Mechanism, regulation, and functional properties of Dictyostelium myosin-1B

Switch-2 determines Mg2+ADP-release kinetics and fine-tunes the duty ratio of Dictyostelium class-1 myosins

Though myosins share a structurally conserved motor domain, single amino acid variations of active site elements, including the P-loop, switch-1 and switch-2, which act as nucleotide sensors, can substantially determine the kinetic signature of a myosin, i.e., to either perform fast movement or enable long-range transport and tension generation. Switch-2 essentially contributes to the ATP hydrolysis reaction and determines product release. With few exceptions, class-1 myosin harbor a tyrosine in the switch-2 consensus sequence DIYGFE, at a position where class-2 myosins and a selection of myosins from other classes have a substitution. Here, we addressed the role of the tyrosine in switch-2 of class-1 myosins as potential determinant of the duty ratio. We generated constitutively active motor domain constructs of two class-1 myosins from the social amoeba Dictyostelium discoideum, namely, Myo1E, a high duty ratio myosin and Myo1B, a low duty ratio myosin. In Myo1E we introduced mutation Y388F and in Myo1B mutation F387Y. The detailed functional characterization by steady-state and transient kinetic experiments, combined with in vitro motility and landing assays revealed an almost reciprocal relationship of a number of critical kinetic parameters and equilibrium constants between wild-type and mutants that dictate the lifetime of the strongly actin-attached states of myosin. The Y-to-F mutation increased the duty ratio of Moy1B by almost one order of magnitude, while the introduction of the phenylalanine in switch-2 of Myo1E transformed the myosin into a low duty ratio motor. These data together with structural considerations propose a role of switch-2 in fine-tuning ADP release through a mechanism, where the class-specific tyrosine together with surrounding residues contributes to the coordination of Mg2+ and ADP. Our results highlight the importance of conserved switch-2 residues in class-1 myosins for efficient chemo-mechanical coupling, revealing that switch-2 is important to adjust the duty ratio of the amoeboid class-1 myosins for performing movement, transport or gating functions.

Drosophila class-I myosins that can impact left-right asymmetry have distinct ATPase kinetics

Myosin-1D (myo1D) is important for Drosophila left-right asymmetry, and its effects are modulated by myosin-1C (myo1C). De novo expression of these myosins in nonchiral Drosophila tissues promotes cell and tissue chirality, with handedness depending on the paralog expressed. Remarkably, the identity of the motor domain determines the direction of organ chirality, rather than the regulatory or tail domains. Myo1D, but not myo1C, propels actin filaments in leftward circles in in vitro experiments, but it is not known if this property contributes to establishing cell and organ chirality. To further explore if there are differences in the mechanochemistry of these motors, we determined the ATPase mechanisms of myo1C and myo1D. We found that myo1D has a 12.5-fold higher actin-activated steady-state ATPase rate, and transient kinetic experiments revealed myo1D has an 8-fold higher MgADP release rate compared to myo1C. Actin-activated phosphate release is rate limiting for myo1C, whereas MgADP release is the rate-limiting step for myo1D. Notably, both myosins have among the tightest MgADP affinities measured for any myosin. Consistent with ATPase kinetics, myo1D propels actin filaments at higher speeds compared to myo1C in in vitro gliding assays. Finally, we tested the ability of both paralogs to transport 50 nm unilamellar vesicles along immobilized actin filaments and found robust transport by myo1D and actin binding but no transport by myo1C. Our findings support a model where myo1C is a slow transporter with long-lived actin attachments, whereas myo1D has kinetic properties associated with a transport motor.

