MYO1A (brush border myosin I) dynamics in the brush border of LLC-PK1-CL4 cells.

The kidney epithelial cell line, LLC-PK1-CL4 (CL4), forms a well ordered brush border (BB) on its apical surface. CL4 cells were used to examine the dynamics of MYO1A (M1A; formerly BB myosin I) within the BB using GFP-tagged MIA (GFP-M1A), MIA motor domain (GFP-MDIQ), and tail domain (GFP-Tail). GFP-beta-actin (GFP-Actin) was used to assess actin dynamics within the BB. GFP-M1A, GFP-Tail, but not GFP-MDIQ localized to the BB, indicating that the tail is sufficient for apical targeting of M1A. GFP-Actin targeted to all the actin domains of the cell including the BB. Fluorescence recovery after photobleaching analysis revealed that GFP-M1A and GFP-Tail turnover in the BB is rapid, approximately 80% complete in <1 min. As expected for an actin-based motor, ATP depletion resulted in significant inhibition of GFP-M1A turnover yet had little effect on GFP-Tail exchange. Rapid turnover of GFP-M1A and GFP-Tail was not due to actin turnover as GFP-Actin turnover in the BB was much slower. These results indicate that the BB population of M1A turns over rapidly, while its head and tail domains interact transiently with the core actin and plasma membrane, respectively. This rapidly exchanging pool of M1A envelops an actin core bundle that, by comparison, is static in structure.

The Abl-related gene (Arg) nonreceptor tyrosine kinase uses two F-actin-binding domains to bundle F-actin.

Abl family nonreceptor tyrosine kinases regulate cellular morphogenesis and motility through functional interactions with the actin cytoskeleton. Although Abl family kinases are known to contain filamentous (F)-actin-binding domains at their C termini, it is unclear how Abl family kinases regulate the structure and/or function of the actin cytoskeleton. We show here that the Abl-related kinase Arg binds with positive cooperativity to F-actin in vitro with binding saturating at a ratio of one Arg/two actin molecules. Measurements of the F-actin-binding properties of Arg deletion mutants led to the identification of a second, previously uncharacterized internal F-actin-binding domain in Arg. Purified Arg can bundle F-actin in vitro, and this bundling activity requires both F-actin-binding domains. An Arg-yellow fluorescent protein fusion protein can induce the formation of actin-rich structures at the lamellipodia of Swiss 3T3 fibroblasts. Both of Arg's F-actin-binding domains are necessary and sufficient for the formation of these actin-rich structures. Together, our data suggest that Arg can use its F-actin-bundling activity to directly regulate actin cytoskeletal structure in vivo.

Myosin-I nomenclature.

We suggest that the vertebrate myosin-I field adopt a common nomenclature system based on the names adopted by the Human Genome Organization (HUGO). At present, the myosin-I nomenclature is very confusing; not only are several systems in use, but several different genes have been given the same name. Despite their faults, we believe that the names adopted by the HUGO nomenclature group for genome annotation are the best compromise, and we recommend universal adoption.

High affinity binding of brain myosin-Va to F-actin induced by calcium in the presence of ATP.

Brain myosin-Va consists of two heavy chains, each containing a neck domain with six tandem IQ motifs that bind four to five calmodulins and one to two essential light chains. Previous studies demonstrated that myosin-Va exhibits an unusually high affinity for F-actin in the presence of ATP and that its MgATPase activity is stimulated by micromolar Ca(2+) in a highly cooperative manner. We demonstrate here that Ca(2+) also induces myosin-Va binding to and cosedimentation with F-actin in the presence of ATP in a similar cooperative manner and calcium concentration range as that observed for the ATPase activity. Neither hydrolysis of ATP nor buildup of ADP was required for Ca(2+)-induced cosedimentation. The Ca(2+)-induced binding was inhibited by low temperature or by 0.6 m NaCl, but not by 1% Triton X-100. Tight binding between myosin-Va and pyrene-labeled F-actin in the presence of ATP and Ca(2+) was also detected by quenching of the pyrene fluorescence. Negatively stained preparations of actomyosin-Va under Ca(2+)-induced binding conditions showed tightly packed F-actin bundles cross-linked by myosin-Va. Our data demonstrate that high affinity binding of myosin-Va and F-actin in the presence of ATP or 5'-O-(thiotriphosphate) is induced by micromolar concentrations of Ca(2+). Since Ca(2+) regulates both the actin binding properties and actin-activated ATPase of myosin-Va over the same concentration range, we suggest that the calcium signal may regulate the mechanism of processivity of myosin Va.

