Myosin VIIB from Drosophila is a high duty ratio motor

Morphologic, cytometric, quantitative transcriptomic and functional characterisation provide insights into the haemocyte immune responses of Pacific abalone (Haliotis discus hannai)

In recent years, the abalone aquaculture industry has been threatened by the bacterial pathogens. The immune responses mechanisms underlying the phagocytosis of haemocytes remain unclear in Haliotis discus hannai. It is necessary to investigate the immune mechanism in response to these bacterial pathogens challenges. In this study, the phagocytic activities of haemocytes in H. discus hannai were examined by flow cytometry combined with electron microscopy and transcriptomic analyses. The results of Vibrio parahaemolyticus, Vibrio alginolyticus and Staphylococcus aureu challenge using electron microscopy showed a process during phagosome formation in haemocytes. The phagocytic rate (PP) of S. aureus was higher than the other five foreign particles, which was about 63%. The PP of Vibrio harveyi was about 43%, the PP peak of V. alginolyticus in haemocyte was 63.7% at 1.5 h. After V. parahaemolyticus and V. alginolyticus challenge, acid phosphatase, alkaline phosphatase, total superoxide dismutase, lysozyme, total antioxidant capacity, catalase, nitric oxide synthase and glutathione peroxidase activities in haemocytes were measured at different times, differentially expressed genes (DEGs) were identified by quantitative transcriptomic analysis. The identified DEGs after V. parahaemolyticus challenge included haemagglutinin/amebocyte aggregation factor-like, supervillin-like isoform X4, calmodulin-like and kyphoscoliosis peptidase-like; the identified DEGs after V. alginolyticus challenge included interleukin-6 receptor subunit beta-like, protein turtle homolog B-like, rho GTPase-activating protein 6-like isoform X2, leukocyte surface antigen CD53-like, calponin-1-like, calmodulin-like, troponin C, troponin I-like isoform X4, troponin T-like isoform X18, tumor necrosis factor ligand superfamily member 10-like, rho-related protein racA-like and haemagglutinin/amebocyte aggregation factor-like. Some immune-related KEGG pathways were significantly up-regulated or down-regulated after challenge, including thyroid hormone synthesis, Th17 cell differentiation signalling pathway, focal adhesion, melanogenesis, leukocyte transendothelial migration, inflammatory mediator regulation of TRP channels, ras signalling pathway, rap1 signalling pathway. This study is the first step towards understanding the H. discus hannai immune system by adapting several tools to gastropods and providing a first detailed morpho-functional study of their haemocytes.

Propagation of F-actin disassembly via Myosin15-Mical interactions

The F-actin cytoskeleton drives cellular form and function. However, how F-actin-based changes occur with spatiotemporal precision and specific directional orientation is poorly understood. Here, we identify that the unconventional class XV myosin [Myosin 15 (Myo15)] physically and functionally interacts with the F-actin disassembly enzyme Mical to spatiotemporally position cellular breakdown and reconstruction. Specifically, while unconventional myosins have been associated with transporting cargo along F-actin to spatially target cytoskeletal assembly, we now find they also target disassembly. Myo15 specifically positions this F-actin disassembly by associating with Mical and using its motor and MyTH4-FERM cargo-transporting functions to broaden Mical's distribution. Myo15's broadening of Mical's distribution also expands and directionally orients Mical-mediated F-actin disassembly and subsequent cellular remodeling, including in response to Semaphorin/Plexin cell surface activation signals. Thus, we identify a mechanism that spatiotemporally propagates F-actin disassembly while also proposing that other F-actin-trafficked-cargo is derailed by this disassembly to directionally orient rebuilding.

Enhanced protein phosphorylation in Apostichopus japonicus intestine triggered by tussah immunoreactive substances might be involved in the regulation of immune-related signaling pathways

The sea cucumber Apostichopus japonicus is an economically important species owing to its high nutritive and medicinal value. In order to avoid the pollution resulting from the overuse of antibiotics in A. japonicus aquaculture, various immunostimulants have been used as an alternative to improve the efficiency of A. japonicus farming. Our previous proteomic investigation has shown that several proteins participating in the immune-related physiology of A. japonicus were differentially expressed in the intestinal tissue in response to tussah immunoreactive substances (TIS). This study further explored the immunostimulation mechanism of TIS in A. japonicus. Phosphoproteomics technology was used to investigate the effect of TIS on protein phosphorylation in the intestine of A. japonicus following feeding with a TIS-supplemented diet. A total of 213 unique phosphoproteins were detected from 225 unique phosphopeptides. KEGG pathway analysis showed that majority of the phosphoproteins are involved in endocytosis, carbon metabolism and spliceosome functional group. Sixteen of the phosphoproteins exhibited differential phosphorylation in response to TIS and 12 of these were found to associate with biological functions. Of these 12 phosphoproteins, eight exhibited enhanced phosphorylation while four displayed reduced phosphorylation. These 12 proteins were further analyzed and all were found to play a role in regulating some aspects of the immune system and the growth of sea cucumbers, especially in phagocytosis, energy metabolism and disease resistance. The findings of this study could therefore shed new light on the immune pathways of sea cucumber that are affected by TIS. This could help us to better understand the underlying mechanism linked to the immunoenhancement of A. japonicus in response to TIS, one that is associated with the change in protein phosphorylation.

