Mapping of the microvillar 110K-calmodulin complex: calmodulin-associated or -free fragments of the 110-kD polypeptide bind F-actin and retain ATPase activity

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.

Regulation and expression of metazoan unconventional myosins

Unconventional myosins are molecular motors that convert adenosine triphosphate (ATP) hydrolysis into movement along actin filaments. On the basis of primary structure analysis, these myosins are represented by at least 15 distinct classes (classes 1 and 3-16), each of which is presumed to play a specific cellular role. However, in contrast to the conventional myosins-2, which drive muscle contraction and cytokinesis and have been studied intensively for many years in both uni- and multicellular organisms, unconventional myosins have only been subject to analysis in metazoan systems for a short time. Here we critically review what is known about unconventional myosin regulation, function, and expression. Several points emerge from this analysis. First, in spite of the high relative conservation of motor domains among the myosin classes, significant differences are found in biochemical and enzymatic properties of these motor domains. Second, the idea that characteristic distributions of unconventional myosins are solely dependent on the myosin tail domain is almost certainly an oversimplification. Third, the notion that most unconventional myosins function as transport motors for membranous organelles is challenged by recent data. Finally, we present a scheme that clarifies relationships between various modes of myosin regulation.

Overlapping distribution of the 130- and 110-kDa myosin I isoforms on rat liver membranes

The biochemical and mechanochemical properties and localization of myosin I suggest the involvement of these small members of the myosin superfamily in some aspects of intracellular motility in higher cells. We have determined by quantitative immunoblotting with isoform-specific antibodies that the 130-kDa myosin I (myr 1 gene product) and 110-kDa myosin I (myr 2 gene product) account for 0.5 and 0.4%, respectively, of total rat liver protein. Immunoblot analyses reveal that the 130- and 110-kDa myosins I are found in several purified subcellular fractions from rat liver. The membrane-associated 130-kDa myosin I is found at the highest concentration in the plasma membrane (28 ng/microg plasma membrane protein) followed by the endoplasmic reticulum-like mitochondria-associated membrane fraction (MAM; 10 ng/microg MAM protein), whereas the 110-kDa myosin I is found at the highest concentration in Golgi (50 ng/¿g Golgi protein) followed by plasma membrane (20 ng/microg) and MAM (7 ng/microg). Our analyses indicate that myosin I is peripherally associated with Golgi and MAM and its presence in these fractions is not a consequence of myosin I bound to contaminating actin filaments. Although found in relatively low concentrations in microsomes, because of the abundance of microsomes, in liver most of the membrane-associated myosin I is associated with microsomes. Neither myosin I isoform is detected in purified mitochondria. This is the first quantitative analysis addressing the cellular distribution of these mammalian class I myosins.

New perspectives on mechanisms involved in generating epithelial cell polarity

Polarized epithelial cells form barriers that separate biological compartments and regulate homeostasis by controlling ion and solute transport between those compartments. Receptors, ion transporters and channels, signal transduction proteins, and cytoskeletal proteins are organized into functionally and structurally distinct domains of the cell surface, termed apical and basolateral, that face these different compartments. This review is about mechanisms involved in the establishment and maintenance of cell polarity. Previous reports and reviews have adopted a Golgi-centric view of how epithelial cell polarity is established, in which the sorting of apical and basolateral membrane proteins in the Golgi complex is a specialized process in polarized cells, and the generation of cell surface polarity is a direct consequence of this process. Here, we argue that events at the cell surface are fundamental to the generation of cell polarity. We propose that the establishment of structural asymmetry in the plasma membrane is the first, critical event, and subsequently, this asymmetry is reinforced and maintained by delivery of proteins that were constitutively sorted in the Golgi. We propose a hierarchy of stages for establishing cell polarity.

Characterization of microtubule binding domains in the Arabidopsis kinesin-like calmodulin binding protein

The kinesin-like calmodulin binding protein (KCBP) is a new member of the kinesin superfamily that appears to be present only in plants. The KCBP is unique in its ability to interact with calmodulin in a Ca2+-dependent manner. To study the interaction of the KCBP with microtubules, we expressed different regions of the Arabidopsis KCBP and used the purified proteins in cosedimentation assays with microtubules. The motor domain with or without the calmodulin binding domain bound to microtubules. The binding of the motor domain containing the calmodulin binding region to microtubules was inhibited by Ca2+-calmodulin. This Ca2+-calmodulin regulation of motor domain interactions with microtubules was abolished in the presence of antibodies specific to the calmodulin binding region. In addition, the binding of the motor domain lacking the calmodulin binding region to microtubules was not inhibited in the presence of Ca2+-calmodulin, suggesting an essential role for the calmodulin binding region in Ca2+-calmodulin modulation. Results of the cosedimentation assays with the N-terminal tail suggest the presence of a second microtubule binding site on the KCBP. However, the interaction of the N-terminal tail region of the KCBP with microtubules was insensitive to ATP. These data on the interaction of the KCBP with microtubules provide new insights into the functioning of the KCBP in plants.