Myosin-VIIb, a novel unconventional myosin, is a constituent of microvilli in transporting epithelia.

Mouse myosin-VIIb, a novel unconventional myosin, was cloned from the inner ear and kidney. The human myosin-VIIb (HGMW-approved symbol MYO7B) sequence and exon structure were then deduced from a human BAC clone. The mouse gene was mapped to chromosome 18, approximately 0.5 cM proximal to D18Mit12. The human gene location at 2q21.1 was deduced from the map location of the BAC and confirmed by fluorescence in situ hybridization. Myosin-VIIb has a conserved myosin head domain, five IQ domains, two MyTH4 domains coupled to two FERM domains, and an SH3 domain. A phylogenetic analysis based on the MyTH4 domains suggests that the coupled MyTH and FERM domains were duplicated in myosin evolution before separation into different classes. Myosin-VIIb is expressed primarily in kidney and intestine, as shown by Northern and immunoblot analyses. An antibody to myosin-VIIb labeled proximal tubule cells of the kidney and enterocytes of the intestine, specifically the distal tips of apical microvilli on these transporting epithelial cells.

The yeast class V myosins, Myo2p and Myo4p, are nonprocessive actin-based motors.

The motor properties of the two yeast class V myosins, Myo2p and Myo4p, were examined using in vitro motility assays. Both myosins are active motors with maximum velocities of 4.5 microm/s for Myo2p and 1.1 microm/s for Myo4p. Myo2p motility is Ca(2+) insensitive. Both myosins have properties of a nonprocessive motor, unlike chick myosin-Va (M5a), which behaves as a processive motor when assayed under identical conditions. Additional support for the idea that Myo2p is a nonprocessive motor comes from actin cosedimentation assays, which show that Myo2p has a low affinity for F-actin in the presence of ATP and Ca(2+), unlike chick brain M5a. These studies suggest that if Myo2p functions in organelle transport, at least five molecules of Myo2p must be present per organelle to promote directed movement.

The light chain composition of chicken brain myosin-Va: calmodulin, myosin-II essential light chains, and 8-kDa dynein light chain/PIN.

Class V myosins are a ubiquitously expressed family of actin-based molecular motors. Biochemical studies on myosin-Va from chick brain indicate that this myosin is a two-headed motor with multiple calmodulin light chains associated with the regulatory or neck domain of each heavy chain, a feature consistent with the regulatory effects of Ca(2+) on this myosin. In this study, the identity of three additional low molecular weight proteins of 23-,17-, and 10 kDa associated with myosin-Va is established. The 23- and 17-kDa subunits are both members of the myosin-II essential light chain gene family, encoded by the chicken L23 and L17 light chain genes, respectively. The 10-kDa subunit is a protein originally identified as a light chain (DLC8) of flagellar and axonemal dynein. The 10-kDa subunit is associated with the tail domain of myosin-Va.

Myosin-V stepping kinetics: a molecular model for processivity.

Myosin-V is a molecular motor that moves processively along its actin track. We have used a feedback-enhanced optical trap to examine the stepping kinetics of this movement. By analyzing the distribution of time periods separating discrete approximately 36-nm mechanical steps, we characterize the number and duration of rate-limiting biochemical transitions preceding each such step. These data show that myosin-V is a tightly coupled motor whose cycle time is limited by ADP release. On the basis of these results, we propose a model for myosin-V processivity.

Compartmentalization of the cell cortex by septins is required for maintenance of cell polarity in yeast.