A heterologous in-cell assay for investigating intermicrovillar adhesion complex interactions reveals a novel protrusion length-matching mechanism

Solute transporting epithelial cells build arrays of microvilli on their apical surface to increase membrane scaffolding capacity and enhance function potential. In epithelial tissues such as the kidney and gut, microvilli are length-matched and assembled into tightly packed "brush borders," which are organized by ∼50-nm thread-like links that form between the distal tips of adjacent protrusions. Composed of protocadherins CDHR2 and CDHR5, adhesion links are stabilized at the tips by a cytoplasmic tripartite module containing the scaffolds USH1C and ANKS4B and the actin-based motor MYO7B. Because several questions about the formation and function of this "intermicrovillar adhesion complex" remain open, we devised a system that allows one to study individual binary interactions between specific complex components and MYO7B. Our approach employs a chimeric myosin consisting of the MYO10 motor domain fused to the MYO7B cargo-binding tail domain. When expressed in HeLa cells, which do not normally produce adhesion complex proteins, this chimera trafficked to the tips of filopodia and was also able to transport individual complex components to these sites. Unexpectedly, the MYO10-MYO7B chimera was able to deliver CDHR2 and CDHR5 to distal tips in the absence of USH1C or ANKS4B. Cells engineered to localize high levels of CDHR2 at filopodial tips acquired interfilopodial adhesion and exhibited a striking dynamic length-matching activity that aligned distal tips over time. These findings deepen our understanding of mechanisms that promote the distal tip accumulation of intermicrovillar adhesion complex components and also offer insight on how epithelial cells minimize microvillar length variability.

Quantitative proteomic analysis in serum of Takifugu rubripes infected with Cryptocaryon irritans

Cryptocaryon irritans can cause cryptocaryonosis (white spot disease) in marine fish but the pathogenesis of the disease is unclear. In this work, we used high-throughput proteomics to identify differentially expressed proteins in the serum of Takifugu rubripes challenged with C. irritans. By using quantitative proteomic assays combined with Tandem Mass Tag-labeled quantitative proteomic analysis, we identified a total of 2088 differentially abundant proteins (1706 proteins were quantified, p < 0.05, fold-change threshold ≥ 2), including 21 up-regulated and 44 down-regulated. Combined with STRING-based functional analysis, we ultimately obtained eight proteins including glucokinase-like, integrin beta-1-like isoform X2, H4, H2A.V, histone H1-like, histone H2AX-like, histone H2B 1/2-like and myosin-9 isoform X1, which could be considered as potential biomarkers for T. rubripes immune responses. Eight proteins that were selected to validate significant differentially expressed genes at the proteomic level were consistent with qPCR at the transcriptomic level. The proteins identified in our work may serve as candidates for elucidating the molecular mechanism of cryptocaryonosis in T. rubripes. Our collective findings could provide new insights into searching for disease-specific targets and biomarkers, which may be effective indicators of C. irritans infection in T. rubripes.

The small EF-hand protein CALML4 functions as a critical myosin light chain within the intermicrovillar adhesion complex

Specialized transporting and sensory epithelial cells employ homologous protocadherin-based adhesion complexes to remodel their apical membrane protrusions into organized functional arrays. Within the intestine, the nutrient-transporting enterocytes utilize the intermicrovillar adhesion complex (IMAC) to assemble their apical microvilli into an ordered brush border. The IMAC bears remarkable homology to the Usher complex, whose disruption results in the sensory disorder type 1 Usher syndrome (USH1). However, the entire complement of proteins that comprise both the IMAC and Usher complex are not yet fully elucidated. Using a protein isolation strategy to recover the IMAC, we have identified the small EF-hand protein calmodulin-like protein 4 (CALML4) as an IMAC component. Consistent with this finding, we show that CALML4 exhibits marked enrichment at the distal tips of enterocyte microvilli, the site of IMAC function, and is a direct binding partner of the IMAC component myosin-7b. Moreover, distal tip enrichment of CALML4 is strictly dependent upon its association with myosin-7b, with CALML4 acting as a light chain for this myosin. We further show that genetic disruption of CALML4 within enterocytes results in brush border assembly defects that mirror the loss of other IMAC components and that CALML4 can also associate with the Usher complex component myosin-7a. Our study further defines the molecular composition and protein-protein interaction network of the IMAC and Usher complex and may also shed light on the etiology of the sensory disorder USH1H.

MyTH4-FERM myosins in the assembly and maintenance of actin-based protrusions

Unconventional myosins are actin-based molecular motors that serve a multitude of roles within the cell. One group of myosin motors, the MyTH4-FERM myosins, play an integral part in building and maintaining finger-like protrusions, which allow cells to interact with their external environment. Suggested to act primarily as transporters, these motor proteins enrich adhesion molecules, actin-regulatory proteins and other factors at the tips of filopodia, microvilli, and stereocilia. Below we review data from biophysical, biochemical, and cell biological studies, which implicate these myosins as central players in the assembly, maintenance and function of actin-based protrusions.