Conformational changes due to calcium-induced calmodulin dissociation in brush border myosin I-decorated F-actin revealed by cryoelectron microscopy and image analysis

Brush border myosin I (BBMI) is a single-headed molecular motor. Its catalytic domain exhibits extensive sequence homology to the catalytic domain of myosin II, while its tail lacks the coiled-coil nature of myosin II. The BBMI tail domain contains at least three IQ motifs and binds calmodulin. Addition of calcium removes one of these calmodulin light chains, with effects on ATPase activity and motility in in vitro assays. Using the techniques of cryoelectron microscopy and helical image analysis we have calculated three-dimensional (3D) maps of BBMI-decorated actin filaments prepared in the presence and absence of calcium. The 3D maps describe a BBMI catalytic domain that is strikingly similar to the catalytic domain of myosin II subfragment 1 (S1), with the exception of a short amino-terminal region of the heavy chain, which is absent from BBMI. The tail domains of BBMI and S1 are highly divergent in structure, continuing on from their respective motor domains with very different geometries. Addition of calcium to BBMI, and the concomitant loss of a calmodulin light chain, results in an extensive reorganization of mass in the tail domain.

Three-dimensional structure of Brush Border Myosin-I at approximately 20 A resolution by electron microscopy and image analysis

Brush Border Myosin-I (BBMI) is a single-headed, unconventional myosin found in the microvilli of intestinal epithelial cells where it forms lateral bridges between the core bundle of actin filaments and the plasma membrane of the microvillus. A three-dimensional (3D) reconstruction of BBMI was made from images of negatively stained, two-dimensional (2D) crystals grown on lipid monolayers formed from mixtures of phosphatidylserine and phosphatidylcholine. The resolution of the 3D map extends to approximately 20 A and allows identification of all of the major structural domains of BBMI. The BBMI molecule is composed of three domains: a globular motor domain, a light-chain-binding domain and a lipid-binding domain. In our map, the putative motor domain is connected to an extended density, which we believe to be the light-chain-binding domain. This long, narrow region has three distinct bends, which may delineate the bound calmodulin light chains. Following the last calmodulin there is density which extends for a short distance across the lipid surface and is presumably the carboxy-terminal lipid-binding domain.

myoA of Aspergillus nidulans encodes an essential myosin I required for secretion and polarized growth

We have identified and cloned a novel essential myosin I in Aspergillus nidulans called myoA. The 1,249-amino acid predicted polypeptide encoded by myoA is most similar to the amoeboid myosins I. Using affinity-purified antibodies against the unique myosin I carboxyl terminus, we have determined that MYOA is enriched at growing hyphal tips. Disruption of myoA by homologous recombination resulted in a diploid strain heterozygous for the myoA gene disruption. We can recover haploids with an intact myoA gene from these strains, but never haploids that are myoA disrupted. These data indicated that myoA encodes an essential myosin I, and this has allowed us to use a unique approach to studying myosin I function. We have developed conditionally null myoA strains in which myoA expression is regulated by the alcA alcohol dehydrogenase promoter. A conditionally lethal strain germinated on inducing medium grows as wild type, displaying polarized growth by apical extension. However, growth of the same myoA mutant strain on repressing medium results in enlarged cells incapable of hyphal extension, and these cells eventually die. Under repressing conditions, this strain also displays reduced levels of secreted acid phosphatase. The mutant phenotype indicates that myoA plays a critical role in polarized growth and secretion.

The unconventional myosin, Myo2p, is a calmodulin target at sites of cell growth in Saccharomyces cerevisiae

Myo2p is an unconventional myosin required for polarized growth in Saccharomyces cerevisiae. Four lines of evidence suggest that (a) Myo2p is a target of calmodulin at sites of cell growth, and (b) the interaction between Myo2p and calmodulin is Ca2+ independent. First, as assessed by indirect immunofluorescence, the distributions of Myo2p and calmodulin are nearly indistinguishable throughout the cell cycle. Second, a genetic analysis indicates that mutations in CMD1 show allele-specific synthetic lethality with the myo2-66 conditional mutation. Mutations that inactivate the Ca(2+)-binding sites of calmodulin have little or no effect on strains carrying myo2-66, whereas an allele with a mutation outside the Ca(2+)-binding sites dramatically increases the severity of the phenotype conferred by myo2-66. Third, Myo2p coimmunoprecipitates with calmodulin in the presence of Ca2+ or EGTA. Finally, we used a modified gel overlay assay to demonstrate direct interaction between calmodulin and fusion proteins containing portions of Myo2p. Calmodulin binds specifically to the region of Myo2p containing six tandem repeats of a motif called an IQ site. Binding occurs in either Ca2+ or EGTA, and only two sites are required to observe binding.

Novel 130-kDa rat liver myosin-1 will translocate actin filaments

We have recently purified and characterized from rat liver, polypeptides of 110-kDa and 130-kDa which possess several characteristics of myosin-1 [Coluccio and Conaty: Cell Motil. Cytoskeleton 24:189-199, 1993]. What roles these myosin-1 molecules play in hepatocytes is not yet defined. One hypothesis is that they are involved in either intracellular transport or locomotion. As a first step in establishing their function, we have investigated whether these molecules are capable of supporting motility in vitro. Our results clearly demonstrate that the isolated 130-kDa-calmodulin complex will translocate filaments at a rate of 0.03-0.05 microns/sec; motility is inhibited in free calcium ion concentrations above 0.1 microM. This inhibition is reversed with the addition of exogenous calmodulin. These results provide supporting evidence of a motile role for the 130-kDa-calmodulin complex in vivo. This is the first demonstration that in higher eukaryotes, myosin-1 from a tissue other than intestine will support motility. Partial peptide sequence analysis indicates that the 130-kDa polypeptide resembles the recently described myr 1 [Ruppert et al.: J. Cell Biol. 120:1393-1403, 1993] or MM1 alpha [Sherr et al.: J. Cell Biol. 1405-1416, 1993] gene product.

