Isolated flagellar outer arm dynein translocates brain microtubules in vitro

Versatile properties of dynein molecules underlying regulation in flagellar oscillation

Dynein is a minus-end-directed motor that generates oscillatory motion in eukaryotic flagella. Cyclic beating, which is the most significant feature of a flagellum, occurs by sliding spatiotemporal regulation by dynein along microtubules. To elucidate oscillation generated by dynein in flagellar beating, we examined its mechanochemical properties under three different axonemal dissection stages. By starting from the intact 9 + 2 structure, we reduced the number of interacting doublets and determined three parameters, namely, the duty ratio, dwell time and step size, of the generated oscillatory forces at each stage. Intact dynein molecules in the axoneme, doublet bundle and single doublet were used to measure the force with optical tweezers. The mean forces per dynein determined under three axonemal conditions were smaller than the previously reported stall forces of axonemal dynein; this phenomenon suggests that the duty ratio is lower than previously thought. This possibility was further confirmed by an in vitro motility assay with purified dynein. The dwell time and step size estimated from the measured force were similar. The similarity in these parameters suggests that the essential properties of dynein oscillation are inherent to the molecule and independent of the axonemal architecture, composing the functional basis of flagellar beating.

Single-molecule imaging and manipulation of biomolecular machines and systems

BACKGROUND

Biological molecular machines support various activities and behaviors of cells, such as energy production, signal transduction, growth, differentiation, and migration.

SCOPE OF REVIEW

We provide an overview of single-molecule imaging methods involving both small and large probes used to monitor the dynamic motions of molecular machines in vitro (purified proteins) and in living cells, and single-molecule manipulation methods used to measure the forces, mechanical properties and responses of biomolecules. We also introduce several examples of single-molecule analysis, focusing primarily on motor proteins and signal transduction systems.

MAJOR CONCLUSIONS

Single-molecule analysis is a powerful approach to unveil the operational mechanisms both of individual molecular machines and of systems consisting of many molecular machines.

GENERAL SIGNIFICANCE

Quantitative, high-resolution single-molecule analyses of biomolecular systems at the various hierarchies of life will help to answer our fundamental question: "What is life?" This article is part of a Special Issue entitled "Biophysical Exploration of Dynamical Ordering of Biomolecular Systems" edited by Dr. Koichi Kato.

Effects of external strain on the regulation of microtubule sliding induced by outer arm dynein of sea urchin sperm flagella

Oscillatory bending movement of eukaryotic flagella is powered by orchestrated activity of dynein motor proteins that hydrolyze ATP and produce microtubule sliding. Although the ATP concentration within a flagellum is kept uniform at a few mmol l−1 level, sliding activities of dyneins are dynamically coordinated along the flagellum in accordance with the phase of bending waves. Thus, at the organellar level the dynein not only generates force for bending but also modulates its motile activity by responding to bending of the flagellum. Single molecule analyses have suggested that dynein at the molecular level, even if isolated from the axoneme, could alter the modes of motility in response to mechanical strain. However, it still remains unknown whether the coordinated activities of multiple dyneins can be modulated directly by mechanical signals. Here, we studied the effects of externally applied strain on the sliding movement of microtubules interacted with ensemble of dynein molecules adsorbed on a glass surface. We found that by bending the microtubules with a glass microneedle, three modes of motility that have not been previously characterized without bending can be induced: those were, stoppage, backward sliding and dissociation. Modification in sliding velocities was also induced by imposed bending. These results suggest that the activities of dyneins interacted with a microtubule can be modified and coordinated through external strain in a quite flexible manner and that such regulatory mechanism may be the basis of flagellar oscillation.

Axonemal Dynein Arms

Axonemal dyneins form the inner and outer rows of arms associated with the doublet microtubules of motile cilia. These enzymes convert the chemical energy released from adenosine triphosphate (ATP) hydrolysis into mechanical work by causing the doublets to slide with respect to each other. Dyneins form two major groups based on the number of heavy-chain motors within each complex. In addition, these enzymes contain other components that are required for assembly of the complete particles and/or for the regulation of motor function in response to phosphorylations status, ligands such as Ca²⁺, changes in cellular redox state and which also apparently monitor and respond to the mechanical state or curvature in which any given motor finds itself. It is this latter property, which is thought to result in waves of motor function propagating along the axoneme length. Here, I briefly describe our current understanding of axonemal dynein structure, assembly, and organization.

The geometric clutch at 20: stripping gears or gaining traction?

It has been 20 years since the geometric clutch (GC) hypothesis was first proposed. The core principle of the GC mechanism is fairly simple. When the axoneme of a eukaryotic flagellum is bent, mechanical stress generates forces transverse to the outer doublets (t-forces). These t-forces can push doublets closer together or pry them apart. The GC hypothesis asserts that changes in the inter-doublet spacing caused by t-forces are responsible for the activation and deactivation of the dynein motors, that creates the beat cycle. A series of computer models utilizing the clutch mechanism has shown that it can simulate ciliary and flagellar beating. The objective of the present review is to assess where things stand with the GC hypothesis in the clarifying light of new information. There is considerable new evidence to support the hypothesis. However, it is also clear that it is necessary to modify some of the original conceptions of the hypothesis so that it can be consistent with the results of recent experimental and ultrastructural studies. In particular, dynein deactivation by t-forces must be able to occur with dyneins that remain attached to the B-subtubule of the adjacent doublet.