Myosin-7b Promotes Distal Tip Localization of the Intermicrovillar Adhesion Complex

Transporting epithelial cells interact with the luminal environment using a tightly packed array of microvilli known as the brush border. During intestinal epithelial differentiation, microvillar packing and organization are driven by cadherin-dependent adhesion complexes that localize to the distal tips of microvilli, where they drive physical interactions between neighboring protrusions. Although enrichment of the "intermicrovillar adhesion complex" (IMAC) at distal tips is required for proper function, the mechanism driving tip accumulation of these factors remains unclear. Here, we report that the actin-based motor myosin-7b (Myo7b) promotes the accumulation of IMAC components at microvillar tips. Myo7b is highly enriched at the tips of microvilli in both kidney and intestinal brush borders, and loss of Myo7b in differentiating intestinal epithelial cells disrupts intermicrovillar adhesion and, thus, brush border assembly. Analysis of cells lacking Myo7b revealed that IMAC components and the resulting intermicrovillar adhesion links are mislocalized along the microvillar axis rather than enriched at the distal tips. We also found that Myo7b motor domains are capable of supporting tip-directed transport. However, motor activity is supplemented by other passive targeting mechanisms that together drive highly efficient IMAC accumulation at the tips. These findings illuminate the molecular basis of IMAC enrichment at microvillar tips and hold important implications for understanding apical morphogenesis in transporting and sensory epithelial tissues.

Myosin XI-I is Mechanically and Enzymatically Unique Among Class-XI Myosins in Arabidopsis

Arabidopsis possesses 13 genes encoding class-XI myosins. Among these, myosin XI-I is phylogenetically distant. To examine the molecular properties of Arabidopsis thaliana myosin XI-I (At myosin XI-I), we performed in vitro mechanical and enzymatic analyses using recombinant constructs of At myosin XI-I. Unlike other biochemically studied class-XI myosins, At myosin XI-I showed extremely low actin-activated ATPase activity (Vmax = 3.7 Pi s(-1) head(-1)). The actin-sliding velocity of At myosin XI-I was 0.25 µm s(-1), >10 times lower than those of other class-XI myosins. The ADP dissociation rate from acto-At myosin XI-I was 17 s(-1), accounting for the low actin-sliding velocity. In contrast, the apparent affinity for actin in the presence of ATP, estimated from Kapp (0.61 µM) of actin-activated ATPase, was extremely high. The equilibrium dissociation constant for actin was very low in both the presence and absence of ATP, indicating a high affinity for actin. To examine At myosin XI-I motility in vivo, green fluorescent protein-fused full-length At myosin XI-I was expressed in cultured Arabidopsis cells. At myosin XI-I localized not only on the nuclear envelope but also on small dots moving slowly (0.23 µm s(-1)) along actin filaments. Our results show that the properties of At myosin XI-I differ from those of other Arabidopsis class-XI myosins. The data suggest that At myosin XI-I does not function as a driving force for cytoplasmic streaming but regulates the organelle velocity, supports processive organelle movement or acts as a tension generator.

Kinetic Adaptations of Myosins for Their Diverse Cellular Functions

Members of the myosin superfamily are involved in all aspects of eukaryotic life. Their function ranges from the transport of organelles and cargos to the generation of membrane tension, and the contraction of muscle. The diversity of physiological functions is remarkable, given that all enzymatically active myosins follow a conserved mechanoenzymatic cycle in which the hydrolysis of ATP to ADP and inorganic phosphate is coupled to either actin-based transport or tethering of actin to defined cellular compartments. Kinetic capacities and limitations of a myosin are determined by the extent to which actin can accelerate the hydrolysis of ATP and the release of the hydrolysis products and are indispensably linked to its physiological tasks. This review focuses on kinetic competencies that - together with structural adaptations - result in myosins with unique mechanoenzymatic properties targeted to their diverse cellular functions.

Kinetic mechanism of Nicotiana tabacum myosin-11 defines a new type of a processive motor

The 175-kDa myosin-11 from Nicotiana tabacum (Nt(175kDa)myosin-11) is exceptional in its mechanical activity as it is the fastest known processive actin-based motor, moving 10 times faster than the structurally related class 5 myosins. Although this ability might be essential for long-range organelle transport within larger plant cells, the kinetic features underlying the fast processive movement of Nt(175kDa)myosin-11 still remain unexplored. To address this, we generated a single-headed motor domain construct and carried out a detailed kinetic analysis. The data demonstrate that Nt(175kDa)myosin-11 is a high duty ratio motor, which remains associated with actin most of its enzymatic cycle. However, different from other processive myosins that establish a high duty ratio on the basis of a rate-limiting ADP-release step, Nt(175kDa)myosin-11 achieves a high duty ratio by a prolonged duration of the ATP-induced isomerization of the actin-bound states and ADP release kinetics, both of which in terms of the corresponding time constants approach the total ATPase cycle time. Molecular modeling predicts that variations in the charge distribution of the actin binding interface might contribute to the thermodynamic fine-tuning of the kinetics of this myosin. Our study unravels a new type of a high duty ratio motor and provides important insights into the molecular mechanism of processive movement of higher plant myosins.