Calcium-calmodulin and regulation of brush border myosin-I MgATPase and mechanochemistry

We examined the Ca(2+)-dependent regulation of brush border (BB) myosin-I by probing the possible roles of the calmodulin (CM) light chains. BB myosin-I MgATPase activity, sensitivity to chymotryptic digestion, and mechanochemical properties were assessed using 1-10 microM Ca2+ and in the presence of exogenously added CM since it has been proposed that this myosin is regulated by calcium-induced CM dissociation from the 119-kD heavy chain. Each of these BB myosin-I properties were dramatically altered by the same threshold of 2-3 microM Ca2+. Enzymatically active NH2-terminal proteolytic fragments of BB myosin-I which lack the CM binding domains (the 78-kD peptide) differ from CM-containing peptides in that the former is completely insensitive to Ca2+. Furthermore, the 78-kD peptide exhibits high levels of MgATPase activity which are comparable to that observed for BB myosin-I in the presence of Ca2+. This suggests that Ca2+ regulates BB myosin-I MgATPase by binding directly to the CM light chains, and that CM acts to repress endogenous MgATPase activity. Ca(2+)-induced CM dissociation from BB myosin-I can be prevented by the addition of exogenous CM. Under these conditions Ca2+ causes a reversible slowing of motility. In contrast, in the absence of exogenous CM, motility is stopped by Ca2+. We demonstrate this reversible slowing is not due to the presence of inactive BB myosin-I molecules exerting a "braking" effect on motile filaments. However, we did observe Ca(2+)-independent slowing of motility by acidic phospholipids, suggesting that factors other than Ca2+ and CM content can affect the mechanochemical properties of BB myosin-I.

The wily ways of a parasite: induction of actin assembly by Listeria

The intracellular pathogen Listeria has a spectacular mode of transport within and between host cells. By inducing host cell actin to assemble from its surface, the bacterium forms a tail composed of many short, crossbridged actin filaments. With this tail Listeria is propelled across the cytoplasm like a comet streaking across the sky. Here we discuss the antics of Listeria and some of the bacterial genes instrumental in maintaining it in the host.

Mammalian myosin I alpha, I beta, and I gamma: new widely expressed genes of the myosin I family

A polymerase chain reaction strategy was devised to identify new members of the mammalian myosin I family of actin-based motors. Using cellular RNA from mouse granular neurons and PC12 cells, we have cloned and sequenced three 1.2-kb polymerase chain reaction products that correspond to novel mammalian myosin I genes designated MMI alpha, MMI beta, MMI gamma. The pattern of expression for each of the myosin I's is unique: messages are detected in diverse tissues including the brain, lung, kidney, liver, intestine, and adrenal gland. Overlapping clones representing full-length cDNAs for MMI alpha were obtained from mouse brain. These encode a 1,079 amino acid protein containing a myosin head, a domain with five calmodulin binding sites, and a positively charged COOH-terminal tail. In situ hybridization reveals that MMI alpha is highly expressed in virtually all neurons (but not glia) in the postnatal and adult mouse brain and in neuroblasts of the cerebellar external granular layer. Expression varies in different brain regions and undergoes developmental regulation. Myosin I's are present in diverse organisms from protozoa to vertebrates. This and the expression of three novel members of this family in brain and other mammalian tissues suggests that they may participate in critical and fundamental cellular processes.

The unconventional myosin encoded by the myoA gene plays a role in Dictyostelium motility

The myoA gene of Dictyostelium is a member of a gene family of unconventional myosins. The myosin Is share homologous head and basic domains, but the myoA gene product lacks the glycine-, proline-, alanine-rich and src homology 3 domains typical of several of the other myosin Is. A mutant strain of Dictyostelium lacking a functional myoA gene was produced by gene targeting, and the motility of this strain in buffer and a spatial gradient of the chemoattractant cyclic AMP was analyzed by computer-assisted methods. The myoA- cells have a normal elongate morphology in buffer but exhibit a decrease in the instantaneous velocity of cellular translocation, an increase in the frequency of lateral pseudopod formation, and an increase in turning. In a spatial gradient, in which the frequency of pseudopod formation is depressed, myoA- cells exhibit positive chemotaxis but still turn several times more frequently than control cells. These results demonstrate that the other members of the unconventional myosin family do not fully compensate for the loss of functional myoA gene product. Surprisingly, the phenotype of the myoA- strain closely resembles that of the myoB- strain, suggesting that both play a role in the frequency of pseudopod formation and turning during cellular translocation.

