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Siegel GJ, Agranoff BW, Albers RW, et al., editors. Basic Neurochemistry: Molecular, Cellular and Medical Aspects. 6th edition. Philadelphia: Lippincott-Raven; 1999.

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Cover of Basic Neurochemistry

Basic Neurochemistry: Molecular, Cellular and Medical Aspects. 6th edition.

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Molecular Motors: Kinesin, Dynein and Myosin

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Correspondence to Scott T. Brady, Department of Cell Biology and Neuroscience, University of Texas Southwestern Medical Center, 5323 Harry Hines Boulevard, Dallas, Texas 75235-9111.

Prior to 1985, the only molecular motors characterized in vertebrate cells were muscle myosins and flagellar dyneins. Myosins had been purified from nervous tissue, but no clear functions were established. Given evidence that fast axonal transport was microtubule-based, many investigators looked for dynein in the cytoplasm, with equivocal success. Moreover, the biochemical properties of fast transport were inconsistent with both myosin and dynein [10]. The pharmacology and biochemistry of fast axonal transport created a picture of organelle transport distinct from muscle contraction or flagellar beating.

The characteristic properties of different molecular motors aid in their identification

One striking difference between fast axonal transport and myosin- or dynein-based motility emerged from studies with ATP analogues. Adenylyl-imidodiphosphate (AMP-PNP), a nonhydrolyzable analogue of ATP, is a weak competitive inhibitor of both myosin and dynein. However, when AMP-PNP is perfused into axoplasm, bidirectional transport stops within minutes [1,28]. Both anterograde and retrograde moving organelles freeze in place on MTs. Inhibition of fast axonal transport by AMP-PNP indicated that fast axonal transport involved a new class of motors [1,28] and suggested that this new motor should have a high affinity for MTs in the presence of AMP-PNP. The polypeptide composition of this new motor molecule was soon defined and it was christened kinesin [29,30]. This discovery raised the possibility of other novel motor molecules, and soon additional classes of molecular motor began to be identified. The proliferation of motor types has transformed our understanding of cellular motility.

All three classes of molecular motor proteins are now known to be large protein families with diverse cellular functions [10]. Both the kinesin family [31,32] and the myosin family [33] have been defined and their proteins grouped into subfamilies. Finally, the elusive cytoplasmic version of dynein was identified and a multigene family of flagellar and cytoplasmic dyneins defined. Members of a given motor protein family share significant homology in their motor domains with the defining member, kinesin, dynein or myosin; but they also contain unique protein domains that are specialized for interaction with different cargoes. This large number of motor proteins may reflect the number of cellular functions that require force generation or movement, ranging from mitosis to morphogenesis to transport of vesicles. Here, we focus on motor proteins that are thought to be important for axonal transport or neuronal function, starting with kinesin.

Kinesins mediate anterograde transport in a variety of organisms and tissues

Since its discovery, much has been learned about the biochemical, pharmacological and molecular properties of kinesin [34]. Kinesin is the most abundant member of the kinesin family in vertebrates and is widely distributed in neuronal and non-neuronal cells. The holoenzyme is a heterotetramer comprising two heavy chains of 115 to 130 kDa and two light chains of 62 to 70 kDa. Structural studies have shown that kinesin is a rod-shaped protein approximately 80 nm in length with two globular heads connected to a fan-like tail by a long stalk. High-resolution electron microscopic immunolocalization of kinesin subunits and molecular genetic studies both indicate that kinesin heavy chains are arranged in parallel with their amino termini, forming the heads and much of the stalk. The kinesin heavy chain heads comprise the motor domains, containing both ATP- and MT-binding motifs. This motor domain is the most highly conserved region within the kinesin family. Binding of kinesin to MTs is stabilized by AMP-PNP, and this property remains a hallmark of kinesins. Kinesin light chains localize to the fan-like tail and may contribute to part of the stalk. The stalk itself is formed by α-helical coiled coil domains that are present in both heavy and light chains. Kinesin light chains appear to be unique to conventional kinesin but are highly conserved across species. Light chains are thought to be involved in organelle binding and may play a role in targeting of kinesin isoforms to different types of MBO [35].

