Slow ADP-dependent acceleration of microtubule translocation produced by an axonemal dynein

IC2 participates in the cooperative activation of outer arm dynein densely attached to microtubules

Ciliary outer-arm dynein (OAD) consists of heavy chains (HCs), intermediate chains (ICs), and light chains (LCs), of which HCs are the motor proteins that produce force. Studies using the green alga Chlamydomonas have revealed that ICs and LCs form a complex (IC/LC tower) at the base of the OAD tail and play a crucial role in anchoring OAD to specific sites on the microtubule. In this study, we isolated a novel slow-swimming Chlamydomonas mutant deficient in the IC2 protein. This mutation, E279K, is in the third of the seven WD repeat domains. No apparent abnormality was observed in electron microscope observations of axonemes or in SDS-PAGE analyses of dynein subunits. To explore the reason for the lowered motility in this mutant, in vitro microtubule sliding experiments were performed, which revealed that the motor activity of the mutant OAD was lowered. In particular, a large difference was observed between wild type (WT) and the mutant in the microtubule sliding velocity in microtubule bundles formed with the addition of OAD: ~35.3 μm/sec (WT) and ~4.3 μm/sec (mutant). From this and other results, we propose that IC2 in an OAD interacts with the β HC of the adjacent OAD, and that an OAD-OAD interaction is important for efficient beating of cilia and flagella.Key words: cilia, axoneme, dynein heavy chain, cooperativity.

Light-Powered Reactivation of Flagella and Contraction of Microtubule Networks: Toward Building an Artificial Cell

Artificial systems capable of self-sustained movement with self-sufficient energy are of high interest with respect to the development of many challenging applications, including medical treatments, but also technical applications. The bottom-up assembly of such systems in the context of synthetic biology is still a challenging task. In this work, we demonstrate the biocompatibility and efficiency of an artificial light-driven energy module and a motility functional unit by integrating light-switchable photosynthetic vesicles with demembranated flagella. The flagellar propulsion is coupled to the beating frequency, and dynamic ATP synthesis in response to illumination allows us to control beating frequency of flagella in a light-dependent manner. In addition, we verified the functionality of light-powered synthetic vesicles in in vitro motility assays by encapsulating microtubules assembled with force-generating kinesin-1 motors and the energy module to investigate the dynamics of a contractile filamentous network in cell-like compartments by optical stimulation. Integration of this photosynthetic system with various biological building blocks such as cytoskeletal filaments and molecular motors may contribute to the bottom-up synthesis of artificial cells that are able to undergo motor-driven morphological deformations and exhibit directional motion in a light-controllable fashion.

Rapid estimation of cytosolic ATP concentration from the ciliary beating frequency in the green alga Chlamydomonas reinhardtii

Determination of cellular ATP levels, a key indicator of metabolic status, is essential for the quantitative analysis of metabolism. The biciliate green alga Chlamydomonas reinhardtii is an excellent experimental organism to study ATP production pathways, including photosynthesis and respiration, particularly because it can be cultured either photoautotrophically or heterotrophically. Additionally, its cellular ATP concentration, [ATP], is reflected in the beating of its cilia. However, the methods currently used for quantifying the cellular ATP levels are time consuming or invasive. In this study, we established a rapid method for estimating cytosolic [ATP] from the ciliary beating frequency in C. reinhardtii. Using an improved method of motility reactivation in demembranated cell models, we obtained calibration curves for [ATP]–ciliary beating frequency over a physiological range of ATP concentrations. These curves allowed rapid estimation of the cytosolic [ATP] in live wild-type cells to be ∼2.0 mM in the light and ∼1.5 mM in the dark: values comparable to those obtained by other methods. Furthermore, we used this method to assess the effects of genetic mutations or inhibitors of photosynthesis or respiration quantitatively and noninvasively. This sensor-free method is a convenient tool for quickly estimating cytosolic [ATP] and studying the mechanism of ATP production in C. reinhardtii or other ciliated organisms.