Myosin-I in mammalian liver

Myosin-I refers to a class of proteins with a molecular weight of approximately 110-kDa, which have characteristics of conventional myosin but are unable to form filaments. Previous studies have implicated myosin-I in motile cellular processes including cell migration and phagocytosis. Although the first example of myosin-I in higher eukaryotes was the intestinal 110K-calmodulin complex, which forms in microvilli the lateral links connecting the core bundle of actin filaments to the membrane, myosin-I has now been shown to be a component of rat kidney and to be present in bovine adrenal gland and brain. We have now purified and characterized two polypeptides from rat liver which have several characteristics of the intestinal 110K-calmodulin complex. Both liver polypeptides are solubilized with ATP and co-elute on gel filtration with calmodulin. The polypeptides, of 110-kDa and 130-kDa, bind calmodulin in 1 mM EGTA. Both polypeptides bind to F-actin in an ATP reversible fashion, and crosslink actin filaments. The purified polypeptides possess an actin-activated Mg(2+)-ATPase activity typical of brush border myosin-I. A polyclonal antiserum directed against the chicken intestinal 110-kDa polypeptide recognizes both rat liver polypeptides, whereas another serum recognizes the 130-kDa but not the 110-kDa rat liver polypeptide. Controlled proteolysis of the purified polypeptides with alpha-chymotrypsin indicates that the two polypeptides are distinct but related. Immunofluorescence microscopy on isolated hepatocytes shows distribution of myosin-I to be vesicular, distributed throughout the cytoplasm, but more concentrated near the nucleus. These data contribute new evidence by several functional criteria that multiple myosin-I molecules are present in higher organisms and may coexist in a single cell type.

Golgi-derived vesicles from developing epithelial cells bind actin filaments and possess myosin-I as a cytoplasmically oriented peripheral membrane protein

In the intestinal brush border, the mechanoenzyme myosin-I links the microvillus core actin filaments with the plasma membrane. Previous immunolocalization shows that myosin-I is associated with vesicles in mature enterocytes (Drenckhahn, D., and R. Dermietzel. 1988. J. Cell Biol. 107:1037-1048) suggesting a potential role mediating vesicle motility. We now report that myosin-I is associated with Golgi-derived vesicles isolated from cells that are rapidly assembling brush borders in intestinal crypts. Crypt cells were isolated in hyperosmotic buffer, homogenized, and fractionated using differential- and equilibrium-density centrifugation. Fractions containing 50-100-nm vesicles, a similar size to those observed in situ, were identified by EM and were shown to contain myosin-I as demonstrated by immunoblotting and immunolabel negative staining. Galactosyltransferase, a marker enzyme for trans-Golgi membranes was present in these fractions, as was alkaline phosphatase, which is an apical membrane targeted enzyme. Galactosyltransferase was also present in vesicles immuno-purified with antibodies to myosin-I. Villin, a marker for potential contamination from fragmented microvilli, was absent. Myosin-I was found to reside on the vesicle "outer" or cytoplasmic surface for it was accessible to exogenous proteases and intact vesicles could be immunolabeled with myosin-I antibodies in solution. The bound myosin-I could be extracted from the vesicles using NaCl, KI and Na2CO3, suggesting that it is a vesicle peripheral membrane protein. These vesicles were shown to bundle actin filaments in an ATP-dependent manner. These results are consistent with a role for myosin-I as an apically targeted motor for vesicle translocation in epithelial cells.

Primary structure and cellular localization of chicken brain myosin-V (p190), an unconventional myosin with calmodulin light chains

Recent biochemical studies of p190, a calmodulin (CM)-binding protein purified from vertebrate brain, have demonstrated that this protein, purified as a complex with bound CM, shares a number of properties with myosins (Espindola, F. S., E. M. Espreafico, M. V. Coelho, A. R. Martins, F. R. C. Costa, M. S. Mooseker, and R. E. Larson. 1992. J. Cell Biol. 118:359-368). To determine whether or not p190 was a member of the myosin family of proteins, a set of overlapping cDNAs encoding the full-length protein sequence of chicken brain p190 was isolated and sequenced. Verification that the deduced primary structure was that of p190 was demonstrated through microsequence analysis of a cyanogen bromide peptide generated from chick brain p190. The deduced primary structure of chicken brain p190 revealed that this 1,830-amino acid (aa) 212,509-D) protein is a member of a novel structural class of unconventional myosins that includes the gene products encoded by the dilute locus of mouse and the MYO2 gene of Saccharomyces cerevisiae. We have named the p190-CM complex "myosin-V" based on the results of a detailed sequence comparison of the head domains of 29 myosin heavy chains (hc), which has revealed that this myosin, based on head structure, is the fifth of six distinct structural classes of myosin to be described thus far. Like the presumed products of the mouse dilute and yeast MYO2 genes, the head domain of chicken myosin-V hc (aa 1-764) is linked to a "neck" domain (aa 765-909) consisting of six tandem repeats of an approximately 23-aa "IQ-motif." All known myosins contain at least one such motif at their head-tail junctions; these IQ-motifs may function as calmodulin or light chain binding sites. The tail domain of chicken myosin-V consists of an initial 511 aa predicted to form several segments of coiled-coil alpha helix followed by a terminal 410-aa globular domain (aa, 1,421-1,830). Interestingly, a portion of the tail domain (aa, 1,094-1,830) shares 58% amino acid sequence identity with a 723-aa protein from mouse brain reported to be a glutamic acid decarboxylase. The neck region of chicken myosin-V, which contains the IQ-motifs, was demonstrated to contain the binding sites for CM by analyzing CM binding to bacterially expressed fusion proteins containing the head, neck, and tail domains. Immunolocalization of myosin-V in brain and in cultured cells revealed an unusual distribution for this myosin in both neurons and nonneuronal cells.(ABSTRACT TRUNCATED AT 400 WORDS)