A Toxoplasma gondii class XIV myosin, expressed in Sf9 cells with a parasite co-chaperone, requires two light chains for fast motility

Many diverse myosin classes can be expressed using the baculovirus/Sf9 insect cell expression system, whereas others have been recalcitrant. We hypothesized that most myosins utilize Sf9 cell chaperones, but others require an organism-specific co-chaperone. TgMyoA, a class XIVa myosin from the parasite Toxoplasma gondii, is required for the parasite to efficiently move and invade host cells. The T. gondii genome contains one UCS family myosin co-chaperone (TgUNC). TgMyoA expressed in Sf9 cells was soluble and functional only if the heavy and light chain(s) were co-expressed with TgUNC. The tetratricopeptide repeat domain of TgUNC was not essential to obtain functional myosin, implying that there are other mechanisms to recruit Hsp90. Purified TgMyoA heavy chain complexed with its regulatory light chain (TgMLC1) moved actin in a motility assay at a speed of ∼1.5 μm/s. When a putative essential light chain (TgELC1) was also bound, TgMyoA moved actin at more than twice that speed (∼3.4 μm/s). This result implies that two light chains bind to and stabilize the lever arm, the domain that amplifies small motions at the active site into the larger motions that propel actin at fast speeds. Our results show that the TgMyoA domain structure is more similar to other myosins than previously appreciated and provide a molecular explanation for how it moves actin at fast speeds. The ability to express milligram quantities of a class XIV myosin in a heterologous system paves the way for detailed structure-function analysis of TgMyoA and identification of small molecule inhibitors.

The promotion of cytoskeleton integration and redox in the haemocyte of shrimp Litopenaeus vannamei after the successive stimulation of recombinant VP28

VP28 protein has been reported to work as a "vaccine" to protect the host from white spot syndrome, but the detailed mechanism of vaccination with VP28 protein in shrimp is still far from well understood. In the present study, whole transcriptomes of shrimp haemocytes were sequenced using the SOLiD4 platform after the successive VP28 stimulation. Eight single-end fragment libraries were constructed and sequenced in the four groups including the VP28-VP28, PBS-VP28, PBS-PBS and BLANK group, and there were 243,949,667 single-end reads with length of 50bp obtained totally, with 14,800 genes further identified. After reads mapping and transcript assembling, 1027, 1539, 1158, 1091 and 1300 genes in five differentially expressed gene lists were obtained in the comparison of VP28-VP28 versus PBS-VP28, VP28-VP28 versus PBS-PBS, VP28-VP28 versus BLANK, PBS-VP28 versus PBS-PBS and PBS-VP28 versus BLANK, respectively. There were 555 differentially expressed genes responsive to the single VP28 stimulation after grouping the PBS-VP28_BLANK and PBS-VP28_PBS-PBS gene lists, and 269 ones responsive to the successive VP28 stimulation after grouping the VP28-VP28_BLANK, VP28-VP28_PBS-PBS and VP28-VP28_PBS-VP28 gene lists. In the GO enrichment analysis of the genes responsive to the single VP28 stimulation, five immune-related GO terms were observed among 14 increased terms, which included defense response to bacterium, response to stimulus, disruption of cells of other organism, killing of cells of other organism and response to bacterium. It was worth noting that the GO terms, response to stimulus and response to stress, were the most common annotation ones which accounted 28.7% and 18.8% of the total differently expressed genes, respectively. For the genes responsive to the successive VP28 stimulation, terms including actin filament-based movement and myosin heavy chain binding were mostly enriched in the Biological Process and Molecular Function category, respectively. In the Cellular Component category, the enriched GO terms were myosin VII complex, myosin V complex, myosin VI complex and myosin II complex. Furthermore, the most abundant GO term was oxidation-reduction process, followed by single-organism transport, neurogenesis and translation for 214 genes only responsive to successive VP28 stimulation. These results collectively indicated that the successive VP28 stimulation could modulate cytoskeleton integration and redox to promote the phagocytosis activity of shrimp haemocytes, which might protect effectively for shrimp against WSSV infection.

Mouse myosin-19 is a plus-end-directed, high-duty ratio molecular motor

Class XIX myosin (Myo19) is a vertebrate-specific unconventional myosin, responsible for the transport of mitochondria. To characterize biochemical properties of Myo19, we prepared recombinant mouse Myo19-truncated constructs containing the motor domain and the IQ motifs using the baculovirus/Sf9 expression system. We identified regulatory light chain (RLC) of smooth muscle/non-muscle myosin-2 as the light chain of Myo19. The actin-activated ATPase activity and the actin-gliding velocity of Myo19-truncated constructs were about one-third and one-sixth as those of myosin-5a, respectively. The apparent affinity of Myo19 to actin was about the same as that of myosin-5a. The RLCs bound to Myo19 could be phosphorylated by myosin light chain kinase, but this phosphorylation had little effect on the actin-activated ATPase activity and the actin-gliding activity of Myo19-truncated constructs. Using dual fluorescence-labeled actin filaments, we determined that Myo19 is a plus-end-directed molecular motor. We found that, similar to that of the high-duty ratio myosin, such as myosin-5a, ADP release rate was comparable with the maximal actin-activated ATPase activity of Myo19, indicating that ADP release is a rate-limiting step for the ATPase cycle of acto-Myo19. ADP strongly inhibited the actin-activated ATPase activity and actin-gliding activity of Myo19-truncated constructs. Based on the above results, we concluded that Myo19 is a high-duty ratio molecular motor moving to the plus-end of the actin filament.