CFAP45 deficiency causes situs abnormalities and asthenospermia by disrupting an axonemal adenine nucleotide homeostasis module

Axonemal dynein ATPases direct ciliary and flagellar beating via adenosine triphosphate (ATP) hydrolysis. The modulatory effect of adenosine monophosphate (AMP) and adenosine diphosphate (ADP) on flagellar beating is not fully understood. Here, we describe a deficiency of cilia and flagella associated protein 45 (CFAP45) in humans and mice that presents a motile ciliopathy featuring situs inversus totalis and asthenospermia. CFAP45-deficient cilia and flagella show normal morphology and axonemal ultrastructure. Proteomic profiling links CFAP45 to an axonemal module including dynein ATPases and adenylate kinase as well as CFAP52, whose mutations cause a similar ciliopathy. CFAP45 binds AMP in vitro, consistent with structural modelling that identifies an AMP-binding interface between CFAP45 and AK8. Microtubule sliding of dyskinetic sperm from Cfap45^−/− mice is rescued with the addition of either AMP or ADP with ATP, compared to ATP alone. We propose that CFAP45 supports mammalian ciliary and flagellar beating via an adenine nucleotide homeostasis module.

The mechanism by which adenosine monophosphate modulates dynein ATPase-mediated ciliary and flagellar beating remains obscure. Here the authors identify an axonemal module including cilia and flagella associated protein 45 that supports adenine nucleotide homeostasis and underlies a human ciliopathy

Force-Generating Mechanism of Axonemal Dynein in Solo and Ensemble

In eukaryotic cilia and flagella, various types of axonemal dyneins orchestrate their distinct functions to generate oscillatory bending of axonemes. The force-generating mechanism of dyneins has recently been well elucidated, mainly in cytoplasmic dyneins, thanks to progress in single-molecule measurements, X-ray crystallography, and advanced electron microscopy. These techniques have shed light on several important questions concerning what conformational changes accompany ATP hydrolysis and whether multiple motor domains are coordinated in the movements of dynein. However, due to the lack of a proper expression system for axonemal dyneins, no atomic coordinates of the entire motor domain of axonemal dynein have been reported. Therefore, a substantial amount of knowledge on the molecular architecture of axonemal dynein has been derived from electron microscopic observations on dynein arms in axonemes or on isolated axonemal dynein molecules. This review describes our current knowledge and perspectives of the force-generating mechanism of axonemal dyneins in solo and in ensemble.

Stochastic Model of T Cell Repolarization during Target Elimination I

Cytotoxic T lymphocytes (T) and natural killer cells are the main cytotoxic killer cells of the human body to eliminate pathogen-infected or tumorigenic cells (i.e., target cells). Once a natural killer or T cell has identified a target cell, they form a tight contact zone, the immunological synapse (IS). One then observes a repolarization of the cell involving the rotation of the microtubule (MT) cytoskeleton and a movement of the MT organizing center (MTOC) to a position that is just underneath the plasma membrane at the center of the IS. Concomitantly, a massive relocation of organelles attached to MTs is observed, including the Golgi apparatus, lytic granules, and mitochondria. Because the mechanism of this relocation is still elusive, we devise a theoretical model for the molecular-motor-driven motion of the MT cytoskeleton confined between plasma membrane and nucleus during T cell polarization. We analyze different scenarios currently discussed in the literature, the cortical sliding and capture-shrinkage mechanisms, and compare quantitative predictions about the spatiotemporal evolution of MTOC position and MT cytoskeleton morphology with experimental observations. The model predicts the experimentally observed biphasic nature of the repositioning due to an interplay between MT cytoskeleton geometry and motor forces and confirms the dominance of the capture-shrinkage over the cortical sliding mechanism when the MTOC and IS are initially diametrically opposed. We also find that the two mechanisms act synergistically, thereby reducing the resources necessary for repositioning. Moreover, it turns out that the localization of dyneins in the peripheral supramolecular activation cluster facilitates their interaction with the MTs. Our model also opens a way to infer details of the dynein distribution from the experimentally observed features of the MT cytoskeleton dynamics. In a subsequent publication, we will address the issue of general initial configurations and situations in which the T cell established two ISs.

Functional characterization of lethal P-loop mutations in Tetrahymena outer arm dynein (Dyh3p)

Mutational analyses of axonemal dyneins are useful for elucidating the molecular mechanism of ciliary motility. This study demonstrates a mutation system for characterizing lethal P-loop mutations in Tetrahymena outer arm dynein (Dyh3p). The viable DYH3-knockout (vKO-DYH3) cells isolated in this study enabled the examination of lethal mutations in P-loops 1 and 2. The P1 mutant dynein localized in the oral apparatus and the proximal region of the cilia, and the P2 mutant dynein localized only in the oral apparatus. Both results are different from the localization of wild-type Dyh3p. In addition, a co-precipitation assay showed that the P1 and P2 mutant dyneins did not dissociate from microtubules in ATP plus vanadate or in no-ATP conditions, in contrast to wild-type Dyh3p. This mutation system is useful for further molecular studies of axonemal dyneins and ciliary motility.