How Listeria exploits host cell actin to form its own cytoskeleton. I. Formation of a tail and how that tail might be involved in movement

After Listeria is phagocytosed by a macrophage, it dissolves the phagosomal membrane and enters the cytoplasm. The Listeria then nucleates actin filaments from its surface. These actin filaments rearrange to form a tail with which the Listeria moves to the macrophage surface as a prelude to spreading. Since individual actin filaments appear to remain in their same positions in the tail in vitro after extraction with detergent, the component filaments must be cross-bridged together. From careful examination of the distribution of actin filaments attached to the surface of Listeria and in the tail, and the fact that during and immediately after division filaments are not nucleated from the new wall formed during septation, we show how a cloud of actin filaments becomes rearranged into a tail simply by the mechanics of growth. From lineage studies we can relate the length of the tail to the age of the surface of Listeria and make predictions as to the ratio of Listeria with varying tail lengths at a particular time after the initial infection. Since we know that division occurs about every 50 min, after 4 h we would predict that if we started with one Listeria in a macrophage, 16 bacteria would be found, two with long tails, two with medium tails, four with tiny tails, and eight with no tails or a ratio of 1:1:2:4. We measured the lengths of the tails on Listeria 4 h after infection in serial sections and confirmed this prediction. By decorating the actin filaments that make up the tail of Listeria with subfragment 1 of myosin we find (a) that the filaments are indeed short (maximally 0.3 microns in length); (b) that the filament length is approximately the same at the tip and the base of the tail; and (c) that the polarity of these filaments is inappropriate for myosin to be responsible or to facilitate movement through the cytoplasm, but the polarity insures that the bacterium will be located at the tip of a pseudopod, a location that is essential for spreading to an adjacent cell. Putting all this information together we can begin to unravel the problem of how the Listeria forms the cytoskeleton and what is the biological purpose of this tail. Two functions are apparent: movement and pseudopod formation.

Purification and characterization of a mammalian myosin I

Myosin I, an actin-dependent force-generating enzyme, has been purified from three mammalian sources: bovine adrenal medulla, adrenal cortex, and brain. The purification procedure includes extraction of tissue with ATP at low ionic strength and coprecipitation with actin, followed by gel filtration on Sepharose 4B, anion-exchange chromatography on Q Sepharose, and affinity chromatography on ATP-agarose. Mammalian myosin I molecules are composed of a heavy chain of 116 kDa and multiple low molecular weight polypeptides identified as calmodulin. The structural and enzymatic properties of adrenal medulla myosin I were further characterized. This enzyme exhibits high K+,EDTA- and Ca(2+)-ATPase specific activities (about 0.2 mumol.min-1 per mg of protein), whereas the Mg(2+)-ATPase activity is very low (1-3 nmol.min-1.mg-1). The Mg(2+)-ATPase of medulla myosin I is activated by F-actin in a Ca(2+)-dependent manner: activity is stimulated 40-fold in the presence of EGTA and 90-fold in the presence of 10 microM Ca2+. Two structural domains of the myosin I heavy chain were identified. A 74-kDa chymotryptic fragment contains the catalytic site, while a 36-kDa polypeptide contains the calmodulin-binding sites. These results indicate that mammalian myosin I is more closely related to myosin I from the avian intestinal brush border than to the enzymes isolated from the protozoans Acanthamoeba and Dictyostelium.

Novel myosins

The traditional view of myosin, drawn from studies of myosins from striated muscles, is that of an elongated two-headed molecule that assembles into filaments. However, biochemical, molecular genetic and genetic studies have uncovered a host of ubiquitous single-headed nonfilamentous myosins known collectively as myosins I. All of the myosins I possess the myosin head domain, the motor portion of muscle myosins they have tail the filament-forming tail domain of muscle myosins they have tail domains that interact variously with membranes, actin and calmodulin. These alternative molecular interactions confer novel motile properties on myosins I, such as the ability to move membranes relative to actin and to move actin relative to actin without having to assemble into filaments. The numerous actin-based movements retained by cells lacking myosin II, the two-headed filamentous form of nonmuscle myosin, may be supported by myosins I.

Limited tissue distribution of the intestinal brush border myosin I protein

A myosinlike 105-110-kilodalton calmodulin-binding protein, brush border myosin I, found in the intestinal brush border has been linked to two seemingly disparate but possibly interacting functions of the brush border, namely, microvillar motility and vitamin D regulated calcium transport. If brush border myosin I were to function primarily as a myosinlike molecule powering cellular or microvillar motility, one might expect it to be found in a variety of tissues with microvilli such as the renal brush border and bile canaliculus. On the other hand, a more specialized function such as participation in vitamin D regulated calcium transport might dictate a more restricted tissue distribution for brush border myosin I. To determine the tissue distribution of brush border myosin I, we purified this protein to apparent homogeneity, generated antisera to it, and used the antisera to localize the protein within the intestinal epithelial cell by immunocytochemistry. We then screened a variety of other tissues (brain, lung, heart, liver, spleen, pancreas, kidney, and skeletal muscle) both for calmodulin-binding proteins as well as for brush border myosin I using Western blots and immunofluorescence. Our results indicate that the intestinal brush border myosin I is limited in its distribution to the intestinal brush border.