A computational model for the formation of lamin-B mitotic spindle envelope and matrix

Recent reports show that, after nuclear envelope breakdown, lamin-B, a component of the nuclear lamina in interphase, localizes around the mitotic spindle as a membranous network. How this process occurs, however, and how it influences mitotic spindle morphogenesis is unclear. Here, we develop a computational model based on a continuum description to represent the abundance and location of various molecular species involved during mitosis, and use the model to test a number of hypotheses regarding the formation of the mitotic matrix. Our model illustrates that freely diffusible nuclear proteins can be captured and transported to the spindle poles by minus-end-directed microtubule (MT) motors. Moreover, simulations show that these proteins can be used to build a shell-like region that envelopes the mitotic spindle, which helps to improve the focusing of the mitotic spindle by spatially restricting MT polymerization and limiting the effective diffusion of the free MTs. Simulations also confirm that spatially dependent regulation of the spindle network through the Ran system improves spindle focusing and morphology. Our results agree with experimental observations that lamin-B reorganizes around the spindle and helps to maintain spindle morphology.

Measuring the regulation of dynein activity during flagellar motility

Flagellar and ciliary motility are driven by the activity of dynein, which produces microtubule sliding within the axonemes. Our goal is to understand how dynein motile activity is regulated to produce the characteristic oscillatory movement of flagella. Analysis of various parameters, such as frequency and shear angle in beating flagella, is important for understanding the time-dependent changes of microtubule sliding amounts along the flagellum. Demembranated flagella can be reactivated in a wide range of ATP concentrations (from 2 μM to several mM) and the beat frequency increases with an increase in ATP. By imposed vibration of a micropipette that caught a sperm head by suction, however, the oscillatory motion can be modulated so as to synchronize to the vibration frequency over a range of 20-70Hz at 2mM ATP. The time-averaged sliding velocity calculated as a product of shear angle and vibration frequency decreases when the imposed frequency is below the undriven flagellar beat frequency, but at higher imposed frequencies, it remains constant. In addition to the role of ATP, the mechanical force of bending is involved in the activation of dynein. In elastase-treated axonemes, bending-dependent regulation of microtubule sliding is achieved. This chapter provides an overview of several approaches, using sea urchin sperm flagella, to studying the measurements in the regulation of dynein activity with or without mechanical force.

Molecular motor proteins and amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disorder affecting motor neurons in the brain, brainstem and spinal cord, which is characterized by motor dysfunction, muscle dystrophy and progressive paralysis. Both inherited and sporadic forms of ALS share common pathological features, however, the initial trigger of neurodegeneration remains unknown. Motor neurons are uniquely targeted by ubiquitously expressed proteins in ALS but the reason for this selectively vulnerability is unclear. However motor neurons have unique characteristics such as very long axons, large cell bodies and high energetic metabolism, therefore placing high demands on cellular transport processes. Defects in cellular trafficking are now widely reported in ALS, including dysfunction to the molecular motors dynein and kinesin. Abnormalities to dynein in particular are linked to ALS, and defects in dynein-mediated axonal transport processes have been reported as one of the earliest pathologies in transgenic SOD1 mice. Furthermore, dynein is very highly expressed in neurons and neurons are particularly sensitive to dynein dysfunction. Hence, unravelling cellular transport processes mediated by molecular motor proteins may help shed light on motor neuron loss in ALS.

Dynein light chain Tctex-type 1 modulates orexin signaling through its interaction with orexin 1 receptor

Orexins (OX-A, OX-B) are neuropeptides involved in the regulation of the sleep-wake cycle, feeding and reward, via activation of orexin receptors 1 and 2 (OX1R, OX2R). The loss of orexin peptides or functional OX2R has been shown to cause the sleep disorder, narcolepsy. Since the regulation of orexin receptors remains largely undefined, we searched for novel protein partners of the intracellular tail of orexin receptors. Using a yeast two-hybrid screening strategy in combination with co-immunoprecipitation experiments, we found interactions between OX1R and the dynein light chains Tctex-type 1 and 3 (Dynlt1, Dynlt3). These interactions were mapped to the C-terminal region of the dynein light chains and to specific residues within the last 10 amino acids of OX1R. Hence, we hypothesized that dynein light chains could regulate orexin signaling. In HEK293 cells expressing OX1R, stimulation with OX-A produced a less sustained extracellular signal-regulated kinases 1/2 (ERK1/2) activation when Dynlt1 was co-expressed, while it was prolonged under reduced Dynlt1 expression. The amount of OX1R located at the plasma membrane as well as the kinetics and extent of OX-A-induced internalization of OX1R (disappearance from membrane) were not altered by Dynlt1. However, Dynlt1 reduced the localization of OX1R in early endosomes following initial internalization. Taken together, these data suggest that Dynlt1 modulates orexin signaling by regulating OX1R, namely its intracellular localization following ligand-induced internalization.