Establishment of a mutation system in Tetrahymena outer arm dynein and P-loop functions of the alpha heavy chain (Dyh3p)

Axonemal dyneins are large AAA+ type motor proteins that exhibit unique motor properties during ciliary beating. This study established a mutation system for Tetrahymena outer arm dynein and characterized four nucleotide-binding loops (P-loops; P1-P4) in the alpha heavy chain (Dyh3p). Macronuclear transformation of the mutant DYH3 genes in DYH3-knockout (KO-DYH3) cells enabled P-loop mutations that abolish the ability of nucleotide binding to be stably maintained in the polyploid genome. This mutation system revealed that the P3 and P4 mutant dyneins rescued lethality in macronuclear KO-DYH3 cells and exhibited normal ciliary localization. Intriguingly, however, an in vitro motility assay showed that the P3 mutation abolished the motor activity of Dyh3p, whereas the P4 mutation did not affect the gliding velocity or gliding index of Dyh3p. In contrast, no P1 or P2 mutant cells were isolated from the KO-DYH3 cells, which suggests that nucleotide binding at the P1 and P2 sites is required for the intracellular function of Dyh3p. This mutation system will be useful for further molecular studies of diverse axonemal dyneins and ciliary motility.

The effects of Ca2+ and ADP on dynein switching during the beat cycle of reactivated bull sperm models

Calcium regulation of flagellar motility is the basis for chemotaxis, phototaxis, and hyperactivation responses in eukaryotic flagellates and spermatozoa. Ca2+ is the internal messenger for these responses, but the coupling between Ca2+ and the motor mechanism that generates the flagellar beat is incompletely understood. We examined the effects of Ca2+ on the flagellar curvature at the switch-points of the beat cycle in bull sperm. The sperm were detergent extracted and reactivated with 0.1 mM adenosine triphosphate (ATP). With their heads immobilized and their tails beating freely it is possible to calculate the bending torque and the transverse force acting on the flagellum at the switch-points. An increase in the free Ca2+ concentration (pCa 8 to pCa 4) significantly decreased the development of torque and t-force in the principal bending direction, while having negligible effect on the reverse bend. The action of Ca2+ was more pronounced when the sperm were also treated with 4 mM adenosine diphosphate (ADP); it was sufficient to change the direction of bending that reaches the greater curvature. We also observed that the curvature of the distal half of the flagellum became locked in one direction in the presence of Ca2+ . This indicates that a subset of the dynein becomes continuously activated by Ca2+ and fails to switch with the beat cycle. Our evidence suggests this subset of dyneins is localized to doublets #1-4 of the axoneme.

The functional expression and motile properties of recombinant outer arm dynein from Tetrahymena

Cilia and flagella are motile organelles that play various roles in eukaryotic cells. Ciliary movement is driven by axonemal dyneins (outer arm and inner arm dyneins) that bind to peripheral microtubule doublets. Elucidating the molecular mechanism of ciliary movement requires the genetic engineering of axonemal dyneins; however, no expression system for axonemal dyneins has been previously established. This study is the first to purify recombinant axonemal dynein with motile activity. In the ciliated protozoan Tetrahymena, recombinant outer arm dynein purified from ciliary extract was able to slide microtubules in a gliding assay. Furthermore, the recombinant dynein moved processively along microtubules in a single-molecule motility assay. This expression system will be useful for investigating the unique properties of diverse axonemal dyneins and will enable future molecular studies on ciliary movement.

Displacement-weighted velocity analysis of gliding assays reveals that Chlamydomonas axonemal dynein preferentially moves conspecific microtubules

In vitro gliding assays, in which microtubules are observed to glide over surfaces coated with motor proteins, are important tools for studying the biophysics of motility. Gliding assays with axonemal dyneins have the unusual feature that the microtubules exhibit large variations in gliding speed despite measures taken to eliminate unsteadiness. Because axonemal dynein gliding assays are usually done using heterologous proteins, i.e., dynein and tubulin from different organisms, we asked whether the source of tubulin could underlie the unsteadiness. By comparing gliding assays with microtubules polymerized from Chlamydomonas axonemal tubulin with those from porcine brain tubulin, we found that the unsteadiness is present despite matching the source of tubulin to the source of dynein. We developed a novel, to our knowledge, displacement-weighted velocity analysis to quantify both the velocity and the unsteadiness of gliding assays systematically and without introducing bias toward low motility. We found that the quantified unsteadiness is independent of tubulin source. In addition, we found that the short Chlamydomonas microtubules translocate significantly faster than their porcine counterparts. By modeling the effect of length on velocity, we propose that the observed effect may be due to a higher rate of binding of Chlamydomonas axonemal dynein to Chlamydomonas microtubules than to porcine microtubules.