Binding of brush border myosin I to phospholipid vesicles

The actin filament core within each microvillus of the intestinal epithelial cell is attached laterally to the plasma membrane by brush border (BB) myosin I, a protein-calmodulin complex belonging to the myosin I class of actin-based mechanoenzymes. In this report, the binding of BB myosin I to pure phospholipid vesicles was examined and characterized. BB myosin I demonstrated saturable binding to liposomes composed of anionic phospholipids, but did not associate with liposomes composed of only neutral phospholipids. The binding of BB myosin I to phosphatidylserine and phosphatidylglycerol vesicles reached saturation at 4-5 x 10(-3) nmol protein/nmol phospholipid, while the apparent dissociation constant was determined to be 1-3 x 10(-7) M. Similar to the free protein, membrane-associated BB myosin I bound F-actin in an ATP-sensitive manner and demonstrated actin-activated Mg-ATPase activity. Immunoblot analysis of peptides generated from controlled proteolysis of vesicle-bound BB myosin I provided structural information concerning the site responsible for the membrane interaction. Immunoblot staining with domain-specific mAbs revealed a series of COOH-terminal, liposome-associated peptides that were protected from digestion, suggesting that the membrane-binding domain is within the carboxy-terminal "tail" of the BB myosin I heavy chain.

A second isoform of chicken brush border myosin I contains a 29-residue inserted sequence that binds calmodulin

Chicken brush border myosin I (CBB-MI) is a single-headed, nonfilamentous, myosin-like mechanoenzyme which, as isolated, has 3 mol of calmodulin (CAM) 'light chains' bound per mole of 119 kDa heavy chain. We have isolated a partial cDNA clone for CBB-MI that encodes the C-terminal approximately 35 kDa of the heavy chain. The sequence of this clone is identical to that of an authentic, near-full-length CBB-MI cDNA clone reported recently, except for an 87-bp/29-residue insertion occurring approximately 32 kDa from the C-terminus. This insert, which is probably generated by an alternate splicing event, is expressed in brush border as part of a message of the size predicted for the CBB-MI heavy chain, although the steady state level of this transcript is approximately 8-fold lower than for transcripts lacking the insert. 125I-CAM overlays of this cDNA clone (expressed as a trpE fusion protein in E. coli) indicate that it binds one more calmodulin than does a second cDNA clone that lacks the 29-residue insert. A synthetic peptide corresponding to the insert sequence binds tightly to CAM-Sepharose, demonstrates a shift and enhancement of fluorescence in the presence of CAM, and binds CAM in solution with a KD of 190 nM (in 100 mM KCl). We conclude that a second, low-abundance isoform of CBB-MI contains an additional (and possibly fourth) CAM binding site as a result of a 29-residue peptide that is inserted into the tail domain by an apparent alternate splicing event.

Calmodulin dissociation regulates brush border myosin I (110-kD-calmodulin) mechanochemical activity in vitro

110-kD-calmodulin, when immobilized on nitrocellulose-coated coverslips, translocates actin filaments at a maximal rate of 0.07-0.1 micron/s at 37 degrees C. Actin activates MgATPase activity greater than 40-fold, with a Km of 40 microM and Vmax of 0.86 s-1 (323 nmol/min/mg). The rate of motility mediated by 110-kD-calmodulin is dependent on temperature and concentration of ATP, but independent of time, actin filament length, amount of enzyme, or ionic strength. Tropomyosin inhibits actin binding by 110-kD-calmodulin in MgATP and inhibits motility. Micromolar calcium slightly increases the rate of motility and increases the actin-activated MgATP hydrolysis of the intact complex. In 0.1 mM or higher calcium, motility ceases and actin-dependent MgATPase activity remains at a low rate not activated by increasing actin concentration. Correlated with these inhibitions of activity, a subset of calmodulin is dissociated from the complex. To determine if calmodulin loss is the cause of calcium inhibition, we assayed the ability of calmodulin to rescue the calcium-inactivated enzyme. Readdition of calmodulin to the nitrocellulose-bound, calcium-inactivated enzyme completely restores motility. Addition of calmodulin also restores actin activation to MgATPase activity in high calcium, but does not affect the activity of the enzyme in EGTA. These results demonstrate that in vitro 110-kD-calmodulin functions as a calcium-sensitive mechanoenzyme, a vertebrate myosin I. The properties of this enzyme suggest that despite unique structure and regulation, myosins I and II share a molecular mechanism of motility.

Assembly of the brush border cytoskeleton: changes in the distribution of microvillar core proteins during enterocyte differentiation in adult chicken intestine

The assembly of the intestinal microvillus cytoskeleton was examined during the differentiation of enterocytes along the crypt-villus axis in adult chicken duodenum using light and electron microscopic immunolocalization techniques. Using antibodies reactive with villin, fimbrin, and the heavy chain (hc) of brush border (BB) myosin I (110K-calmodulin complex) and rhodamine-conjugated phalloidin as a probe for F-actin, we determined that while actin, villin, and fimbrin were all localized apically along the entire axis, BB myosin I (hc) did not assume this localization until the crypt-villus transition zone. In addition to their localization at the BB surface, all four proteins were present at significant levels along the lateral margins of enterocytes along the entire crypt-villus axis, suggesting that these proteins may be involved in the organization and function of the basolateral membrane cytoskeleton as well. The pattern of expression of the microvillar core proteins along the crypt-villus axis in the adult was comparable to that seen in the intestine of the late stage chicken embryo and suggests that a common program for brush border assembly may be used in both modes of enterocyte differentiation.