Phenamacril is a reversible and noncompetitive inhibitor of Fusarium class I myosin

The cyanoacrylate compound phenamacril (also known as JS399-19) is a recently identified fungicide that exerts its antifungal effect on susceptible Fusarium species by inhibiting the ATPase activity of their myosin class I motor domains. Although much is known about the antifungal spectrum of phenamacril, the exact mechanism behind the phenamacril-mediated inhibition remains to be resolved. Here, we describe the characterization of the effect of phenamacril on purified myosin motor constructs from the model plant pathogen and phenamacril-susceptible species Fusarium graminearum, phenamacril-resistant Fusarium species, and the mycetozoan model organism Dictyostelium discoideum Our results show that phenamacril potently (IC50 ∼360 nm), reversibly, and noncompetitively inhibits ATP turnover, actin binding during ATP turnover, and motor activity of F. graminearum myosin-1. Phenamacril also inhibits the ATPase activity of Fusarium avenaceum myosin-1 but has little or no inhibitory effect on the motor activity of Fusarium solani myosin-1, human myosin-1c, and D. discoideum myosin isoforms 1B, 1E, and 2. Our findings indicate that phenamacril is a species-specific, noncompetitive inhibitor of class I myosin in susceptible Fusarium sp.

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.

Various Themes of Myosin Regulation

Members of the myosin superfamily are actin-based molecular motors that are indispensable for cellular homeostasis. The vast functional and structural diversity of myosins accounts for the variety and complexity of the underlying allosteric regulatory mechanisms that determine the activation or inhibition of myosin motor activity and enable precise timing and spatial aspects of myosin function at the cellular level. This review focuses on the molecular basis of posttranslational regulation of eukaryotic myosins from different classes across species by allosteric intrinsic and extrinsic effectors. First, we highlight the impact of heavy and light chain phosphorylation. Second, we outline intramolecular regulatory mechanisms such as autoinhibition and subsequent activation. Third, we discuss diverse extramolecular allosteric mechanisms ranging from actin-linked regulatory mechanisms to myosin:cargo interactions. At last, we briefly outline the allosteric regulation of myosins with synthetic compounds.

MicroRNA-363 targets myosin 1B to reduce cellular migration in head and neck cancer

Background

Squamous cell carcinoma of the head and neck (SCCHN) remains a prevalent and devastating disease. Recently, there has been an increase in SCCHN cases that are associated with high-risk human papillomavirus (HPV) infection. The clinical characteristics of HPV-positive and HPV-negative SCCHN are known to be different but their molecular features are only recently beginning to emerge. MicroRNAs (miRNAs, miRs) are small, non-coding RNAs that are likely to play significant roles in cancer initiation and progression where they may act as oncogenes or tumor suppressors. Previous studies in our laboratory showed that miR-363 is overexpressed in HPV-positive compared to HPV-negative SCCHN cell lines, and the HPV type 16-E6 oncoprotein upregulates miR-363 in SCCHN cell lines. However, the functional role of miR-363 in SCCHN in the context of HPV infection remains to be elucidated.

Methods

We analyzed miR-363 levels in SCCHN tumors with known HPV-status from The Cancer Genome Atlas (TCGA) and an independent cohort from our institution. Cell migration studies were conducted following the overexpression of miR-363 in HPV-negative cell lines. Bioinformatic tools and a luciferase reporter assay were utilized to confirm that miR-363 targets the 3’-UTR of myosin 1B (MYO1B). MYO1B mRNA and protein expression levels were evaluated following miR-363 overexpression in HPV-negative SCCHN cell lines. Small interfering RNA (siRNA) knockdown of MYO1B was performed to assess the phenotypic implication of reduced MYO1B expression in SCCHN cell lines.

Results

MiR-363 was found to be overexpressed in HPV-16-positive compared to the HPV-negative SCCHN tumors. Luciferase reporter assays performed in HPV-negative JHU028 cells confirmed that miR-363 targets one of its two potential binding sites in the 3’UTR of MYO1B. MYO1B mRNA and protein levels were reduced upon miR-363 overexpression in four HPV-negative SCCHN cell lines. Increased miR-363 expression or siRNA knockdown of MYO1B expression reduced Transwell migration of SCCHN cell lines, indicating that the miR-363-induced migration attenuation of SCCHN cells may act through MYO1B downregulation.