The mechanism of dynein motility: insight from crystal structures of the motor domain

Dynein is a large cytoskeletal motor protein that belongs to the AAA+ (ATPases associated with diverse cellular activities) superfamily. While dynein has had a rich history of cellular research, its molecular mechanism of motility remains poorly understood. Here we describe recent X-ray crystallographic studies that reveal the architecture of dynein's catalytic ring, mechanical linker element, and microtubule binding domain. This structural information has given rise to new hypotheses on how the dynein motor domain might change its conformation in order to produce motility along microtubules.

Molecular organization and force-generating mechanism of dynein

Dynein, which is a minus-end-directed microtubule motor, is crucial to a range of cellular processes. The mass of its motor domain is about 10 times that of kinesin, the other microtubule motor. Its large size and the difficulty of expressing and purifying mutants have hampered progress in dynein research. Recently, however, electron microscopy, X-ray crystallography and single-molecule nanometry have shed light on several key unsolved questions concerning how the dynein molecule is organized, what conformational changes in the molecule accompany ATP hydrolysis, and whether two or three motor domains are coordinated in the movements of dynein. This minireview describes our current knowledge of the molecular organization and the force-generating mechanism of dynein, with emphasis on findings from electron microscopy and single-molecule nanometry.

Assays of cell and axonemal motility in Chlamydomonas reinhardtii

Chlamydomonas, an organism that offers a variety of flagella-deficient mutants, has been very important for studies of cilia and flagella. Motility assessment of mutant flagella at various levels helps us understand the function of specific axonemal proteins and structures for flagellar function. Measurements of gross cell movements are useful to assess the overall flagellar activity, analyses of demembranated and reactivated cells ("cell models") enable us to study the regulatory mechanism, and measurements of microtubule sliding velocity in vitro provide important information about dynein-microtubule interactions. This chapter describes fundamental techniques for these measurements.

Analysis of the role of nucleotides in axonemal dynein function

Axonemal dynein in flagella and cilia is a motor molecule that produces microtubule sliding, powered by the energy of ATP hydrolysis. Our goal is to understand how dynein motile activity is controlled to produce the characteristic oscillatory movement of flagella. ATP, the energy source for dynein, is also important as a regulator of dynein activity. Among the four nucleotide-binding sites of a dynein heavy chain, one is the primary ATP hydrolyzing site while the others are noncatalytic sites and thought to perform regulatory functions. Stable binding of both ATP and ADP to these regulatory sites is probably essential for the chemomechanical energy transduction in dynein. Although the ATP concentration in beating flagella is physiologically high and constant, at any moment in the oscillatory cycle some dynein molecules are active while others are not, and the motile activity of dynein oscillates temporally and spatially in the axoneme. It is likely that the basic mechanism underlying the highly dynamic control of dynein activity involves the ATP-dependent inhibition and ADP-dependent activation (or release of inhibition) of dynein. How the inhibition and activation can be induced in beating flagella is still unknown. It seems, however, that the mechanical force of bending is involved in the activation of dynein, probably through the control of noncatalytic nucleotide binding to dynein. This chapter provides an overview of several approaches, using sea urchin sperm flagella, to studying the roles of ATP and ADP in the regulation of dynein activity with or without the mechanical signal of bending.

Ratchetlike properties of in vitro microtubule translocation by a Chlamydomonas inner-arm dynein species c in the presence of flow

To investigate the force generation properties of Chlamydomonas axonemal inner-arm dyneins in response to external force, we analyzed microtubule gliding on dynein-coated surfaces under shear flow. When inner-arm dynein c was used, microtubule translocation in the downstream direction accelerated with increasing flow speed in a manner that depended on the dynein density and ATP concentration. In contrast, the microtubule translocation velocity in the upstream direction was unaffected by the flow speed. The number of microtubules on the glass surface was almost constant with and without flow, suggesting that gliding acceleration was not simply caused by weakened dynein-microtubule binding. With other inner-arm dynein species, the microtubule gliding velocity was unaffected by the flow regardless of the flow direction or nucleotide concentration. The flow-generated force acting on a single dynein was estimated to be as small as approximately 0.03 pN/dynein. These results indicate that dynein c possesses a ratchetlike property that allows acceleration only in one direction by a very small external force. This property should be important for slow- and fast-moving dyneins to function simultaneously within the axoneme.