Dysregulation of axonal transport and motorneuron diseases

MNDs (motorneuron diseases) are neurodegenerative disorders in which motorneurons located in the motor cortex, in the brainstem and in the spinal cord are affected. These diseases in their inherited or sporadic forms are mainly characterized by motor dysfunctions, occasionally associated with cognitive and behavioural alterations. Although these diseases show high variability in onset, progression and clinical symptoms, they share common pathological features, and motorneuronal loss invariably leads to muscle weakness and atrophy. One of the most relevant aspect of these disorders is the occurrence of defects in axonal transport, which have been postulated to be either a direct cause, or a consequence, of motorneuron degeneration. In fact, due to their peculiar morphology and high energetic metabolism, motorneurons deeply rely on efficient axonal transport processes. Dysfunction of axonal transport is known to adversely affect motorneuronal metabolism, inducing progressive degeneration and cell death. In this regard, the understanding of the fine mechanisms at the basis of the axonal transport process and of their possible alterations may help shed light on MND pathological processes. In the present review, we will summarize what is currently known about the alterations of axonal transport found to be either causative or a consequence of MNDs.

Kinesin-5, a mitotic microtubule-associated motor protein, modulates neuronal migration

Kinesin-5 (also called Eg5 or kif11) is a homotetrameric motor protein that functions by modulating microtubule (MT)-MT interactions. In the case of mitosis, kinesin-5 slows the rate of separation of the half-spindles. In the case of the axon, kinesin-5 limits the frequency of transport of short MTs, and also limits the rate of axonal growth. Here we show that experimental inhibition of kinesin-5 in cultured migratory neurons results in a faster but more randomly moving neuron with a shorter leading process. As is the case with axons of stationary neurons, short MT transport frequency is notably enhanced in the leading process of the migratory neuron when kinesin-5 is inhibited. Conversely, overexpression of kinesin-5, both in culture and in developing cerebral cortex, causes migration to slow and even cease. Regions of anti-parallel MT organization behind the centrosome were shown to be especially rich in kinesin-5, implicating these regions as potential sites where kinesin-5 forces may be especially relevant. We posit that kinesin-5 acts as a "brake" on MT-MT interactions that modulates the advance of the entire MT apparatus. In so doing, kinesin-5 regulates the rate and directionality of neuronal migration and possibly the cessation of migration when the neuron reaches its destination.

Mouse cytoplasmic dynein intermediate chains: identification of new isoforms, alternative splicing and tissue distribution of transcripts

Background

Intracellular transport of cargoes including organelles, vesicles, signalling molecules, protein complexes, and RNAs, is essential for normal function of eukaryotic cells. The cytoplasmic dynein complex is an important motor that moves cargos along microtubule tracks within the cell. In mammals this multiprotein complex includes dynein intermediate chains 1 and 2 which are encoded by two genes, Dync1i1 and Dync1i2. These proteins are involved in dynein cargo binding and dynein complexes with different intermediate chains bind to specific cargoes, although the mechanisms to achieve this are not known. The DYNC1I1 and DYNC1I2 proteins are translated from different splice isoforms, and specific forms of each protein are essential for the function of different dynein complexes in neurons.

Methodology/Principal Findings

Here we have undertaken a systematic survey of the dynein intermediate chain splice isoforms in mouse, basing our study on mRNA expression patterns in a range of tissues, and on bioinformatics analysis of mouse, rat and human genomic and cDNA sequences. We found a complex pattern of alternative splicing of both dynein intermediate chain genes, with maximum complexity in the embryonic and adult nervous system. We have found novel transcripts, including some with orthologues in human and rat, and a new promoter and alternative non-coding exon 1 for Dync1i2.

Conclusions/Significance

These data, including the cloned isoforms will be essential for understanding the role of intermediate chains in the cytoplasmic dynein complex, particularly their role in cargo binding within individual tissues including different brain regions.

LIS1 and NudE induce a persistent dynein force-producing state

Cytoplasmic dynein is responsible for many aspects of cellular and subcellular movement. LIS1, NudE, and NudEL are dynein interactors initially implicated in brain developmental disease but now known to be required in cell migration, nuclear, centrosomal, and microtubule transport, mitosis, and growth cone motility. Identification of a specific role for these proteins in cytoplasmic dynein motor regulation has remained elusive. We find that NudE stably recruits LIS1 to the dynein holoenzyme molecule, where LIS1 interacts with the motor domain during the prepowerstroke state of the dynein crossbridge cycle. NudE abrogates dynein force production, whereas LIS1 alone or with NudE induces a persistent-force dynein state that improves ensemble function of multiple dyneins for transport under high-load conditions. These results likely explain the requirement for LIS1 and NudE in the transport of nuclei, centrosomes, chromosomes, and the microtubule cytoskeleton as well as the particular sensitivity of migrating neurons to reduced LIS1 expression.

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.

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.