Partial deduced sequence of the 110-kD-calmodulin complex of the avian intestinal microvillus shows that this mechanoenzyme is a member of the myosin I family

The actin bundle within each microvillus of the intestinal brush border is laterally tethered to the membrane by bridges composed of the protein complex, 110-kD-calmodulin. Previous studies have shown that avian 110-kD-calmodulin shares many properties with myosins including mechanochemical activity. In the present study, a cDNA molecule encoding 1,000 amino acids of the 110-kD protein has been sequenced, providing direct evidence that this protein is a vertebrate homologue of the tail-less, single-headed myosin I first described in amoeboid cells. The primary structure of the 110-kD protein (or brush border myosin I heavy chain) consists of two domains, an amino-terminal "head" domain and a 35-kD carboxy-terminal "tail" domain. The head domain is homologous to the S1 domain of other known myosins, with highest homology observed between that of Acanthamoeba myosin IB and the S1 domain of the protein encoded by bovine myosin I heavy chain gene (MIHC; Hoshimaru, M., and S. Nakanishi. 1987. J. Biol. Chem. 262:14625-14632). The carboxy-terminal domain shows no significant homology with any other known myosins except that of the bovine MIHC. This demonstrates that the bovine MIHC gene most probably encodes the heavy chain of bovine brush border myosin I (BBMI). A bacterially expressed fusion protein encoded by the brush border 110-kD cDNA binds calmodulin. Proteolytic removal of the carboxy-terminal domain of the fusion protein results in loss of calmodulin binding activity, a result consistent with previous studies on the domain structure of the 110-kD protein. No hydrophobic sequence is present in the molecule indicating that chicken BBMI heavy chain is probably not an integral membrane protein. Northern blot analysis of various chicken tissue indicates that BBMI heavy chain is preferentially expressed in the intestine.

Characterization of intestinal microvillar membrane disks: detergent-resistant membrane sheets enriched in associated brush border myosin I (110K-calmodulin)

The actin bundle within each microvillus of the intestinal brush border (BB) is tethered laterally to the membrane by bridges composed of BB myosin I. Avian BB myosin I, formerly termed 110K-calmodulin, consists of a heavy chain with an apparent Mr of 110 kD and three to four molecules of calmodulin "light chains." Recent studies have shown that this complex shares many properties with myosin including mechanochemical activity. In this report, the isolation and characterization of a membrane fraction enriched in bound BB myosin I is described. This membrane fraction, termed microvillar membrane disks, was purified from ATP extracts of nonionic detergent-treated microvilli prepared from avian intestinal BBs. Ultrastructural analysis revealed that these membranes are flat, disk-shaped sheets with protrusions which are identical in morphology to purified BB myosin I. The disks exhibit actin-activated Mg-ATPase activity and bind and cross-link actin filaments in an ATP-dependent fashion. The mechanochemical activity of the membrane disks was assessed using the Nitella bead movement assay (Sheetz, M. P., and J. A. Spudich. 1983. Nature [Lond.]. 303:31-35). These preparations were shown to be free of significant contamination by conventional BB myosin. Latex beads coated with microvillar membrane disks move in a myosin-like fashion along Nitella actin cables at rates of 12-60 nm/s (average rate of 33 nm/s); unlike purified BB myosin I, the movement of membrane disk-coated beads was most reproducibly observed in buffers containing low Ca2+.

The 110-kD protein-calmodulin complex of the intestinal microvillus (brush border myosin I) is a mechanoenzyme

The 110-kD protein-calmodulin complex (110K-CM) of the intestinal brush border serves to laterally tether microvillar actin filaments to the plasma membrane. Results from several laboratories have demonstrated that this complex shares many enzymatic and structural properties with myosin. The mechanochemical potential of purified avian 110K-CM was assessed using the Nitella bead motility assay (Sheetz, M. P., and J. A. Spudich. 1983. Nature (Lond.). 303:31-35). Under low Ca2+ conditions, 110K-CM-coated beads bound to actin cables, but no movement was observed. Using EGTA/calcium buffers (approximately 5-10 microM free Ca2+) movement of 110K-CM-coated beads along actin cables (average rate of approximately 8 nm/s) was observed. The movement was in the same direction as that for beads coated with skeletal muscle myosin. The motile preparations of 110K-CM were shown to be free of detectable contamination by conventional brush border myosin. Based on these and other observations demonstrating the myosin-like properties of 110K-CM, we propose that this complex be named "brush border myosin I."