Considerable evidence implicates kinesin as a motor molecule for fast axonal transport. Kinesin is an MT-activated ATPase with minimal basal activity [34]. MTs will glide across kinesin-coated glass surfaces with motor movement toward the plus end [10,30]. Since axonal MTs have a uniform polarity with their plus ends distal, the directionality of kinesin is consistent with an anterograde transport motor. Immunofluorescence and electron microscopy have shown that kinesin is associated with MBOs that are, in turn, associated with MTs [36,37] (Fig. 28-7). While these properties of kinesin were consistent with a role in axonal transport of MBOs, they were insufficient to prove the hypothesis that kinesin was the fast axonal transport motor. Such proof came from inhibition of kinesin function by antibodies against both heavy and light chains of kinesin [10,35]. Since kinesin light chains are associated only with conventional kinesin, the ability of kinesin light chain antibodies to inhibit transport is compelling evidence that kinesin is involved in fast axonal transport. Finally, reduction of kinesin levels using antisense oligonucleotides and gene deletion also implicates kinesin in axonal transport processes [31].

Figure 28-7. Axonally transported vesicles and the axonal cytoskeleton in longitudinal section.

Figure 28-7

Axonally transported vesicles and the axonal cytoskeleton in longitudinal section. Quick-freeze, deep-etch electron micrograph of a region of rat spinal cord neurite rich in membrane-bound organelles and microtubules. Arrows point to rod-shaped structures (more...)

In neurons and non-neuronal cells, kinesin is associated with a variety of MBOs, ranging from synaptic vesicles to mitochondria to lysosomes. In addition to its role in fast axonal transport and related phenomena in non-neuronal cells, kinesin appears to be involved in constitutive cycling of membranes between the Golgi complex and endoplasmic reticulum [38]. However, kinesin is not associated with all cellular membranes. For example, the nucleus, membranes of the Golgi complex and the plasma membrane appear to lack kinesin. Kinesin interactions with membranes are thought to involve the light chains and carboxyl termini of heavy chains. However, neither this selectivity nor the molecular basis for binding of kinesin and other motors to membranes is well understood.

Cloning and immunochemical studies of kinesin subunits have demonstrated that multiple isoforms of kinesin heavy and light chains exist in the brain. At least two heavy chain genes are expressed in mammals, a ubiquitous kinesin heavy chain gene expressed in all tissues and a neuron-specific heavy chain gene that is expressed only in brain [34]. At least two different genes exist for the kinesin light chains as well, and differential splicing for one of these light chain genes generates three different light chain polypeptides. These different kinesin isoforms are differentially expressed in tissues. Heterogeneity in kinesin heavy and light chains may regulate the transport of different MBO types and ensure that organelles reach their correct destinations in the axon. Radiolabeled isoforms of the kinesin heavy chains move down the axon at different net rates that correlate with different MBO types, such as synaptic vesicles and mitochondria [39]. These observations suggest that kinesin isoforms exist to transport different types of organelles.

Multiple members of the kinesin superfamily are expressed in the nervous system

Kinesin has been purified and then cloned from many species, including Drosophila, squid, sea urchin, chicken, rat and human. Conventional kinesin is highly conserved. Once the sequence of the kinesin motor domain was available, related proteins with homology only in the motor domain began to be identified. Kinesin-related proteins (KRPs) were first identified in yeast and fungal mutants with defective cell division, but many others are now known [31,32].