Bending-induced switching of dynein activity in elastase-treated axonemes of sea urchin sperm--roles of Ca2+ and ADP

Flagellar beating is caused by microtubule sliding, driven by the activity of dynein, between adjacent two of the nine doublet microtubules. An essential process in the regulation of dynein is to alternate its activity (switching) between the two sides of the central pair microtubules. The switching of dynein activity can be detected, in an in vitro system using elastase-treated axonemes of sea urchin sperm flagella, as a reversal of the relative direction of ATP-induced sliding between the two bundles of doublets at high Ca(2+) (10(-4) M) at pH 7.8-8.0. The reversal is triggered by externally applied bending of the doublet bundle. However, the mechanism of this bending-induced reversal (or backward sliding) remains unclear. To understand how the switching of dynein activity in flagella can be induced by bending, we studied the roles of ADP, which is an important factor for the dynein motile activity, and of Ca(2+) in the bending-induced reversal of microtubule sliding between two bundles of doublets at pH 7.5 and 7.2. We found that the reversal of sliding direction was induced regardless of the concentrations of Ca(2+) at low pH, but occurred more frequently at low Ca(2+) (<10(-9) M) than at high Ca(2+). At pH 7.5, an application of ADP increased the frequency of occurrence of backward sliding at high as well as low concentrations of Ca(2+). The results indicate that ADP-dependent activation of dynein, probably resulting from ADP-binding to dynein, is involved in the regulation of the bending-induced switching of dynein activity in flagella.

Central pair apparatus enhances outer-arm dynein activities through regulation of inner-arm dyneins

The beating of eukaryotic cilia and flagella is controlled by multiple species of inner-arm and outer-arm dyneins. To clarify the regulation on axonemal beating by nucleotide conditions and central-pair microtubules, microtubule sliding in disintegrating Chlamydomonas axonemes of various mutants and in vitro microtubule gliding by isolated axonemal dyneins were examined. In the in vitro motility assays with outer-arm dyneins (alphabeta and gamma), microtubule translocation velocity decreased at high concentrations of ATP, while this inhibition was canceled by the simultaneous presence of ADP or ribose-modified analogues, mantATP/ADP. In contrast, motility of inner-arm dyneins was rather insensitive to these nucleotides. The velocity of sliding disintegration in axonemes lacking the central pair was less than that in wild-type axonemes at high ATP concentrations, but was overcome by the presence of ADP or mantATP/ADP. While these nucleotides did not activate the sliding velocity in other mutant axonemes, they increased the extent of sliding, except for axonemes lacking outer-arm dynein. Experiments with axonemes lacking inner-arm dynein f using casein kinase 1 inhibitor suggest that the regulation of outer-arm dynein by the central pair is effected through the activation of inner-arm dynein f, and possibly by other interactions. These results indicate that the central pair activates outer-arm dyneins on specific outer-doublet, resulting in amplification of the axonemal bending force.

Kinetic models for the coordinated stepping of cytoplasmic dynein

To generate processive motion along a polymer track requires that motor proteins couple their ATP hydrolysis cycle with conformational changes in their structural subunits. Numerous experimental and theoretical efforts have been devoted to establishing how this chemomechanical coupling occurs. However, most processive motors function as dimers. Therefore a full understanding of the motor's performance also requires knowledge of the coordination between the chemomechanical cycles of the two heads. We consider a general two-headed model for cytoplasmic dynein that is built from experimental measurements on the chemomechanical states of monomeric dynein. We explore different possible scenarios of coordination that simultaneously satisfy two main requirements of the dimeric protein: high processivity (long run length) and high motor velocity (fast ATP turnover). To demonstrate the interplay between these requirements and the necessity for coordination, we first develop and analyze a simple mechanical model for the force-induced stepping in the absence of ATP. Next we use a simplified model of dimeric dynein's chemomechanical cycle to establish the kinetic rules that must be satisfied for the model to be consistent with recent data for the motor's performance from single molecule experiments. Finally, we use the results of these investigations to develop a full model for dimeric dynein's chemomechanical cycle and analyze this model to make experimentally testable predictions.