LIS1 and NDEL1 coordinate the plus-end-directed transport of cytoplasmic dynein

LIS1 was first identified as a gene mutated in human classical lissencephaly sequence. LIS1 is required for dynein activity, but the underlying mechanism is poorly understood. Here, we demonstrate that LIS1 suppresses the motility of cytoplasmic dynein on microtubules (MTs), whereas NDEL1 releases the blocking effect of LIS1 on cytoplasmic dynein. We demonstrate that LIS1, cytoplasmic dynein and MT fragments co-migrate anterogradely. When LIS1 function was suppressed by a blocking antibody, anterograde movement of cytoplasmic dynein was severely impaired. Immunoprecipitation assay indicated that cytoplasmic dynein forms a complex with LIS1, tubulins and kinesin-1. In contrast, immunoabsorption of LIS1 resulted in disappearance of co-precipitated tubulins and kinesin. Thus, we propose a novel model of the regulation of cytoplasmic dynein by LIS1, in which LIS1 mediates anterograde transport of cytoplasmic dynein to the plus end of cytoskeletal MTs as a dynein-LIS1 complex on transportable MTs, which is a possibility supported by our data.

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.

Interaction of the DYNLT (TCTEX1/RP3) light chains and the intermediate chains reveals novel intersubunit regulation during assembly of the dynein complex

The cytoplasmic dynein 1 cargo binding domain is formed by five subunits including the intermediate chain and the DYNLT, DYNLL, and DYNLRB light chain families. Six isoforms of the intermediate chain and two isoforms of each of the light chain families have been identified in mammals. There is evidence that different subunit isoforms are involved in regulating dynein function, in particular linking dynein to different cargoes. However, it is unclear how the subunit isoforms are assembled or if there is any specificity to their interactions. Co-immunoprecipitation using DYNLT-specific antibodies reveals that dynein complexes with DYNLT light chains also contain the DYNLL and DYNLRB light chains. The DYNLT light chains, but not DYNLL light chains, associate exclusively with the dynein complex. Yeast two-hybrid and co-immunoprecipitation assays demonstrate that both members of the DYNLT family are capable of forming homodimers and heterodimers. In addition, both homodimers of the DYNLT family bind all six intermediate chain isoforms. However, DYNLT heterodimers do not bind to the intermediate chain. Thus, whereas all combinations of DYNLT light chain dimers can be made, not all of the possible combinations of the isoforms are utilized during the assembly of the dynein complex.

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.

Kinetic characterization of tail swing steps in the ATPase cycle of Dictyostelium cytoplasmic dynein

According to the power stroke model of dynein deduced from electron microscopic and fluorescence resonance energy transfer studies, the power stroke and the recovery stroke are expected to take place at the two isomerization steps of the ATPase cycle at the primary ATPase site. Here, we have conducted presteady-state kinetic analyses of these two isomerization steps with the single-headed motor domain of Dictyostelium cytoplasmic dynein by employing fluorescence resonance energy transfer to probe ATPase steps at the primary site and tail positions. Our results show that the recovery stroke at the first isomerization step proceeds quickly ( approximately 180 s(-1)), whereas the power stroke at the second isomerization step is very slow ( approximately 0.2 s(-1)) in the absence of microtubules, and that the presence of microtubules accelerates the second but not the first step. Moreover, a comparison of the microtubule-induced acceleration of the power stroke step and that of steady-state ATP hydrolysis implies the intriguing possibility that microtubules simultaneously accelerate the ATPase activity not only at the primary site but also at other site(s) in the motor domain.

How dynein helps the cell find its center: a servomechanical model

Cytoplasmic dynein is the major minus-end-directed microtubule motor protein in interphase cells. In addition to its well-established roles in vesicular transport and chromosome dynamics, cytoplasmic dynein also associates with the cell cortex. From this site, it appears to pull on the cytoplasmic microtubule network, influencing mitotic spindle orientation, nuclear position and other aspects of cell polarity and organization. Recent evidence indicates that the cell has the remarkable ability to calculate is geometric center, and, with the help of dynein, to position the centrosome at this central site. Here, we outline models to account for this behavior.

Genetic analysis of the cytoplasmic dynein subunit families

Cytoplasmic dyneins, the principal microtubule minus-end-directed motor proteins of the cell, are involved in many essential cellular processes. The major form of this enzyme is a complex of at least six protein subunits, and in mammals all but one of the subunits are encoded by at least two genes. Here we review current knowledge concerning the subunits, their interactions, and their functional roles as derived from biochemical and genetic analyses. We also carried out extensive database searches to look for new genes and to clarify anomalies in the databases. Our analysis documents evolutionary relationships among the dynein subunits of mammals and other model organisms, and sheds new light on the role of this diverse group of proteins, highlighting the existence of two cytoplasmic dynein complexes with distinct cellular roles.

Design and regulation of the AAA+ microtubule motor dynein

Dyneins are highly complex molecular motors that transport their attached cargo towards the minus end of microtubules. These enzymes are required for many essential motile activities within the cytoplasm and also power eukaryotic cilia and flagella. Each dynein contains one or more heavy chain motor units that consist of an N-terminal stem domain that is involved in cargo attachment, and six AAA+ domains (AAA1-6) plus a C-terminal globular segment that are arranged in a heptameric ring. At least one AAA+ domain (AAA1) is capable of ATP binding and hydrolysis, and the available data suggest that one or more additional domains also may bind nucleotide. The ATP-sensitive microtubule binding site is located at the tip of a 10nm coiled coil stalk that emanates from between AAA4 and AAA5. The function of this motor both in the cytoplasm and the flagellum must be tightly regulated in order to result in useful work. Consequently, dyneins also contain a series of additional components that serve to define the cargo-binding properties of the enzyme and which act as sensors to transmit regulatory inputs to the motor units. Here we describe the two basic dynein designs and detail the various regulatory systems that impinge on this motor within the eukaryotic flagellum.