Formation and maintenance of specialized plasma membrane domains are crucial for many biological processes, such as cell polarization and signaling. During isotropic bud growth, the yeast cell periphery is divided into two domains: the bud surface, an active site of exocytosis and growth, and the relatively quiescent surface of the mother cell. We found that cells lacking septins at the bud neck failed to maintain the exocytosis and morphogenesis factors Spa2, Sec3, Sec5, and Myo2 in the bud during isotropic growth. Furthermore, we found that septins were required for proper regulation of actin patch stability; septin-defective cells permitted to enter isotropic growth lost actin and growth polarity. We propose that septins maintain cell polarity by specifying a boundary between cortical domains.

Role of actin and Myo2p in polarized secretion and growth of Saccharomyces cerevisiae.

We examined the role of the actin cytoskeleton in secretion in Saccharomyces cerevisiae with the use of several quantitative assays, including time-lapse video microscopy of cell surface growth in individual living cells. In latrunculin, which depolymerizes filamentous actin, cell surface growth was completely depolarized but still occurred, albeit at a reduced level. Thus, filamentous actin is necessary for polarized secretion but not for secretion per se. Consistent with this conclusion, latrunculin caused vesicles to accumulate at random positions throughout the cell. Cortical actin patches cluster at locations that correlate with sites of polarized secretion. However, we found that actin patch polarization is not necessary for polarized secretion because a mutant, bee1Delta(las17Delta), which completely lacks actin patch polarization, displayed polarized growth. In contrast, a mutant lacking actin cables, tpm1-2 tpm2Delta, had a severe defect in polarized growth. The yeast class V myosin Myo2p is hypothesized to mediate polarized secretion. A mutation in the motor domain of Myo2p, myo2-66, caused growth to be depolarized but with only a partial decrease in the level of overall growth. This effect is similar to that of latrunculin, suggesting that Myo2p interacts with filamentous actin. However, inhibition of Myo2p function by expression of its tail domain completely abolished growth.

The mouse neurological mutant flailer expresses a novel hybrid gene derived by exon shuffling between Gnb5 and Myo5a.

Exon shuffling is thought to be an important mechanism for evolution of new genes. Here we show that the mouse neurological mutation flailer (flr) expresses a novel gene that combines the promoter and first two exons of guanine nucleotide binding protein beta 5 (Gnb5) with the C-terminal exons of the closely linked Myosin 5A (MyoVA) gene (Myo5a). The flailer protein, which is expressed predominantly in brain, contains the N-terminal 83 amino acids of Gnb5 fused in-frame with the C-terminal 711 amino acids of MyoVA, including the globular tail domain that binds organelles for intracellular transport. Biochemical and genetic studies indicate that the flailer protein competes with wild-type MyoVA in vivo, preventing the localization of smooth endoplasmic reticulum vesicles in the dendritic spines of cerebellar Purkinje cells. The flailer protein thus has a dominant-negative mechanism of action with a recessive mode of inheritance due to the dependence of competitive binding on the ratio between mutant and wild-type proteins. The chromosomal arrangement of Myo5a upstream of Gnb5 is consistent with non-homologous recombination as the mutational mechanism. To our knowledge, flailer is the first example of a mammalian mutation caused by germ line exon shuffling between unrelated genes.

Localization of unconventional myosins V and VI in neuronal growth cones.

Class V and VI myosins, two of the six known classes of actin-based motor genes expressed in vertebrate brain (Class I, II, V, VI, IX, and XV), have been suggested to be organelle motors. In this report, the neuronal expression and subcellular localization of chicken brain myosin V and myosin VI is examined. Both myosins are expressed in brain during embryogenesis. In cultured dorsal root ganglion (DRG) neurons, immunolocalization of myosin V and myosin VI revealed a similar distribution for these two myosins. Both are present within cell bodies, neurites and growth cones. Both of these myosins exhibit punctate labeling patterns that are found in the same subcellular region as microtubules in growth cone central domains. In peripheral growth cone domains, where individual puncta are more readily resolved, we observe a similar number of myosin V and myosin VI puncta. However, less than 20% of myosin V and myosin VI puncta colocalize with each other in the peripheral domains. After live cell extraction, punctate staining of myosin V and myosin VI is reduced in peripheral domains. However, we do not detect such changes in the central domains, suggesting that these myosins are associated with cytoskeletal/organelle structures. In peripheral growth cone domains myosin VI exhibits a higher extractability than myosin V. This difference between myosin V and VI was also found in a biochemical growth cone particle preparation from brain, suggesting that a significant portion of these two motors has a distinct subcellular distribution.