Insights into the mechanism of ADP action on flagellar motility derived from studies on bull sperm

Adenosine diphosphate (ADP) is known to have interesting effects on flagellar motility. Permeabilized and reactivated bull sperm exhibit a marked reduction in beating frequency and a greatly increased beat amplitude in the presence of 1-4 mM ADP. In this study we examined the force production of sperm reactivated with 0.1 mM ATP with and without 1 mM ADP and found that there is little or no resulting change in the stalling force produced by a bull sperm flagella in response to ADP. Because bull sperm bend to a higher curvature after ADP treatment we explored the possibility that ADP-treated sperm flagella are more flexible. We measured the stiffness of 50 muM sodium vanadate treated bull sperm in the presence of 4 mM ADP, but found no change in the passive flagellar stiffness. When we analyzed the torque that develops in ADP-treated sperm at the point of beat reversal we found that the torque developed by the flagellum is significantly increased. Our torque estimates also allow us to calculate the transverse force (t-force) acting on the flagellum at the point of beat direction reversal. We find that the t-force at the switch-point of the beat is increased significantly in the ADP treated condition, averaging 0.7 +/- 0.29 nN/microm in 0.1 mM ATP and increasing to 2.9 +/- 1.2 nN/microm in 0.1 mM ATP plus 4 mM ADP. This suggests that ADP is exerting its effect on the beat by increasing the tenacity of dynein attachment at the B-subtubule. This could be a direct result of a regulatory effect of ADP on the binding affinity of dynein for the B-subtubule of the outer doublets. This result could also help to explain a number of previous experimental observations, as discussed.

Molecular mechanism of force generation by dynein, a molecular motor belonging to the AAA+ family

Dynein is an AAA+ (ATPase associated with various cellular activities)-type motor complex that utilizes ATP hydrolysis to actively drive microtubule sliding. The dynein heavy chain (molecular mass >500 kDa) contains six tandemly linked AAA+ modules and exhibits full motor activities. Detailed molecular dissection of this motor with unique architecture was hampered by the lack of an expression system for the recombinant heavy chain, as a result of its large size. However, the recent success of recombinant protein expression with full motor activities has provided a method for advances in structure-function studies in order to elucidate the molecular mechanism of force generation.

Clockwise translocation of microtubules by flagellar inner-arm dyneins in vitro

Cilia and flagella are equipped with multiple species of dyneins that have diverse motor properties. To assess the properties of various axonemal dyneins of Chlamydomonas, in vitro microtubule translocation by isolated dyneins was examined with and without flow of the medium. With one inner-arm dynein species, dynein c, most microtubules became aligned parallel to the flow and translocated downstream after the onset of flow. When the flow was stopped, the gliding direction was gradually randomized. In contrast, with inner-arm dyneins d and g, microtubules tended to translocate at a shallow right angle to the flow. When the flow was stopped, each microtubule turned to the right, making a curved track. The clockwise translocation was not accompanied by lateral displacement, indicating that these dyneins generate torque that bends the microtubule. The torque generated by these dyneins in the axoneme may modulate the relative orientation between adjacent doublet microtubules and lead to more efficient functioning of total dyneins.

Inhibition by ATP and activation by ADP in the regulation of flagellar movement in sea urchin sperm

ATP and ADP are known to play inhibitory and activating roles, respectively, in the regulation of dynein motile activity of flagella. To elucidate how these nucleotide functions are related to the regulation of normal flagellar beating, we examined their effects on the motility of reactivated sea urchin sperm flagella at low pH. At pH 7.0-7.2 which is lower than the physiological pH of 8, about 90% of reactivated flagella were motionless at 1 mM ATP, while about 60% were motile at 0.02 mM ATP. The motionless flagella at 1 mM ATP maintained a single large bend or an S-shaped bend, indicating formation of dynein crossbridges in the axoneme. The ATP-dependent inhibition of flagellar movement was released by ADP, and was absent in outer arm-depleted flagella. Similar inhibition was also observed at 0.02 mM ATP when demembranated flagella were reactivated in the presence of Li+ or pretreated with protein phosphatase 1 (PP1). ADP also released this type of ATP-inhibition. In PP1-pretreated axonemes the binding of a fluorescent analogue of ADP to dynein decreased. Under elastase-treatment at pH 8.0, the beating of demembranated flagella at 1 mM ATP and 0.02 mM ATP lasted for approximately 100 and 45 s, respectively. The duration of beating at 0.02 mM ATP was prolonged by Li+, and that at 1 mM ATP was shortened by removal of outer arms. These results indicate that the regulation of on/off switching of dynein motile activity of flagella involves ATP-induced inhibition and ADP-induced activation, probably through phosphorylation/dephosphorylation of outer arm-linked protein(s).