Disruption of genes encoding predicted inner arm dynein heavy chains causes motility phenotypes in Tetrahymena

The multi-dynein hypothesis [Asai, 1995: Cell Motil Cytoskeleton 32:129-132] states: (1) there are many different dynein HC isoforms; (2) each isoform is encoded by a different gene; (3) different isoforms have different functions. Many studies provide evidence in support of the first two statements [Piperno et al., 1990: J Cell Biol 110:379-389; Kagami and Kamiya, 1992: J Cell Sci 103:653-664; Gibbons, 1995: Cell Motil Cytoskeleton 32:136-144; Porter et al., 1996: Genetics 144:569-585; Xu et al., 1999: J Eukaryot Microbiol 46:606-611] and there is evidence that outer arms and inner arms play different roles in flagellar beating [Brokaw and Kamiya, 1987: Cell Motil. Cytoskeleton 8:68-75]. However, there are few studies rigorously testing in vivo whether inner arm dyneins, especially the 1-headed inner arm dyneins, play unique roles. This study tested the third tenet of the multi-dynein hypothesis by introducing mutations into three inner arm dynein HC genes (DYH8, 9 and 12) that are thought to encode HCs associated with 1-headed inner arm dyneins. Southern blots, Northern blots, and RT-PCR analyses indicate that all three mutants (KO-8, 9, and 12) are complete knockouts. Each mutant swims slower than the wild-type cells. The beat frequency of KO-8 cells is lower than that of the wild-type cells while the beat frequencies of KO-9 and KO-12 are not different from that of wild-type cells. Our results suggest that each inner arm dynein HC is essential for normal cell motility and cannot be replaced functionally by other dynein HCs and that not all of the 1-headed inner arm dyneins play the same role in ciliary motility. Thus, the results of our study support the multi-dynein hypothesis [Asai, 1995: Cell Motil Cytoskeleton 32:129-132].

Functional diversity of axonemal dyneins as studied in Chlamydomonas mutants

Cilia and flagella of most organisms are equipped with two kinds of motor protein complex, the inner and outer dynein arms. The two arms were previously thought to be similar to each other, but recent studies using Chlamydomonas mutants indicate that they differ significantly in subunit structure and arrangement within the axoneme. For example, whereas the outer dynein arm exists as a single protein complex containing three heavy chains, the inner dynein arm comprises seven different subspecies each containing one or two discrete heavy chains. Furthermore, the two kinds of arms appear to differ in function also. Most strikingly, our studies suggest that inner-arm dynein, but not outer-arm dynein, is under the control of the central pair microtubules and radial spokes. The axoneme thus appears to be equipped with two rather distinct systems for beating: one involving inner-arm dyneins, the central pair and radial spokes, and the other involving outer-arm dynein alone.

Microtubule displacements at the tips of living flagella

We have observed that the flagellar axoneme of the Chinese hamster spermatozoon undergoes periodic changes in length at the same frequency as the flagellar beat. The amplitude of the length oscillation recorded at the tip is maximally about 0.5 microm or 0.2% of the total length. In some favourable cells, it was possible to see the opposing "halves" of the axoneme moving at the tip in a reciprocating manner and 180 degrees out-of-phase. This behaviour, when analysed quantitatively, is broadly consistent with predictions made from the sliding-doublet theory of ciliary and flagellar motility and thus it constitutes an additional verification of the theory, for the first time in a living cell. However, on close examination, there is a partial mismatch between the timing of the length oscillation and the phase of the beat cycle. We deduce from this that there is some sliding at the base of the flagellum, sliding that is accommodated by elastic compression of the connecting piece. Micrographic evidence for such compression is presented.

Molecular motors and their functions in plants

Molecular motors that hydrolyze ATP and use the derived energy to generate force are involved in a variety of diverse cellular functions. Genetic, biochemical, and cellular localization data have implicated motors in a variety of functions such as vesicle and organelle transport, cytoskeleton dynamics, morphogenesis, polarized growth, cell movements, spindle formation, chromosome movement, nuclear fusion, and signal transduction. In non-plant systems three families of molecular motors (kinesins, dyneins, and myosins) have been well characterized. These motors use microtubules (in the case of kinesines and dyneins) or actin filaments (in the case of myosins) as tracks to transport cargo materials intracellularly. During the last decade tremendous progress has been made in understanding the structure and function of various motors in animals. These studies are yielding interesting insights into the functions of molecular motors and the origin of different families of motors. Furthermore, the paradigm that motors bind cargo and move along cytoskeletal tracks does not explain the functions of some of the motors. Relatively little is known about the molecular motors and their roles in plants. In recent years, by using biochemical, cell biological, molecular, and genetic approaches a few molecular motors have been isolated and characterized from plants. These studies indicate that some of the motors in plants have novel features and regulatory mechanisms. The role of molecular motors in plant cell division, cell expansion, cytoplasmic streaming, cell-to-cell communication, membrane trafficking, and morphogenesis is beginning to be understood. Analyses of the Arabidopsis genome sequence database (51% of genome) with conserved motor domains of kinesin and myosin families indicates the presence of a large number (about 40) of molecular motors and the functions of many of these motors remain to be discovered. It is likely that many more motors with novel regulatory mechanisms that perform plant-specific functions are yet to be discovered. Although the identification of motors in plants, especially in Arabidopsis, is progressing at a rapid pace because of the ongoing plant genome sequencing projects, only a few plant motors have been characterized in any detail. Elucidation of function and regulation of this multitude of motors in a given species is going to be a challenging and exciting area of research in plant cell biology. Structural features of some plant motors suggest calcium, through calmodulin, is likely to play a key role in regulating the function of both microtubule- and actin-based motors in plants.