Reassociation of microvillar core proteins: making a microvillar core in vitro

Intestinal epithelia have a brush border membrane of numerous microvilli each comprised of a cross-linked core bundle of 15-20 actin filaments attached to the surrounding membrane by lateral cross-bridges; the cross-bridges are tilted with respect to the core bundle. Isolated microvillar cores contain actin (42 kD) and three other major proteins: fimbrin (68 kD), villin (95 kD), and the 110K-calmodulin complex. The addition of ATP to detergent-treated isolated microvillar cores has previously been shown to result in loss of the lateral cross-bridges and a corresponding decrease in the amount of the 110-kD polypeptide and calmodulin associated with the core bundle. This provided the first evidence to suggest that these lateral cross-bridges to the membrane are comprised at least in part by a 110-kD polypeptide complexed with calmodulin. We now demonstrate that purified 110K-calmodulin complex can be readded to ATP-treated, stripped microvillar cores. The resulting bundles display the same helical and periodic arrangement of lateral bridges as is found in vivo. In reconstitution experiments, actin filaments incubated in EGTA with purified fimbrin and villin form smooth-sided bundles containing an apparently random number of filaments. Upon addition of 110K-calmodulin complex, the bundles, as viewed by electron microscopy of negatively stained images, display along their entire length helically arranged projections with the same 33-nm repeat of the lateral cross-bridges found on microvilli in vivo; these bridges likewise tilt relative to the bundle. Thus, reconstitution of actin filaments with fimbrin, villin, and the 110K-calmodulin complex results in structures remarkably similar to native microvillar cores. These data provide direct proof that the 110K-calmodulin is the cross-bridge protein and indicate that actin filaments bundled by fimbrin and villin are of uniform polarity and lie in register. The arrangement of the cross-bridge arms on the bundle is determined by the structure of the core filaments as fixed by fimbrin and villin; a contribution from the membrane is not required.

Microinjection into Acanthamoeba castellanii of monoclonal antibodies to myosin-II slows but does not stop cell locomotion

To study the in vivo role of myosin-II in Acanthamoeba castellanii, motile cells were microinjected with monoclonal antibodies raised against the myosin-II heavy chain. All injected cells underwent a transient shock response. It was found that although injection of buffer alone or of an endogenous Acanthamoeba protein decreased the motility of injected cells from 7 microns/min to approximately 3 microns/min, injection of monoclonal antibodies specific for myosin-II decreased motility further to approximately 0.8 micron/min. This effect was seen whether or not the monoclonal antibody to myosin-II inhibited the actomyosin-II MgATPase activity in vitro. Levels of antibody far in excess of endogenous myosin-II concentrations could not completely block amoeboid movement. The morphology of moving antimyosin-II-injected cells was unusual, suggesting a greater defect in the ability to retract the trailing edge of the cell rather than to extend the leading edge. Endosomes frequently disappeared from injected cells, and although buffer-injected cells rapidly recovered visible endosomes (50% recovery at 5 min), endosomes were not seen in antimyosin-II-injected cells until, on the average, approximately 50 min after injection. Injection of a nonspecific antibody or of a nonspecific exogenous protein (ovalbumin) also decreased the mobility of the injected cells beyond that of buffer-injected cells (to approximately 1 micron/min). These cells tended to recover endosomes more rapidly (approximately 25 min) than cells injected with antimyosin-II monoclonal antibodies. The inability of antibodies to myosin-II to inhibit completely any of the movements studied suggests that although myosin-II probably plays a role in these motilities, the cell either routinely uses or can draw upon another cytoplasmic motor to maintain locomotion, organelle movement, contractile vacuole activity, and endocytosis.

Structural and immunological characterization of the myosin-like 110-kD subunit of the intestinal microvillar 110K-calmodulin complex: evidence for discrete myosin head and calmodulin-binding domains

The actin bundle within each microvillus of the intestinal brush border is tethered laterally to the membrane by spirally arranged bridges. These bridges are thought to be composed of a protein complex consisting of a 110-kD subunit and multiple molecules of bound calmodulin (CM). Recent studies indicate that this complex, termed 110K-CM, is myosin-like with respect to its actin binding and ATPase properties. In this study, possible structural similarity between the 110-kD subunit and myosin was examined using two sets of mAbs; one was generated against Acanthamoeba myosin II and the other against the 110-kD subunit of avian 110K-CM. The myosin II mAbs had been shown previously to be cross-reactive with skeletal muscle myosin, with the epitope(s) localized to the 50-kD tryptic fragment of the subfragment-1 (S1) domain. The 110K mAbs (CX 1-5) reacted with the 110-kD subunit as well as with the heavy chain of skeletal but not with that of smooth or brush border myosin. All five of these 110K mAbs reacted with the 25-kD, NH2-terminal tryptic fragment of chicken skeletal S1, which contains the ATP-binding site of myosin. Similar tryptic digestion of 110K-CM revealed that these five mAbs all reacted with a 36-kD fragment of 110K (as well as larger 90- and 54-kD fragments) which by photoaffinity labeling was shown to contain the ATP-binding site(s) of the 110K subunit. CM binding to these same tryptic digests of 110K-CM revealed that only the 90-kD fragment retained both ATP- and CM-binding domains. CM binding was observed to several tryptic fragments of 60, 40, 29, and 18 kD, none of which contain the myosin head epitopes. These results suggest structural similarity between the 110K and myosin S1, including those domains involved in ATP- and actin binding, and provide additional evidence that 110K-CM is a myosin. These studies also support the results of Coluccio and Bretscher (1988. J. Cell Biol. 106:367-373) that the calmodulin-binding site(s) and the myosin head region of the 110-kD subunit lie in discrete functional domains of the molecule.