The roles of noncatalytic ATP binding and ADP binding in the regulation of dynein motile activity in flagella

The regulation of dynein activity to produce microtubule sliding in flagella has not been well understood. To gain more insight into the roles of ATP and ADP in the regulation, we examined the effects of fluorescent ATP analogues and fluorescent ADP analogues on the ATPase activity and motile activity of dynein. 21S dynein purified from the outer arms of sea urchin sperm flagella hydrolyzed BODIPY(R) FL ATP (FL-ATP) at 78% of the rate for ATP hydrolysis. FL-ATP at 0.1-1 mM, however, induced neither microtubule translocation on a dynein-coated glass surface nor sliding disintegration of elastase-treated axonemes. Direct observation of single molecules of the fluorescent analogues showed that both the ATP and ADP analogues were stably bound to dynein over several minutes (dissociation rates = 0.0038-0.0082/s). When microtubule translocation on 21S dynein was induced by ATP, the initial increase of the mean velocity was accelerated by preincubation of the dynein with ADP. Similar increase was also induced by the preincubation with the ADP analogues. Even after preincubation with ADP, FL-ATP did not induce sliding disintegration of elastase-treated axonemes. After preincubation with a nonhydrolyzable ATP analogue, AMPPNP (adenosine 5'-(beta:gamma-imido)triphosphate), however, FL-ATP induced sliding disintegration in approximately 10% of the axonemes. These results indicate that both noncatalytic ATP binding and stable ADP binding, in addition to ATP hydrolysis, are involved in the regulation of the chemo-mechanical transduction in axonemal dynein.

Induction of beating by imposed bending or mechanical pulse in demembranated, motionless sea urchin sperm flagella at very low ATP concentrations

A basic feature of the movement of eukaryotic flagella is oscillation. Although flagellar oscillation is thought to be regulated by a self-regulatory feedback system including the mechanical signal of bending itself, the mechanism regulating the dynein motile activity to produce oscillation is not well understood. To elucidate the mechanism, we developed a new experimental system which allowed us to analyze the conditions necessary for the induction of oscillation. When a mechanical signal of bending or a pulse was applied by micromanipulation to a demembranated motionless sea urchin sperm flagellar axoneme at very low ATP concentrations (1-3 microM), a localized pair of bends was induced. The bend formation was often followed by further responses including propagation of the distal bend of paired bends, growth and propagation of the paired bends, and cyclical beating. The beating was induced at 2.0 microM or higher concentrations of ATP, but appeared even at 1.5 microM ATP if a few muM of ADP was also present. When the proximal half of a flagellum was attached to a microneedle, beating could not be induced in the distal free region at 2 microM ATP. These results suggest that mechanical signal is involved in the mechanism regulating the motile activity of dynein to produce oscillation. Our results also showed that the presence of a small amount of ADP and the axial difference along the flagellum are factors essential for the induction of flagellar oscillation.

Model for unidirectional movement of axonemal and cytoplasmic dynein molecules

A model for the unidirectional movement of dynein is presented based on the structural observations and biochemical experimental results available. In this model, the binding affinity of dynein for microtubule (MT) is independent of its nucleotide state and the change between strong and weak MT-binding is determined naturally by the variation of relative orientation between the stalk and MT, as the stalk rotates following nucleotide-state transition. Thus the enigmatic communication from the adenosine triphosphate (ATP)-binding site in the globular domain to the far MT-binding site in the tip of the stalk, which is a prerequisite in conventional models, is not required. Using the present model, the previous experimental results such as the effect of ATP and adenosine diphosphate (ADP) bindings on dissociation of dynein from MT, the movement of single-headed axonemal dyneins at saturating ATP concentration, the load dependence of step-size for the movement of two-headed cytoplasmic dyneins and the dependence of stall force on ATP concentration can be well explained.