AAA domains and organization of the dynein motor unit

Dyneins contain one-three microtubule motor units that are each derived from the C-terminal globular head of a heavy chain. The N-terminal regions of the heavy chains form stems that are required for intra-dynein associations. The microtubule-binding sites are located at the terminus of a short stalk that emanates from each globular head. Recent electron microscopic analysis indicates that the dynein head has a heptameric toroidal organization. This finding is echoed by the identification of six AAA (ATPases associated with cellular activities) domains and a seventh unrelated unit within this heavy chain region. At least two of these AAA domains can bind nucleotide, although only one appears able to hydrolyze ATP. Several other AAA domain proteins exhibit a similar annular organization of six AAA units. Detailed structural information is available for several AAA proteins, including N-ethylmaleimide-sensitive vesicle-fusion protein and the RuvB motor involved in DNA migration and resolution of Holliday junctions. The resulting structural parallels allow intriguing predictions to be made concerning dynein organization and motor function.

The dynein microtubule motor

Dyneins are large multi-component microtubule-based molecular motors involved in many fundamental cellular processes including vesicular transport, mitosis and ciliary/flagellar beating. In order to achieve useful work, these enzymes must contain motor, cargo-binding and regulatory components. The ATPase and microtubule motor domains are located within the very large dynein heavy chains that form the globular heads and stems of the complex. Cargo-binding activity involves the intermediate chains and several classes of light chain that associate in a subcomplex at the base of the soluble dynein particle. Regulatory control of dynein motor function is thought to involve the phosphorylation of various components as well as a series of light chain proteins that are directly associated with the heavy chains. These latter polypeptides have a variety of intriguing attributes, including redox-sensitive vicinal dithiols and Ca(2+)-binding, suggesting that the activity of individual dyneins may be subject to multiple regulatory inputs. Recent molecular, genetic and structural studies have revealed insight into the roles played by these various components and the mechanisms of dynein-based motility.

Cytoplasmic dynein and microtubule transport in the axon: the action connection

The neuron uses two families of microtubule-based motors for fast axonal transport, kinesin, and cytoplasmic dynein. Cytoplasmic dynein moves membranous organelles from the distal regions of the axon to the cell body. Because dynein is synthesized in the cell body, it must first be delivered to the axon tip. It has recently been shown that cytoplasmic dynein is moved from the cell body along the axon by two different mechanisms. A small amount is associated with fast anterograde transport, the membranous organelles moved by kinesin. Most of the dynein is transported in slow component b, the actin-based transport compartment. Dynactin, a protein complex that binds dynein, is also transported in slow component b. The dynein in slow component b binds to microtubules in an ATP-dependent manner in vitro, suggesting that this dynein is enzymatically active. The finding that functionally active dynein, and dynactin, are associated with the actin-based transport compartment suggests a mechanism whereby dynein anchored to the actin cytoskeleton via dynactin provides the motive force for microtubule movement in the axon.

Unidirectional movement of fluorescent microtubules on rows of dynein arms of disintegrated axonemes

Tetramethylrhodamine-labelled microtubules were observed to move on rows of dynein arms of sea urchin sperm axonemes exposed by elastase-induced sliding disintegration. The microtubules moved towards the flagellar tip at a velocity of 3.1+/−2.1 microm second-1 (mean +/− s.d., n=53) in the presence of 0.1 mM ATP at 22 degrees C, but none moved towards the sperm head. We also examined the polarity of microtubule binding to axonemal doublet microtubules in the absence of ATP by using microtubules brightly labelled at their minus-ends. In 140 of 210 microtubules studied, they bound to axonemal microtubules with a parallel polarity. These results suggest that tightly packed dynein arms on the outer doublet microtubules of sperm axoneme preferentially bind microtubules to themselves with the same polarity as that of the axoneme and that they generate a force to move only these microtubules in the direction away from the sperm head.

Bi-directional movement of actin filaments along long bipolar tracks of oriented rabbit skeletal muscle myosin molecules

In actomyosin in vitro motility assays, orientation of myosin molecules affects their interaction with actin. We obtained long tracks of myosin molecules with uniform orientation. Bipolar filaments about 50 microm long were made from myosin rod prepared from molluscan smooth muscles, to which rabbit skeletal-muscle myosin bound, creating long synthetic thick-filaments. Movement of F-actin toward their center was much faster (4.7 +/- 0.6 microm s(-1)) than in the opposite direction (1.9 +/- 0.2 microm s(-1)), indicating that myosin molecules were arranged in the same orientation along each half of the bipolar filament. These complex thick-filaments permit measurement of actin movement over 20 microm of oriented skeletal myosin tracks facilitating mechanistic studies of actomyosin motility.