All of the members of the large kinesin family of proteins have well-conserved motor domains, but KRPs are highly variable in sequence and structure. Even the position of the motor domain varies. Kinesin and many other family members have amino-terminal motor domains, but other KRPs have motor domains at the carboxyl terminus and some have centrally located motor domains. This variation in structure has functional significance. To date, all tested amino and central motor domain proteins move toward the MT plus end, while carboxyl motor domain proteins move toward the MT minus end. Many KRPs are known only from their sequences and expression, but a few have been examined for function. A number of KRPs are involved in various steps of cell division, but precise cellular functions are still being defined for most of these new motors.

Systematic cloning strategies based on the conserved motor domain sequences have identified a remarkable number of KRPs expressed in the brain [32]. Members of several KRP families expressed in the brain have been implicated in forms of MBO transport. Kinesin-inhibiting factor 3 (KIF3) and its homologue KRP85/95 have been implicated in membrane trafficking in mouse brain and sea urchin, respectively. This KRP has been purified from sea urchin and found to be a heterotrimer with two related KRP motor subunits and a larger accessory subunit that may serve a purpose analogous to that of kinesin light chains. The unc-104 gene product in the nematode Caenorhabditis elegans was originally proposed as a synaptic vesicle motor because unc-104 mutants had defects in synaptic vesicle localization. Subsequently, two related proteins, KIF1A and KIF1B, were cloned from mouse brain and their distributions examined. KIF1A was neuron-specific and appeared to be associated with synaptic vesicles, while KIF1B was ubiquitously expressed and reported to be associated with mitochondria. Since kinesin has also been localized on mitochondria, some MBOs may have multiple motor types or there may be subsets of MBOs with specific motors. Several other KRPs expressed in nervous tissue have been implicated in the transport of MBO types that were previously shown to have conventional kinesin. The extent to which these KRPs reflect unique transport mechanisms rather than functional redundancy within the kinesin family is not known.

Curiously, functions proposed for some brain KRPs [32] are very different from functions proposed for similar or identical KRPs in non-neuronal cells. For example, members of the MCAK/KIF2 family have been implicated in both mitotic spindle function and axonal membrane transport. Similarly, mouse KIF4 was reported to associate with unidentified MBOs in neurites, but its chicken homologue chromokinesin was shown to bind to chromosomal DNA and to mediate chromosomal movements in the mitotic spindle. Finally, CHO1 was originally found to have a role in mitotic spindle function, but members of the CHO1 family were subsequently implicated in the transport of MTs into dendrites [40]. Transfection of insect cells with CHO1 fusion proteins induced formation of tapering dendrite-like processes containing MTs with both plus- and minus-end distal MTs. This suggests that CHO1 or a related KRP may participate in organizing the unique MT cytoskeleton of dendrites.

Although the last to be discovered, the kinesin family of motor proteins has proven to be remarkably diverse. So far, there are at least 14 distinct subfamilies in the kinesin family, and more are likely to emerge, all with homology in the motor domain. Within a subfamily, however, the more extensive sequence similarities are presumed to reflect related functions. At present, many questions remain about the function of these various motors in the nervous system.

Cytoplasmic dyneins may have multiple roles in the neuron

The identification of kinesin as a plus-end-directed microtubule motor suggested that it is involved in anterograde transport but left the identity of the retrograde motor an open question. Since flagellar dynein was known to be a minus-end-directed motor, interest in cytoplasmic dyneins was renewed. Identification of the cytoplasmic form of dynein in nervous tissue was an indirect result of the discovery of kinesin.

Although dynein binding to MTs is not stabilized by AMP-PNP, both cytoplasmic dynein and kinesin associate with MTs in nucleotide-depleted extracts and both are released by addition of ATP. Early studies with ATP-free MT extracts showed that they are substantially enriched in a minor high-molecular-weight MAP, MAP1c. Biochemical analysis showed that MAP1c was not in fact related to MAP1a and -1b but, instead, was closely related to flagellar dynein heavy chains. This discovery led to the purification and characterization of brain cytoplasmic dynein [41]. Like flagellar dyneins, cytoplasmic dynein is a high-molecular-weight protein complex comprising two heavy chains and multiple light and intermediate chains that form a complex of more than 1,200 kDa.