Conclusions

These findings demonstrate that the overexpression of miR-363 reduces cellular migration in head and neck cancer and reveal the biological relationship between miR-363, myosin 1b, and HPV-positive SCCHN.

Electronic supplementary material

The online version of this article (doi:10.1186/s12885-015-1888-3) contains supplementary material, which is available to authorized users.

The Dictyostelium prestalk inducer differentiation-inducing factor-1 (DIF-1) triggers unexpectedly complex global phosphorylation changes

Differentiation-inducing factor-1 (DIF-1) is a polyketide that induces Dictyostelium amoebae to differentiate as prestalk cells. We performed a global quantitative screen for phosphorylation changes that occur within the first minutes after addition of DIF-1, using a triple-label SILAC approach. This revealed a new world of DIF-1-controlled signaling, with changes in components of the MAPK and protein kinase B signaling pathways, components of the actinomyosin cytoskeletal signaling networks, and a broad range of small GTPases and their regulators. The results also provide evidence that the Ca(2+)/calmodulin-dependent phosphatase calcineurin plays a role in DIF-1 signaling to the DimB prestalk transcription factor. At the global level, DIF-1 causes a major shift in the phosphorylation/dephosphorylation equilibrium toward net dephosphorylation. Of interest, many of the sites that are dephosphorylated in response to DIF-1 are phosphorylated in response to extracellular cAMP signaling. This accords with studies that suggest an antagonism between the two inducers and also with the rapid dephosphorylation of the cAMP receptor that we observe in response to DIF-1 and with the known inhibitory effect of DIF-1 on chemotaxis to cAMP. All MS data are available via ProteomeXchange with identifier PXD001555.

Aberrant Myosin 1b Expression Promotes Cell Migration and Lymph Node Metastasis of HNSCC

Lymph node metastasis is the major clinicopathologic feature associated with poor prognosis in patients with head and neck squamous cell carcinoma (HNSCC). Here, web-based bioinformatics meta-analysis was performed to elucidate the molecular mechanism of lymph node metastasis of human HNSCC. Preferential upregulation of Myosin 1b (MYO1B) transcript in HNSCC datasets was identified. Myo1b mRNA was highly expressed in human HNSCC cells and patient tissue specimens compared with their normal counterparts as shown by quantitative PCR (qPCR) analyses. Immunohistochemistry (IHC)-detected Myo1b expression was significantly correlated with lymph node metastases in patients with oral cancer of the tongue. HNSCC with high expression of Myo1b and chemokine receptor 4 (CCR4), another metastasis-associated molecule, was strongly associated with lymph node metastasis. RNA interference (RNAi) of Myo1b in HNSCC cells, SAS and HSC4, significantly inhibited migratory and invasive abilities through decreased large protrusion formation of cell membranes. Finally, Myo1b knockdown in SAS cells significantly inhibited in vivo cervical lymph node metastases in a cervical lymph node metastatic mouse model system.

IMPLICATIONS

Myo1b is functionally involved in lymph node metastasis of human HNSCC through enhanced cancer cell motility and is an attractive target for new diagnostic and therapeutic strategies for patients with HNSCC.

Use of pyrene-labelled actin to probe actin-myosin interactions: kinetic and equilibrium studies

Studying the dynamics of the interaction between actin and myosin and how this is modulated by ATP and other nucleotides is fundamental to any understanding of myosin motor protein activity. The fluorescent label pyrene, covalently attached to actin (at Cys 374), has been one of the most useful optical probes to report myosin binding to actin. The unique spectral features of pyrene make it sensitive to changes in the microenvironment of the probe and allow to monitor processes such as conformational changes and protein-protein interactions. Here we describe how to make and use pyrene-labelled actin and describe a set of fluorescence stopped-flow measurements that allow the actin-myosin interaction to be explored at protein concentrations from μM to nM for many of the known myosin motors.

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.