A model for flagellar motility

Experimental investigation has provided a wealth of structural, biochemical, and physiological information regarding the motile mechanism of eukaryotic flagella/cilia. This chapter surveys the available literature, selectively focusing on three major objectives. First, it attempts to identify those conserved structural components essential to providing motile function in eukaryotic axonemes. Second, it examines the relationship between these structural elements to determine the interactions that are vital to the mechanism of flagellar/ciliary beating. Third, the vital principles of these interactions are incorporated into a tractable theoretical model, referred to as the Geometric Clutch, and this hypothetical scheme is examined to assess its compatibility with experimental observations.

In vitro motility from recombinant dynein heavy chain

The dyneins are a class of motor protein involved in ciliary and flagellar motility, organelle transport, and chromosome segregation. Because of their large size and subunit complexity, relatively little is known about their mechanisms of force production and regulation. We report here on the expression and analysis of the entire rat cytoplasmic dynein heavy chain (Mr 532,000). Full-length cDNAs were constructed from a series of partial clones and tagged at the C terminus with either a FLAG-epitope tag or a His6-tag. The recombinant polypeptides were expressed either in insect cells by baculovirus infection or in COS-7 cells by transient transfection. The recombinant protein was mostly soluble and showed good microtubule binding. It exhibited a broad sedimentation profile, indicative of the formation of dimers as well as higher order multimers. Good microtubule gliding motility activity was observed in assays of heavy chain expressed in either insect or COS-7 cells. Average microtubule gliding velocities of 1.2-1.8 microm/sec were observed, comparable with the rates determined for calf brain cytoplasmic dynein. These results represent the first indication that recombinant heavy chain alone is capable of force production, and should lead to rapid progress in defining the dynein motor domain.

Microtubules and microtubule motors: mechanisms of regulation

Microtubule-based motility is precisely regulated, and the targets of regulation may be the motor proteins, the microtubules, or both components of this intricately controlled system. Regulation of microtubule behavior can be mediated by cell cycle-dependent changes in centrosomal microtubule nucleating ability and by cell-specific, microtubule-associated proteins (MAPs). Changes in microtubule organization and dynamics have been correlated with changes in phosphorylation. Regulation of motor proteins may be required both to initiate movement and to dictate its direction. Axonemal and cytoplasmic dyneins as well as kinesin can be phosphorylated and this modification may affect the motor activities of these enzymes or their ability to interact with organelles. A more complete understanding of how motors can be modulated by phosphorylation, either of the motor proteins or of other associated substrates, will be necessary in order to understand how bidirectional transport is regulated.

Anterograde to retrograde reversal of fast axonal transport within cold blocked and rewarmed intact axons

The possibility that anterograde to retrograde reversal of axonal transport might take place in mid axon at a site distant from any nerve termination was investigated in sciatic nerve preparations from Xenopus laevis. The nerve, containing a pulse of anterogradely transported protein labeled with [35S]methionine, was kept in a two-compartment temperature controlled chamber. One compartment containing the proximal nerve was maintained at room temperature throughout the duration of an experiment while the second compartment containing the distal nerve, and separated from the first by a thermal barrier, was initially cooled to 3-4 degrees C and later warmed to room temperature. Transport of labeled proteins in the nerve was detected with a position-sensitive detector of ionizing radiation. With the distal portion of the nerve cold, the pulse of labeled protein transported up to the thermal barrier and stopped. When the distal part of the nerve was warmed to room temperature, retrograde and anterograde pulses of label propagated away from the thermal barrier with no time delay. The retrograde pulse could be collected on the distal side of a proximally placed tie and could be eliminated by treatment of the proximal nerve with vinblastine or dinitrophenol. Functional and structural evidence indicated that the cold block and thermal barrier were not destructive to the axons. Electron microscopy showed that the numerical density of axonal microtubules distal to the cold block was decreased about seven fold during the cold treatment and that this decrease could be prevented by 10 mumol/l taxol.(ABSTRACT TRUNCATED AT 250 WORDS)

C/A dynein isolated from sea urchin sperm flagellar axonemes. Enzymatic properties and interaction with microtubules

C/A dynein is a novel dynein isolated from sea urchin sperm flagellar axonemes. It is composed of C and A heavy chains and some additional lower molecular mass polypeptide chains. The characterization of ATPase activity and the interaction of this dynein with microtubules polymerized from calf brain tubulin were investigated in this study. The ATPase activity of C/A dynein (0.3-0.4 mumol Pi/min per mg) was about one half that of outer arm 21 S dynein (0.6-0.8 mumol Pi/min per mg) at 25 degrees C. Vanadate inhibited the ATPase activity with a half-maximal inhibition at 1 microM. C/A dynein absorbed to the glass surface was able to translocate the microtubules towards its plus end. The velocity of the microtubule movement in the presence of 1 mM ATP was 4.0 to 4.5 microns/s at 22 degrees C. C/A dynein binds to and bundles the microtubules even in the presence of ATP. Cross-bridges were found between adjacent microtubules in the bundle with an axial periodicity of about 24 nm. The ATPase activity of C/A dynein was enhanced up to several-fold by the microtubules at concentration as low as 1 mg/ml. On the other hand, 21 S dynein bound to the microtubules with 24 nm axial periodicity only in the absence of ATP. Its ATPase activity was not activated by the microtubules. From these results, it is concluded that the manner of interaction with microtubules of C/A dynein is different from that of the outer arm dynein.