As with the kinesins, dynein heavy chains are a multigene family with multiple flagellar and cytoplasmic dynein genes [32]. The 530-kDa dynein heavy chain contains the ATPase activity and MT-binding domains. There may be 10 to 15 dynein heavy chain genes in an organism, but the size of the dynein heavy chain has slowed genetic analyses. At present, dynein genes are grouped as members of either flagellar or cytoplasmic dynein subfamilies. The three intermediate, 74 kDa; four light-intermediate, 55 kDa; and variable number of light chains present in dyneins may also have flagellar and cytoplasmic forms.

The two or more cytoplasmic dynein heavy chain genes could be involved in different cellular functions, but much dynein functional diversity may be due to its many associated polypeptides. The intermediate and light chains of cytoplasmic dynein are thought to be important for both regulation and interactions of dynein with other cellular structures. In addition, a second protein complex, known as dynactin, copurifies with cytoplasmic dynein under some conditions. The dynactin complex is similar in size to dynein and contains ten subunits that include p150Glued; dynamitin, an actin-related protein; and two actin-capping polypeptides, among others [32]. The p150Glued polypeptide interacts with both dynein intermediate chains and the actin-related subunits. Dynamitin may play a role in the binding of cytoplasmic dynein to different types of cargo. Finally, actin-related protein (Arp1) forms a short filament that may include actin as well as the capping proteins. This short filament may interact with both p150Glued and components of the membrane cytoskeleton like spectrin. Dynactin may mediate cytoplasmic dynein binding to selected cargoes, including the Golgi complex and the membrane cytoskeleton. The wide range of functions associated with cytoplasmic dynein is matched by its complexity and its ability to interact with accessory factors. Additional proposed functions include a role in mitosis and in anchoring and localizing the Golgi complex.

A number of studies have implicated cytoplasmic dynein as playing a role in retrograde axonal transport [10,32]. In vitro motility studies demonstrate that cytoplasmic dynein generates force toward the minus end of MTs, consistent with a retrograde motor. Dynein immunoreactivities have been associated with MBOs, and cytoplasmic dynein accumulates on the distal side of a nerve ligation coincidentally with retrogradely transported MBOs. Finally, retrograde transport has been reported to be more sensitive than anterograde transport to ultraviolet light (UV)-vanadate treatment. Since exposure of dynein to UV irradiation in the presence of vanadate and ADP cleaves the dynein heavy chain, this has been a signature of dyneins, although other ATP-binding proteins may be affected as well.

In the nervous system, the most frequent role proposed for dynein is as a motor for retrograde axonal transport, but its properties are also consistent with a motor for slow axonal transport [10]. Consistent with this possibility, a study on the axonal transport of radiolabeled cytoplasmic dynein indicated that most cytoplasmic dynein and dynactin moved with SCb [42]. The ability of dynactin to interact with both cytoplasmic dynein and the membrane cytoskeleton suggests a model in which dynactin links dynein to the membrane cytoskeleton, providing an anchor for dynein-mediated movement of axonal MTs. Some role for the membrane cytoskeleton in the mechanisms of slow axonal transport is likely since neurons require interaction with a solid substrate for neurite growth. Taken together, these studies suggest that cytoplasmic dynein has a wide variety of functions in the nervous system, from anchoring the Golgi to retrograde transport to transport of MTs. Cytoplasmic dynein appears to have adapted to fulfill many cellular functions that require minus-end-directed MT movements.

Different classes of myosin are important for neuronal function

Myosins are remarkably diverse in structure and function. To date, 14 subfamilies of myosin have been defined by sequence homologies [33]. The brain is an abundant source of nonmuscle myosins and one of the earliest studied. Despite their abundance and variety, the roles of myosins in neural tissues have only recently begun to be defined.