A simple theoretical model explains dynein's response to load

Recent experiment showed that cytoplasmic dynein 1, a molecular motor responsible for cargo transport in cells, functions as a gear in response to external load. In the presence of vanishing or small external load, dynein walks with 24- or 32-nm steps, whereas at high external load, its step size is reduced to 8 nm. A simple model is proposed to account for this property of dynein. The model assumes that the chemical energy of ATP hydrolysis is used through a loose coupling between the chemical reaction and the translocation of dynein along microtubule. This loose chemomechanical coupling is represented by the loosely coupled motions of dynein along two different reaction coordinates. The first reaction coordinate is tightly coupled to the chemical reaction and describes the protein conformational changes that control the chemical processes, including ATP binding and hydrolysis, and ADP-Pi release. The second coordinate describes the translocation of dynein along microtubule, which is directly subject to the influence of the external load. The model is used to explain the experimental data on the external force dependence of the dynein step size as well as the ATP concentration dependence of the stall force. A number of predictions, such as the external force dependence of speed of translocation, ATP hydrolysis rate, and dynein step sizes, are made based on this theoretical model. This model provides a simple understanding on how a variable chemomechanical coupling ratio can be achieved and used to optimize the biological function of dynein.

Modeling the signaling endosome hypothesis: why a drive to the nucleus is better than a (random) walk

Background

Information transfer from the plasma membrane to the nucleus is a universal cell biological property. Such information is generally encoded in the form of post-translationally modified protein messengers. Textbook signaling models typically depend upon the diffusion of molecular signals from the site of initiation at the plasma membrane to the site of effector function within the nucleus. However, such models fail to consider several critical constraints placed upon diffusion by the cellular milieu, including the likelihood of signal termination by dephosphorylation. In contrast, signaling associated with retrogradely transported membrane-bounded organelles such as endosomes provides a dephosphorylation-resistant mechanism for the vectorial transmission of molecular signals. We explore the relative efficiencies of signal diffusion versus retrograde transport of signaling endosomes.

Results

Using large-scale Monte Carlo simulations of diffusing STAT-3 molecules coupled with probabilistic modeling of dephosphorylation kinetics we found that predicted theoretical measures of STAT-3 diffusion likely overestimate the effective range of this signal. Compared to the inherently nucleus-directed movement of retrogradely transported signaling endosomes, diffusion of STAT-3 becomes less efficient at information transfer in spatial domains greater than 200 nanometers from the plasma membrane.

Conclusion

Our model suggests that cells might utilize two distinct information transmission paradigms: 1) fast local signaling via diffusion over spatial domains on the order of less than 200 nanometers; 2) long-distance signaling via information packets associated with the cytoskeletal transport apparatus. Our model supports previous observations suggesting that the signaling endosome hypothesis is a subset of a more general hypothesis that the most efficient mechanism for intracellular signaling-at-a-distance involves the association of signaling molecules with molecular motors that move along the cytoskeleton. Importantly, however, cytoskeletal association of membrane-bounded complexes containing ligand-occupied transmembrane receptors and downstream effector molecules provides the ability to regenerate signals at any point along the transmission path. We conclude that signaling endosomes provide unique information transmission properties relevant to all cell architectures, and we propose that the majority of relevant information transmitted from the plasma membrane to the nucleus will be found in association with organelles of endocytic origin.

Multiple ATP-hydrolyzing sites that potentially function in cytoplasmic dynein

Cytoplasmic dynein is a minus-end-directed microtubule motor involved in numerous essential processes within eukaryotic cells, such as nuclear segregation and trafficking of intracellular particles. The motor domain of the dynein heavy chain comprises six tandemly linked AAA (ATPase associated with diverse cellular activities) modules (AAA1-AAA6). The first four modules include nucleotide-binding sites (Walker A or P-loop motifs), and each of the four sites appears to bind ATP. However, the role and the function of each binding site are unknown. Especially, the question of which P-loops are ATP-hydrolyzing sites has not been answered, because it is difficult to measure the ATPase activity of each P-loop. Here, we purified several truncated Saccharomyces cerevisiae cytoplasmic dynein fragments and their mutants expressed in Escherichia coli and then measured their ATPase activities. Our results suggest that there are multiple ATP-binding sites that have abilities to hydrolyze ATP in cytoplasmic dynein. Furthermore, a single AAA module is insufficient for ATP hydrolysis, and the adjacent module facing the ATP-binding site is necessary for ATP-hydrolyzing activity.