Molecular characterization and subcellular localization of Arabidopsis class VIII myosin, ATM1

Land plants possess myosin classes VIII and XI. Although some information is available on the molecular properties of class XI myosins, class VIII myosins are not characterized. Here, we report the first analysis of the enzymatic properties of class VIII myosin. The motor domain of Arabidopsis class VIII myosin, ATM1 (ATM1-MD), and the motor domain plus one IQ motif (ATM1-1IQ) were expressed in a baculovirus system and characterized. ATM1-MD and ATM1-1IQ had low actin-activated Mg(2+)-ATPase activity (Vmax = 4 s(-1)), although their affinities for actin were high (Kactin = 4 μM). The actin-sliding velocities of ATM1-MD and ATM1-1IQ were 0.02 and 0.089 μm/s, respectively, from which the value for full-length ATM1 is calculated to be ∼0.2 μm/s. The results of actin co-sedimentation assay showed that the duty ratio of ATM1 was ∼90%. ADP dissociation from the actin·ATM1 complex (acto-ATM1) was extremely slow, which accounts for the low actin-sliding velocity, low actin-activated ATPase activity, and high duty ratio. The rate of ADP dissociation from acto-ATM1 was markedly biphasic with fast and slow phase rates (5.1 and 0.41 s(-1), respectively). Physiological concentrations of free Mg(2+) modulated actin-sliding velocity and actin-activated ATPase activity by changing the rate of ADP dissociation from acto-ATM1. GFP-fused full-length ATM1 expressed in Arabidopsis was localized to plasmodesmata, plastids, newly formed cell walls, and actin filaments at the cell cortex. Our results suggest that ATM1 functions as a tension sensor/generator at the cell cortex and other structures in Arabidopsis.

Characterization of myosin light chain gene up-regulated in the large yellow croaker immunity by interaction with RanGTPase

RanGTPases are highly conserved in eukaryotes from yeast to human and have been implicated in many aspects of nuclear structure and function. In our previous study, it was revealed that the RanGTPase was up-regulated in large yellow croaker challenged by pathogen. However, the mechanism of RanGTPase in immunity remains unclear. In this investigation, on the basis of protein interaction, it was found that RanGTPase interacted with myosin light chain (designated as LycMLC), a crucial protein in the process of phagocytosis. Furthermore, it was found and characterized in this marine fish for the first time. The full-length cDNA of LycMLC was 771bp, including a 5'-terminal untranslated region (UTR) of 36bp, 3'-terminal UTR of 279bp and an open reading frame (ORF) of 456bp encoding a polypeptide of 151 amino acids. RT-PCR analysis indicated that LycMLC gene was constitutively expressed in the 9 tissues examined, including kidney, liver, gill, muscle, spleen, skin, heart, intestine and blood. The result of quantitative real-time PCR analysis revealed the highest expression in muscle and the weakest expression in skin. Time course analysis showed that LycMLC expression was obviously up-regulated in blood after immunization with either poly I:C or formalin-inactive Gram-negative bacteria Vibrio parahaemolyticus. It indicated that the highest expression was 4.5 times (at 24h) as much as that in the control (P<0.05) challenged by poly I:C and 5.0 times (at 24h) challenged by bacteria. These results suggested that LycMLC might play an important role in large yellow croaker defense against the pathogen infection. Therefore our study revealed a novel pathway concerning immunity of RanGTPase by the direct interaction with the cytoskeleton protein, which would help to better understand the molecular events in immune response against pathogen infection in fish.

Shaking the myosin family tree: biochemical kinetics defines four types of myosin motor

Although all myosin motors follow the same basic cross-bridge cycle, they display a large variety in the rates of transition between different states in the cycle, allowing each myosin to be finely tuned for a specific task. Traditionally, myosins have been classified by sequence analysis into a large number of sub-families (∼35). Here we use a different method to classify the myosin family members which is based on biochemical and mechanical properties. The key properties that define the type of mechanical activity of the motor are duty ratio (defined as the fraction of the time myosin remains attached to actin during each cycle), thermodynamic coupling of actin and nucleotide binding to myosin and the degree of strain-sensitivity of the ADP release step. Based on these properties we propose to classify myosins into four different groups: (I) fast movers, (II) slow/efficient force holders, (III) strain sensors and (IV) gates.

Functional characterization of the human myosin-7a motor domain

Myosin-7a participates in auditory and visual processes. Defects in MYO7A, the gene encoding the myosin-7a heavy chain, are causative for Usher syndrome 1B, the most frequent cause of deaf-blindness in humans. In the present study, we performed a detailed kinetic and functional characterization of the isolated human myosin-7a motor domain to elucidate the details of chemomechanical coupling and the regulation of motor function. A rate-limiting, slow ADP release step causes long lifetimes of strong actin-binding intermediates and results in a high duty ratio. Moreover, our results reveal a Mg²⁺-sensitive regulatory mechanism tuning the kinetic and mechanical properties of the myosin-7a motor domain. We obtained direct evidence that changes in the concentration of free Mg²⁺ ions affect the motor properties of human myosin-7a using an in vitro motility assay system. Our results suggest that in a cellular environment, compartment-specific fluctuations in free Mg²⁺ ions can mediate the conditional switching of myosin-7a between cargo moving and tension bearing modes.