Drosophila quail, a villin-related protein, bundles actin filaments in apoptotic nurse cells.

Drosophila Quail protein is required for the completion of fast cytoplasm transport from nurse cells to the oocyte, an event critical for the production of viable oocytes. The abundant network of cytoplasmic filamentous actin, established at the onset of fast transport, is absent in quail mutant egg chambers. Previously, we showed that Quail is a germline-specific protein with sequence homology to villin, a vertebrate actin-regulating protein. In this study, we combined biochemical experiments with observations in egg chambers to define more precisely the function of this protein in the regulation of actin-bundle assembly in nurse cells. We report that recombinant Quail can bind and bundle filamentous actin in vitro in a manner similar to villin at a physiological calcium concentration. In contrast to villin, Quail is unable to sever or cap filamentous actin, or to promote nucleation of new actin filaments at a high calcium concentration. Instead, Quail bundles the filaments regardless of the calcium concentration. In vivo, the assembly of nurse-cell actin bundles is accompanied by extensive perforation of the nurse-cell nuclear envelopes, and both of these phenomena are manifestations of nurse-cell apoptosis. To investigate whether free calcium levels are affected during apoptosis, we loaded egg chambers with the calcium indicator Indo-1. Our observations indicate a rise in free calcium in the nurse-cell cytoplasm coincident with the permeabilization of the nuclear envelopes. We also show that human villin expressed in the Drosophila germline could sense elevated cytoplasmic calcium; in nurse cells with reduced levels of Quail protein, villin interfered with actin-bundle stability. We conclude that Quail efficiently assembles actin filaments into bundles in nurse cells and maintains their stability under fluctuating free calcium levels. We also propose a developmental model for the fast phase of cytoplasm transport incorporating findings presented in this study.

An invasion-associated Salmonella protein modulates the actin-bundling activity of plastin.

The entry of Salmonella typhimurium into nonphagocytic cells requires a panel of bacterial effector proteins that are delivered to the host cell via a type III secretion system. These proteins modulate host-cell signal-transduction pathways and the actin cytoskeleton to induce membrane ruffling and bacterial internalization. One of these bacterial effectors, termed SipA, is an actin-binding protein that is required for efficient Salmonella entry into host cells. We report here that SipA forms a complex with T-plastin on bacterial infection. Formation of such a complex, which requires the presence of F-actin, results in a marked increase in the actin-bundling activity of T-plastin. We also report that T-plastin is recruited to S. typhimurium-induced membrane ruffles by a CDC42-dependent signaling process and is required for bacterial entry. We propose that modulation of the actin-bundling activity of T-plastin by SipA results in the stabilization of the actin filaments at the point of bacterial-host cell contact, which leads to more efficient Salmonella internalization.

Myosin-V is a processive actin-based motor.

Class-V myosins, one of 15 known classes of actin-based molecular motors, have been implicated in several forms of organelle transport, perhaps working with microtubule-based motors such as kinesin. Such movements may require a motor with mechanochemical properties distinct from those of myosin-II, which operates in large ensembles to drive high-speed motility as in muscle contraction. Based on its function and biochemistry, it has been suggested that myosin-V may be a processive motor like kinesin. Processivity means that the motor undergoes multiple catalytic cycles and coupled mechanical advances for each diffusional encounter with its track. This allows single motors to support movement of an organelle along its track. Here we provide direct evidence that myosin-V is indeed a processive actin-based motor that can move in large steps approximating the 36-nm pseudo-repeat of the actin filament.

The tail of a yeast class V myosin, myo2p, functions as a localization domain.