The association of myosin IB with actin waves in dictyostelium requires both the plasma membrane-binding site and actin-binding region in the myosin tail

F-actin structures and their distribution are important determinants of the dynamic shapes and functions of eukaryotic cells. Actin waves are F-actin formations that move along the ventral cell membrane driven by actin polymerization. Dictyostelium myosin IB is associated with actin waves but its role in the wave is unknown. Myosin IB is a monomeric, non-filamentous myosin with a globular head that binds to F-actin and has motor activity, and a non-helical tail comprising a basic region, a glycine-proline-glutamine-rich region and an SH3-domain. The basic region binds to acidic phospholipids in the plasma membrane through a short basic-hydrophobic site and the Gly-Pro-Gln region binds F-actin. In the current work we found that both the basic-hydrophobic site in the basic region and the Gly-Pro-Gln region of the tail are required for the association of myosin IB with actin waves. This is the first evidence that the Gly-Pro-Gln region is required for localization of myosin IB to a specific actin structure in situ. The head is not required for myosin IB association with actin waves but binding of the head to F-actin strengthens the association of myosin IB with waves and stabilizes waves. Neither the SH3-domain nor motor activity is required for association of myosin IB with actin waves. We conclude that myosin IB contributes to anchoring actin waves to the plasma membranes by binding of the basic-hydrophobic site to acidic phospholipids in the plasma membrane and binding of the Gly-Pro-Gln region to F-actin in the wave.

ATP turnover by individual myosin molecules hints at two conformers of the myosin active site

Coupling of ATP hydrolysis to structural changes in the motor domain is fundamental to the driving of motile functions by myosins. Current understanding of this chemomechanical coupling is primarily based on ensemble average measurements in solution and muscle fibers. Although important, the averaging could potentially mask essential details of the chemomechanical coupling, particularly for mixed populations of molecules. Here, we demonstrate the potential of studying individual myosin molecules, one by one, for unique insights into established systems and to dissect mixed populations of molecules where separation can be particularly challenging. We measured ATP turnover by individual myosin molecules, monitoring appearance and disappearance of fluorescent spots upon binding/dissociation of a fluorescent nucleotide to/from the active site of myosin. Surprisingly, for all myosins tested, we found two populations of fluorescence lifetimes for individual myosin molecules, suggesting that termination of fluorescence occurred by two different paths, unexpected from standard kinetic schemes of myosin ATPase. In addition, molecules of the same myosin isoform showed substantial intermolecular variability in fluorescence lifetimes. From kinetic modeling of our two fluorescence lifetime populations and earlier solution data, we propose two conformers of the active site of myosin, one that allows the complete ATPase cycle and one that dissociates ATP uncleaved. Statistical analysis and Monte Carlo simulations showed that the intermolecular variability in our studies is essentially due to the stochastic behavior of enzyme kinetics and the limited number of ATP binding events detectable from an individual myosin molecule with little room for static variation among individual molecules, previously described for other enzymes.

Regulation and control of myosin-I by the motor and light chain-binding domains

Members of the myosin-I family of molecular motors are expressed in many eukaryotes, where they are involved in a multitude of critical processes. Humans express eight distinct members of the myosin-I family, making it the second largest family of myosins expressed in humans. Despite the high degree of sequence conservation in the motor and light chain-binding domains (LCBDs) of these myosins, recent studies have revealed surprising diversity of function and regulation arising from isoform-specific differences in these domains. Here we review the regulation of myosin-I function and localization by the motor and LCBDs.

Kinetic properties and small-molecule inhibition of human myosin-6

Myosin-6 is an actin-based motor protein that moves its cargo towards the minus-end of actin filaments. Mutations in the gene encoding the myosin-6 heavy chain and changes in the cellular abundance of the protein have been linked to hypertrophic cardiomyopathy, neurodegenerative diseases, and cancer. Here, we present a detailed kinetic characterization of the human myosin-6 motor domain, describe the effect of 2,4,6-triiodophenol on the interaction of myosin-6 with F-actin and nucleotides, and show how addition of the drug reduces the number of myosin-6-dependent vesicle fusion events at the plasma membrane during constitutive secretion.