22S axonemal dynein is preassembled and functional prior to being transported to and attached on the axonemes

In an earlier study we reported the isolation of a cytoplasmic dynein from the cytosol of Paramecium multimicronucleatum. In this study we report the isolation and characterization of two cytosolic axonemal dyneins (22S and 12S) as well as a 19S cytoplasmic dynein from the cytosol of whole or deciliated cells using preformed bovine brain microtubules. These three dynein species were characterized according to mass, morphology, vanadate photocleavage patterns, CTPase/ATPase ratios, Km and Vmax values, temperature optima and reactivity with a mAb. For comparison, 22S and 12S axonemal dyneins (ADs) were also isolated and purified from the demembranated axonemes. The 22S and 12S soluble dyneins appear to be related to ciliary ADs in that the 22S soluble dynein is three-headed while the 12S is a one-headed dynein, as determined by negative staining. Ciliary ADs and their corresponding 22S and 12S soluble dyneins isolated from the cytosol also have similar Km and Vmax values as well as vanadate photocleavage patterns and temperature optima. A mAb raised against the soluble 22S dynein reacted with the 22S ciliary dyneins but not the 12S axonemal or the 19S cytoplasmic dynein. All isolated dyneins supported similar microtubule gliding rates but had different ionic requirements for the translocation buffer. These results suggest that: (i) the two soluble 22S and 12S dyneins are precursor molecules of the ciliary dyneins, (ii) the subunits of the outer arm dynein are already assembled in the cytosol as a three-headed bouquet, and (iii) the 22S and 12S soluble dyneins are functional prior to being transported and attached to the axonemes of the cilia.

A Chlamydomonas outer arm dynein mutant with a truncated beta heavy chain

A new allele of the Chlamydomonas oda4 flagellar mutant (oda4-s7) possessing abnormal outer dynein arms was isolated. Unlike the previously described oda4 axoneme lacking all three (alpha, beta, and gamma) outer-arm dynein heavy chains, the oda4-s7 axoneme contains the alpha and gamma heavy chains and a novel peptide with a molecular mass of approximately 160 kD. The peptide reacts with a mAb (18 beta B) that recognizes an epitope on the NH2-terminal part of the beta heavy chain. These observations indicate that this mutant has a truncated beta heavy chain, and that the NH2-terminal part of the beta heavy chain is important for the stable assembly of the outer arms. In averaged electron microscopic images of outer arms from cross sections of axonemes, the mutant outer arm lacks its mid-portion, producing a forked appearance. Together with our previous finding that the mutant oda11 lacks the alpha heavy chain and the outermost portion of the arm (Sakakibara, H., D. R. Mitchell, and R. Kamiya. 1991. J. Cell Biol. 113:615-622), this result defines the approximate locations of the three outer arm heavy chains in the axonemal cross section. The swimming velocity of oda4-s7 is 65 +/- 8 microns/s, close to that of oda4 which lacks the entire outer arm (62 +/- 8 microns/s) but significantly lower than the velocities of wild type (194 +/- 23 microns/s) and oda11 (119 +/- 17 microns/s). Thus, the lack of the beta heavy chain impairs outer-arm function more seriously than does the lack of the alpha heavy chain, suggesting that the alpha and beta chains play different roles in outer arm function.

Translocation and rotation of microtubules caused by multiple species of Chlamydomonas inner-arm dynein

Dynein was extracted from outer arm-less axonemes of the mutant oda1 and fractionated by high-pressure liquid chromatography on a MonoQ column into seven distinct subspecies (named a-g). Each subspecies contained one or two heavy chains and several medium-sized and light chains; by vanadate/UV-induced photocleavage and SDS- polyacrylamide gel electrophoresis, eight distinct heavy chains were identified. Analysis of the mutant axonemes indicated that the subspecies f (containing two heavy chains) is missing in the inner-arm mutant ida1 and the subspecies a, c and d are missing in the mutant ida4. Six subspecies (all but f) supported microtubule translocation with the maximal rate ranging from 2 to 12 micrometre s-1 and the apparent Km for ATP ranging from about 10 to 100 micromolar. All the subspecies translocated microtubules with the plus end leading, indicating that all the inner-arm dyneins are minus end-directed motors. Five subspecies (all but b and f) displayed microtubule rotation during translocation at rates of up to about 10 Hz. Unexpectedly, the Km values for ATP for translocation and rotation did not always agree; because of this, the pitch of the movement was variable with some subspecies. These observations indicate that axonemes are equipped with several inner-arm subspecies and that torque generation is a feature common to many of them.