocated only occasionally and for only a short period of time. They were counted as non-motile microtubules. For the experiments with the control IgG2a, the motion of microtubules was followed for more than 20 s.
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| RESULTS AND DISCUSSION |
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When the control IgG2a that does not cross-react to GFP was used in place of the anti-GFP antibody, most of the microtubules did not slide on the addition of ATP, although some of them (18%) slid very slowly with an average velocity of 0.3 µm/s (Fig. 3b). These results showed that the HG380 molecules trapped with anti-GFP antibodies on the glass surface supported the smooth, robust sliding of microtubules.
To further confirm this notion, we performed mock experiments by using an HG380 derivative in which the microtubule-binding site at the end of the stalk (6, 26) was deleted (Fig. 1, HG380
MT). The deletion mutant was transiently expressed as in the case of HG380. The expression level of HG380
MT was very similar to that of HG380 (Fig. 2b, HSS). HG380
MT was first purified by nickel-nitrilotriacetic acid beads and then co-precipitated with microtubules. Unlike the case of HG380, however, HG380
MT did not co-precipitate with microtubules as expected, because it lacked the microtubule-binding site. Thus, only a small amount of proteins was released from the precipitated microtubules on the addition of ATP (Fig. 2b, +ATP sup). When this ATP-released protein mixture was used for the motility assay in place of HG380, virtually no microtubule was trapped on the glass surface. The result ensured that microtubules on the glass surface covered with HG380 were actually trapped there by binding to this recombinant protein. This result eliminated the possibility that the endogenous Dictyostelium dynein that might be present in the HG380 preparation or the dynein that might be present in the brain tubulin preparation drove the microtubule sliding. Thus, based on these control experiments, we have established that the HG380 molecules were the motor proteins that drove the robust, minus-end-directed sliding of microtubules.
The HG380 protein had a high basal ATPase activity (4.4 s1), which was further stimulated >15-fold on the addition of 40 µM microtubules (Fig. 4). By fitting the microtubule-activated activities measured in the presence of various concentrations of microtubules with a hyperbola, the maximal ATPase activity stimulated by microtubules (kcat) was calculated to be 160 s1. The microtubule concentration needed for the half-maximal stimulation (KMT) was 68 µM. Although this KMT value seems to be unusually high compared with other MT-based motor proteins, this value for HG380 was highly dependent on ionic strength, implying that MT and HG380 are held together in the presence of ATP by weak ionic interactions.
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In summary, we have established an easy and reproducible method for generating recombinant dyneins with full motor activities: the single-headed recombinant fragment HG380 retains the ability to drive the robust sliding of microtubules and exhibits high microtubule-activated ATPase activity.
| FOOTNOTES |
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To whom correspondence should be addressed. Tel. and Fax: 81-3-5454-6751; E-mail: sutoh{at}bio.c.u-tokyo.ac.jp.
1 The abbreviations and trivial names used are: GFP, green fluorescent protein; HG380, the 380-kDa fragment of dynein heavy chain fused with His tag and GFP; GST·GFP, GFP fused with glutathione S-transferase. ![]()
| ACKNOWLEDGMENTS |
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| REFERENCES |
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