Molecular basis of dynamic relocalization of Dictyostelium myosin IB

Class I myosins have a single heavy chain comprising an N-terminal motor domain with actin-activated ATPase activity and a C-terminal globular tail with a basic region that binds to acidic phospholipids. These myosins contribute to the formation of actin-rich protrusions such as pseudopodia, but regulation of the dynamic localization to these structures is not understood. Previously, we found that Acanthamoeba myosin IC binds to acidic phospholipids in vitro through a short sequence of basic and hydrophobic amino acids, BH site, based on the charge density of the phospholipids. The tail of Dictyostelium myosin IB (DMIB) also contains a BH site. We now report that the BH site is essential for DMIB binding to the plasma membrane and describe the molecular basis of the dynamic relocalization of DMIB in live cells. Endogenous DMIB is localized uniformly on the plasma membrane of resting cells, at active protrusions and cell-cell contacts of randomly moving cells, and at the front of motile polarized cells. The BH site is required for association of DMIB with the plasma membrane at all stages where it colocalizes with phosphoinositide bisphosphate/phosphoinositide trisphosphate (PIP(2)/PIP(3)). The charge-based specificity of the BH site allows for in vivo specificity of DMIB for PIP(2)/PIP(3) similar to the PH domain-based specificity of other class I myosins. However, DMIB-head is required for relocalization of DMIB to the front of migrating cells. Motor activity is not essential, but the actin binding site in the head is important. Thus, dynamic relocalization of DMIB is determined principally by the local PIP(2)/PIP(3) concentration in the plasma membrane and cytoplasmic F-actin.

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.

Identification and characterization of an unusual class I myosin involved in vesicle traffic in Trypanosoma brucei

Myosins are a multimember family of motor proteins with diverse functions in eukaryotic cells. African trypanosomes possess only two candidate myosins and thus represent a useful system for functional analysis of these motors. One of these candidates is an unusual class I myosin (TbMyo1) that is expressed at similar levels but organized differently during the life cycle of Trypanosoma brucei. This myosin localizes to the polarized endocytic pathway in bloodstream forms of the parasite. This organization is actin dependent. Knock down of TbMyo1 results in a significant reduction in endocytic activity, a cessation in cell division and eventually cell death. A striking morphological feature in these cells is an enlargement of the flagellar pocket, which is consistent with an imbalance in traffic to and from the surface. In contrast TbMyo1 is distributed throughout procyclic forms of the tsetse vector and a loss of ∼90% of the protein has no obvious effects on growth or morphology. These results reveal a life cycle stage specific requirement for this myosin in essential endocytic traffic and represent the first description of the involvement of a motor protein in vesicle traffic in these parasites.

Functional adaptation of the switch-2 nucleotide sensor enables rapid processive translocation by myosin-5

Active site loops that are conserved across superfamilies of myosins, kinesins, and G proteins play key roles in allosteric coupling of NTP hydrolysis to interaction with track filaments or effector proteins. In this study, we investigated how the class-specific natural variation in the switch-2 active site loop contributes to the motor function of the intracellular transporter myosin-5. We used single-molecule, rapid kinetic and spectroscopic experiments and semiempirical quantum chemical simulations to show that the class-specific switch-2 structure including a tyrosine (Y439) in myosin-5 enables rapid processive translocation along actin filaments by facilitating Mg(2+)-dependent ADP release. Using wild-type control and Y439 point mutant myosin-5 proteins, we demonstrate that the translocation speed precisely correlates with the kinetics of nucleotide exchange. Switch-2 variants can thus be used to fine-tune translocation speed while maintaining high processivity. The class-specific variation of switch-2 in various NTPase superfamilies indicates its general role in the kinetic tuning of Mg(2+)-dependent nucleotide exchange.