Myosin motor function: the ins and outs of actin-based membrane protrusions

Cells build plasma membrane protrusions supported by parallel bundles of F-actin to enable a wide variety of biological functions, ranging from motility to host defense. Filopodia, microvilli and stereocilia are three such protrusions that have been the focus of intense biological and biophysical investigation in recent years. While it is evident that actin dynamics play a significant role in the formation of these organelles, members of the myosin superfamily have also been implicated as key players in the maintenance of protrusion architecture and function. Based on a simple analysis of the physical forces that control protrusion formation and morphology, as well as our review of available data, we propose that myosins play two general roles within these structures: (1) as cargo transporters to move critical regulatory components toward distal tips and (2) as mediators of membrane-cytoskeleton adhesion.

Ran GTPase regulates hemocytic phagocytosis of shrimp by interaction with myosin

Ran GTPases, one family of small G protein superfamily, have been widely demonstrated to be involved in the transport system between cytoplasm and the nucleus. However, the function of Ran GTPase in immunity remains unclear. In our study, it was found that the Ran GTPase (designated as PjRan) was up-regulated in virus-resistant shrimp, indicating that the PjRan might be implicated in the innate immune system against virus infection. On the basis of protein interactions, it was found that the PjRan interacted with myosin, a crucial protein in the process of phagocytosis to form a protein complex. The RNAi and mRNA assays showed that the PjRan could regulate shrimp hemocytic phagocytosis. Further data evidenced that the depletion of PjRan by RNAi caused a significant increase of virus copies, and the overexpression of PjRan resulted in a significant decrease of virus copies, suggesting that the PjRan participated in the antiviral immunity by regulating phagocytosis. Therefore, our study revealed a completely novel aspect of Ran GTPase in phagocytosis by the direct interaction with the cytoskeleton protein and presented a novel pathway concerning to antiviral immunity, which will help to better understand the molecular events in immune response against virus infection in invertebrates.

A FERM domain autoregulates Drosophila myosin 7a activity

Full-length Drosophila myosin 7a (myosin 7a-FL) has a complex tail containing a short predicted coiled coil followed by a MyTH4-FERM domain, an SH3 domain, and a C-terminal MyTH4-FERM domain. Myosin 7a-FL expressed in Sf9 cells is monomeric despite the predicted coiled coil. We showed previously that Subfragment-1 (S1) from this myosin has MgATPase of V(max) approximately 1 s(-1) and K(ATPase) approximately 1 microM actin. We find that myosin 7a-FL has V(max) similar to S1 but K(ATPase) approximately 30 microM. Thus, at low actin concentrations (5 microM), the MgATPase of S1 is fully activated, whereas that of myosin 7a-FL is low, suggesting that the tail regulates activity. Electron microscopy of myosin 7a-FL with ATP shows the tail is tightly bent back against the motor domain. Myosin 7a-FL extends at either high ionic strength or without ATP, revealing the motor domain, lever, and tail. A series of C-terminal truncations show that deletion of 99 aa (the MyTH7 subdomain of the C-terminal FERM domain) is sufficient to abolish bending, and the K(ATPase) is then similar to S1. This region is highly conserved in myosin 7a. We found that a double mutation in it, R2140A-K2143A, abolishes bending and reduces K(ATPase) to S1 levels. In addition, the expressed C-terminal FERM domain binds actin with K(d) approximately 30 microM regardless of ATP, similar to the K(ATPase) value for myosin 7a-FL. We propose that at low cellular actin concentrations, myosin 7a-FL is bent and inactive, but at high actin concentrations, it is unfolded and active because the C-terminal FERM domain binds to actin.

Human myosin Vc is a low duty ratio, nonprocessive molecular motor

Myosin Vc is the product of one of the three genes of the class V myosin found in vertebrates. It is widely found in secretory and glandular tissues, with a possible involvement in transferrin trafficking. Transient and steady-state kinetic studies of human myosin Vc were performed using a truncated, single-headed construct. Steady-state actin-activated ATPase measurements revealed a V(max) of 1.8 +/- 0.3 s(-1) and a K(ATPase) of 43 +/- 11 microm. Unlike previously studied vertebrate myosin Vs, the rate-limiting step in the actomyosin Vc ATPase pathway is the release of inorganic phosphate (~1.5 s(-1)), rather than the ADP release step (~12.0-16.0 s(-1)). Nevertheless, the ADP affinity of actomyosin Vc (K(d) = 0.25 +/- 0.02 microm) reflects a higher ADP affinity than seen in other myosin V isoforms. Using the measured kinetic rates, the calculated duty ratio of myosin Vc was approximately 10%, indicating that myosin Vc spends the majority of the actomyosin ATPase cycle in weak actin-binding states, unlike the other vertebrate myosin V isoforms. Consistent with this, a fluorescently labeled double-headed heavy meromyosin form showed no processive movements along actin filaments in a single molecule assay, but it did move actin filaments at a velocity of approximately 24 nm/s in ensemble assays. Kinetic simulations reveal that the high ADP affinity of actomyosin Vc may lead to elevations of the duty ratio of myosin Vc to as high as 64% under possible physiological ADP concentrations. This, in turn, may possibly imply a regulatory mechanism that may be sensitive to moderate changes in ADP concentration.