Myosin II is in the same subfamily as myosins in muscle thick filaments, and it forms large, two-headed myosins with two light chains per heavy chain. Although myosin II is abundantly expressed in the brain, little is known about its function in the nervous system. In other nonmuscle cells, myosin II has been implicated in many types of cellular contractility and may serve a similar function in developing neurons. However, myosin II remains abundant in the mature nervous system, where examples of cell contractility are less common.

The second myosin type identified in nervous tissue was the myosin I family. Myosin I was first described in protists and subsequently purified from brain. Myosin I is a single-headed myosin with a short tail that uses calmodulin as a light chain [33]. Myosin I in many cell types has been implicated in both endocytosis and exocytosis, so it may play an important role in delivery and recycling of receptors. Myosin I is enriched in microvilli and may be involved in some aspects of growth cone motility, along with myosins from other subfamilies. In both cases, it may link MF bundles to the plasma membrane through a membrane-binding domain. Recently, the myosin I family has been implicated in mechanotransduction by the stereocilia of hair cells in the inner ear and vestibular apparatus. A myosin I isoform, myosin Iβ, has been localized to the tips of stereocilia, where it appears to mediate sensory adaptation by opening and closing the stretch-activated calcium ion channel (see Box 47-1).

Two other myosin types have been implicated in hearing and vestibular function [33]. The defect in the Snell's waltzer mouse was found to be a mutation in a myosin VI gene which produces degeneration of the cochlea and vestibular apparatus. Myosin VI is localized to the cuticular plate of the hair cell under stereocilia. Similarly, mutations in a myosin VII gene are responsible for the shaker-1 mouse and several human genetic deafness disorders. This myosin, myosin VIIa, is found in a band near the base of the stereocilia, distinct from distributions of myosin Iβ and myosin VI.

Another myosin type that plays a role in nervous tissue is myosin V [33]. Of the myosins identified in brain, myosin I and myosin V are the strongest candidates to act as an organelle motor and myosin V has been reported in association with vesicles purified from squid axoplasm. Myosin V is the product of the mouse dilute locus. Mice carrying the mutant dilute allele show defects in pigment granule movement that result in a dilution of the coat color. These mice also exhibit complex neurological defects that may be due to altered endoplasmic reticulum localization in dendrites. Curiously, a mutation in a form of myosin V found in yeast is suppressed by a KRP gene, suggesting an interaction between these two motor molecules. Finally, there is evidence that myosin V plays a role in growth cone motility, where it is enriched in filopodia.

Matching motors to physiological functions may be difficult

The three classes of motors are similar in their biochemical and pharmacological sensitivities in many respects [10]. However, some hallmark features can be used to identify a motor. In the case of kinesin, the most distinctive characteristic is stabilization of binding to MTs by AMP-PNP. The affinity of myosin for MFs and of dynein for MTs is weakened by treatment with either ATP or AMP-PNP. As a result, if a process is frozen in place by AMP-PNP, kinesins are likely to be involved. If kinesin is not involved in a process which requires MTs, dyneins are likely to be involved. Similarly, processes requiring MFs suggest that myosins are required. In the case of fast axonal transport, we know that MTs are required, and this process is completely inhibited by AMP-PNP, implicating the kinesin family. The development of new pharmacological and immunochemical probes specific for different motors will facilitate future studies.

While many motor proteins are found in nervous tissue, there are few instances in which we fully understand their cellular functions. The proliferation of different motor molecules and the existence of numerous isoforms raise the possibility that some physiological activities require multiple motors. There may be cases in which motors serve a redundant role to ensure that the physiological activity is maintained in the event of a loss of one motor protein. Finally, the existence of so many different types of motor molecules suggests that novel physiological activities requiring molecular motors may be as yet unrecognized.

By agreement with the publisher, this book is accessible by the search feature, but cannot be browsed.

Copyright © 1999, American Society for Neurochemistry.
Bookshelf ID: NBK27955


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