Myo2p is a yeast class V myosin that functions in membrane trafficking. To investigate the function of the carboxyl-terminal-tail domain of Myo2p, we have overexpressed this domain behind the regulatable GAL1 promoter (MYO2DN). Overexpression of the tail domain of Myo2p results in a dominant-negative phenotype that is phenotypically similar to a temperature-sensitive allele of myo2, myo2-66. The tail domain of Myo2p is sufficient for localization at low- expression levels and causes mislocalization of the endogenous Myo2p from sites of polarized cell growth. Subcellular fractionation of polarized, mechanically lysed yeast cells reveals that Myo2p is present predominantly in a 100,000 x g pellet. The Myo2p in this pellet is not solubilized by Mg++-ATP or Triton X-100, but is solubilized by high salt. Tail overexpression does not disrupt this fractionation pattern, nor do mutations in sec4, sec3, sec9, cdc42, or myo2. These results show that overexpression of the tail domain of Myo2p does not compete with the endogenous Myo2p for assembly into a pelletable structure, but does compete with the endogenous Myo2p for a factor that is necessary for localization to the bud tip.

Role of the S. typhimurium actin-binding protein SipA in bacterial internalization.

Entry of the bacterium Salmonella typhimurium into host cells requires membrane ruffling and rearrangement of the actin cytoskeleton. Here, it is shown that the bacterial protein SipA plays a critical role in this process. SipA binds directly to actin, decreases its critical concentration, and inhibits depolymerization of actin filaments. These activities result in the spatial localization and more pronounced outward extension of the Salmonella-induced membrane ruffles, thereby facilitating bacterial uptake.

Cloning and characterization of mouse brush border myosin-I in adult and embryonic intestine.

Brush border myosin-I is a class I myosin with calmodulin light chains that has been identified in several vertebrate species. In chicken, it is exclusively expressed in intestinal epithelial cells where it forms spirally arrayed bridges that tether the microvillar actin bundle to the membrane. To facilitate future knockout strategies, we have isolated mouse brush border myosin-I cDNA and genomic clones. The deduced primary structure of mouse brush border myosin-I is homologous to other known brush border myosins-I. Northern blot, immunoblot, and immunolocalization studies indicate that the intestine-specific and subcellular localization profile of mouse brush border myosin-I are comparable to that determined for other brush border myosins-I. Northern analysis during embryogenesis revealed a 3.9-kb transcript first detected in 15-day embryos. This is in marked contrast to chicken, where brush border myosin-I expression begins early in embryogenesis. In situ localization in 17-day embryos indicated that RNA expression is restricted to the intestine. Protein expression is first detected in 16-day embryos with decreasing levels observed in a proximal to distal fashion. Immunolocalization in embryonic intestine revealed that brush border myosin-I is evenly distributed on both apical and basolateral membrane domains. There is also pronounced localization to a supranuclear region, presumably the Golgi apparatus. This suggests that brush border myosin-I may be targeted to the plasma membrane on Golgi-derived vesicles rather than by direct targeting to microvillar actin cores.

Human brush border myosin-I and myosin-Ic expression in human intestine and Caco-2BBe cells.

The human intestinal cell line, Caco-2BBe, has been established as an excellent model system for analysis of the enterocyte cytoskeleton including that of the actin rich apical brush border. To facilitate its use for functional analysis of a major component of the brush border, brush border myosin-I, human cDNAs encoding the heavy chain of this class I myosin were isolated and sequenced. The identity of this myosin as human brush border myosin-I was verified based on similarity with other vertebrate sequences, as well as its expression profile at both the RNA and protein levels. Localization of the protein in human intestine along the crypt-villus axis closely resembles that previously determined for brush border myosin-I in chicken, and is quite distinct from that of myosin-Ic, another myosin-I expressed in human intestine and Caco-2BBe cells. In immature cells of the crypt, brush border myosin-I staining is low, and there is significant cytosolic and basolateral localization, while villus cells stain much more intensely, and the protein is primarily localized to the brush border. Localization of myosin-Ic is essentially the inverse of brush border myosin-I in that crypt cells exhibit higher levels of staining, while villus cells have very low levels of myosin-Ic. The expression of both myosins-I was also examined during cell-contact induced differentiation of Caco-2BBe cells where expression and changes in localization closely resemble those that accompany differentiation of enterocyte in vivo.