Kinetic mechanism of the fastest motor protein, Chara myosin

Chara corallina class XI myosin is by far the fastest molecular motor. To investigate the molecular mechanism of this fast movement, we performed a kinetic analysis of a recombinant motor domain of Chara myosin. We estimated the time spent in the strongly bound state with actin by measuring rate constants of ADP dissociation from actin.motor domain complex and ATP-induced dissociation of the motor domain from actin. The rate constant of ADP dissociation from acto-motor domain was >2800 s(-1), and the rate constant of ATP-induced dissociation of the motor domain from actin at physiological ATP concentration was 2200 s(-1). From these data, the time spent in the strongly bound state with actin was estimated to be <0.82 ms. This value is the shortest among known values for various myosins and yields the duty ratio of <0.3 with a V(max) value of the actin-activated ATPase activity of 390 s(-1). The addition of the long neck domain of myosin Va to the Chara motor domain largely increased the velocity of the motility without increasing the ATP hydrolysis cycle rate, consistent with the swinging lever model. In addition, this study reveals some striking kinetic features of Chara myosin that are suited for the fast movement: a dramatic acceleration of ADP release by actin (1000-fold) and extremely fast ATP binding rate.

Powering membrane traffic in endocytosis and recycling

Early in evolution, the diversification of membrane-bound compartments that characterize eukaryotic cells was accompanied by the elaboration of molecular machineries that mediate intercompartmental communication and deliver materials to specific destinations. Molecular motors that move on tracks of actin filaments or microtubules mediate the movement of organelles and transport between compartments. The subjects of this review are the motors that power the transport steps along the endocytic and recycling pathways, their modes of attachment to cargo and their regulation.

Human myosin III is a motor having an extremely high affinity for actin

Myosin IIIA is expressed in photoreceptor cells and thought to play a critical role in phototransduction processes, yet its function on a molecular basis is largely unknown. Here we clarified the kinetic mechanism of the ATPase cycle of human myosin IIIA. The steady-state ATPase activity was markedly activated approximately 10-fold with very low actin concentration. The rate of ADP off from actomyosin IIIA was 10 times greater than the overall cycling rate, thus not a rate-determining step. The rate constant of the ATP hydrolysis step of the actin-dissociated form was very slow, but the rate was markedly accelerated by actin binding. The dissociation constant of the ATP-bound form of myosin IIIA from actin is submicromolar, which agrees well with the low K(actin). These results indicate that ATP hydrolysis predominantly takes place in the actin-bound form for actomyosin IIIA ATPase reaction. The obtained K(actin) was much lower than the previously reported one, and we found that the autophosphorylation of myosin IIIA dramatically increased the K(actin), whereas the V(max) was unchanged. Our kinetic model indicates that both the actin-attached hydrolysis and the P(i) release steps determine the overall cycle rate of the dephosphorylated form. Although the stable steady-state intermediates of actomyosin IIIA ATPase reaction are not typical strong actin-binding intermediates, the affinity of the stable intermediates for actin is much higher than conventional weak actin binding forms. The present results suggest that myosin IIIA can spend a majority of its ATP hydrolysis cycling time on actin.

Myosin at work: motor adaptations for a variety of cellular functions

Cells have evolved multiple mechanisms to overcome the effects of entropy and diffusion to create a highly ordered environment. For cells to function properly, some components must be anchored to provide a framework or structure. Others must be rapidly transported over long distances to generate asymmetries in cell morphology and composition. To accomplish long-range transport, cells cannot rely on diffusion alone as many large organelles and macromolecular complexes are essentially immobilized by the dense meshwork of the cytosol. One strategy used by cells to overcome diffusion is to harness the free energy liberated by ATP hydrolysis through molecular motors. Myosins are a family of actin based molecular motors that have evolved a variety of ways to contribute to cellular organization through numerous modifications to the manner they convert that free energy into mechanical work.

Dimerized Drosophila myosin VIIa: a processive motor

The molecular mechanism of processive movement of single myosin molecules from classes V and VI along their actin tracks has recently attracted extraordinary attention. Another member of the myosin superfamily, myosin VII, plays vital roles in the sensory function of Drosophila and mammals. We studied the molecular mechanism of Drosophila myosin VIIa, using transient kinetics and single-molecule motility assays. Myosin VIIa moves along actin filaments as a processive, double-headed single molecule when dimerized by the inclusion of a leucine zipper at the C terminus of the coiled-coil domain. Its motility is approximately 8-10 times slower than that of myosin V, and its step size is 30 nm, which is consistent with the presence of five IQ motifs in its neck region. The kinetic basis for the processive motility of myosin VIIa is the relative magnitude of the release rate constants of phosphate (fast) and ADP (slow) as in myosins V and VI. The ATPase pathway is rate-limited by a reversible interconversion between two distinct ADP-bound actomyosin states, which results in high steady-state occupancy of a strongly actin-bound myosin species. The distinctive features of myosin VIIa (long run lengths, slow motility) will be very useful in video-based single-molecule applications. In cells, this kinetic behavior would allow myosin VIIa to exert and hold tension on actin filaments and, if dimerized, to function as a processive cargo transporter.

Drosophila myosin VIIA is a high duty ratio motor with a unique kinetic mechanism

Mutations of myosin VIIA cause deafness in various species from human and mice to Zebrafish and Drosophila. We analyzed the kinetic mechanism of the ATPase cycle of Drosophila myosin VIIA by using a single-headed construct with the entire neck domain. The steady-state ATPase activity (0.06 s(-1)) was markedly activated by actin to yield V(max) and K(ATPase) of 1.72 s(-1) and 3.2 microm, respectively. The most intriguing finding is that the ATP hydrolysis predominantly takes place in the actin-bound form (actin-attached hydrolysis) for the actomyosin VIIA ATPase reaction. The ATP hydrolysis rate was much faster for the actin-attached form than the dissociated form, in contrast to other myosins reported so far. Both the ATP hydrolysis step and the phosphate release step were significantly faster than the entire ATPase cycle rate, thus not rate-determining. The rate of ADP dissociation from actomyosin VIIA was 1.86 s(-1), which was comparable with the overall ATPase cycle rate, thus assigned to be a rate-determining step. The results suggest that Drosophila myosin VIIA spends the majority of the ATPase cycle in an actomyosin.ADP form, a strong actin binding state. The duty ratio calculated from our kinetic model was approximately 0.9. Therefore, myosin VIIA is classified to be a high duty ratio motor. The present results suggested that myosin VIIA can be a processive motor to serve cargo trafficking in cells once it forms a dimer structure.

What limits the velocity of fast-skeletal muscle contraction in mammals?

In rat skeletal muscle the unloaded shortening velocity (Vo) is defined by the myosin isoform expressed in the muscle fibre. In 2001 we suggested that ADP release from actomyosin in solution (controlled by k(-AD)) was of the right size to limit Vo. However, to compare mechanical and solution kinetic data required a series of corrections to compensate for the differences in experimental conditions (0.5 M KCl, 22 degrees C for kinetic assays of myosin, 200 mM ionic strength, 12 degrees C to measure Vo). Here, a method was developed to prepare heavy meromyosin (HMM) from pure myosin isoforms isolated from single muscle fibres and to study k(-AD) (determined from the affinity of the acto-myosin complex for ADP, KAD) and the rate of ATP-induced acto-HMM dissociation (controlled by K1k+2) under the same experimental condition used to measure Vo). In fast-muscle myosin isolated from a wide range of mammalian muscles, k(-AD) was found to be too fast to limit Vo, whereas K1k+2 was of the right magnitude for ATP-induced dissociation of the cross-bridge to limit shortening velocity. The result was unexpected and prompted further experiments using the stopped-flow approach on myosin subfragment-1 (S1) and HMM obtained from bulk preparations of rabbit and rat muscle. These confirmed that the rate of cross-bridge dissociation by ATP limits the velocity of contraction for fast myosin II isoforms at 12 degrees C, while k(-AD) limits the velocity of slow myosin II isoforms. Extrapolating our data to 37 degrees C suggests that at physiological temperature the rate of ADP dissociation may limit Vo for both isoforms.

Myosin-X: a molecular motor at the cell's fingertips

Research in several areas, including unconventional myosins and deafness genes, has converged recently on a group of myosins whose tails contain myosin tail homology 4 (MyTH4) and band 4.1, ezrin, radixin, moesin (FERM) domains. Although these 'MyTH-FERM' myosins are not present in yeast and plants, they are present in slime molds, worms, flies and mammals, where they mediate interactions between the cytoskeleton and the plasma membrane. The most broadly distributed MyTH-FERM myosin in vertebrate cells appears to be myosin-X (Myo10). This myosin can act as a link to integrins and microtubules, stimulate the formation of filopodia and undergo a novel form of motility within filopodia.

Vertebrate myosin VIIb is a high duty ratio motor adapted for generating and maintaining tension

Kinetic adaptation of muscle and non-muscle myosins plays a central role in defining the unique cellular functions of these molecular motor enzymes. The unconventional vertebrate class VII myosin, myosin VIIb, is highly expressed in polarized cells and localizes to highly ordered actin filament bundles such as those found in the microvilli of the intestinal brush border and kidney. We have cloned mouse myosin VIIb from a cDNA library, expressed and purified the catalytic motor domain, and characterized its actin-activated ATPase cycle using quantitative equilibrium and kinetic methods. The myosin VIIb steady-state ATPase activity is slow (approximately 1 s(-1)), activated by very low actin filament concentrations (K(ATPase) approximately 0.7 microm), and limited by ADP release from actomyosin. The slow ADP dissociation rate constant generates a long lifetime of the strong binding actomyosin.ADP states. ADP and actin binding is uncoupled, which enables myosin VIIb to remain strongly bound to actin and ADP at very low actin concentrations. In the presence of 2 mm ATP and 2 microm actin, the duty ratio of myosin VIIb is approximately 0.8. The enzymatic properties of actomyosin VIIb are suited for generating and maintaining tension and favor a role for myosin VIIb in anchoring membrane surface receptors to the actin cytoskeleton. Given the high conservation of vertebrate class VII myosins, deafness phenotypes arising from disruption of normal myosin VIIa function are likely to reflect a loss of tension in the stereocilia of inner ear hair cells.