The Conserved Lysine-265 Allosterically Modulates Nucleotide- and Actin-binding Site Coupling in Myosin-2

Motor properties of Myosin 5c are modulated by tropomyosin isoforms and inhibited by pentabromopseudilin

Myosin 5c (Myo5c) is a motor protein that is produced in epithelial and glandular tissues, where it plays an important role in secretory processes. Myo5c is composed of two heavy chains, each containing a generic motor domain, an elongated neck domain consisting of a single α-helix with six IQ motifs, each of which binds to a calmodulin (CaM) or a myosin light chain from the EF-hand protein family, a coiled-coil dimer-forming region and a carboxyl-terminal globular tail domain. Although Myo5c is a low duty cycle motor, when two or more Myo5c-heavy meromyosin (HMM) molecules are linked together, they move processively along actin filaments. We describe the purification and functional characterization of human Myo5c-HMM co-produced either with CaM alone or with CaM and the essential and regulatory light chains Myl6 and Myl12b. We describe the extent to which cofilaments of actin and Tpm1.6, Tpm1.8 or Tpm3.1 alter the maximum actin-activated ATPase and motile activity of the recombinant Myo5c constructs. The small allosteric effector pentabromopseudilin (PBP), which is predicted to bind in a groove close to the actin and nucleotide binding site with a calculated ΔG of -18.44 kcal/mol, inhibits the motor function of Myo5c with a half-maximal concentration of 280 nM. Using immunohistochemical staining, we determined the distribution and exact localization of Myo5c in endothelial and endocrine cells from rat and human tissue. Particular high levels of Myo5c were observed in insulin-producing β-cells located within the pancreatic islets of Langerhans.

Allosteric modulation of cardiac myosin mechanics and kinetics by the conjugated omega-7,9 trans-fat rumenic acid

KEY POINTS

Direct binding of rumenic acid to the cardiac myosin-2 motor domain increases the release rate for orthophosphate and increases the Ca²⁺ responsiveness of cardiac muscle at low load. Physiological cellular concentrations of rumenic acid affect the ATP turnover rates of the super-relaxed and disordered relaxed states of β-cardiac myosin, leading to a net increase in myocardial metabolic load. In Ca²⁺ -activated trabeculae, rumenic acid exerts a direct inhibitory effect on the force-generating mechanism without affecting the number of force-generating motors. In the presence of saturating actin concentrations rumenic acid binds to the β-cardiac myosin-2 motor domain with an EC₅₀ of 200 nM. Molecular docking studies provide information about the binding site, the mode of binding, and associated allosteric communication pathways. Free rumenic acid may exceed thresholds in cardiomyocytes above which contractile efficiency is reduced and interference with small molecule therapeutics, targeting cardiac myosin, occurs.

ABSTRACT

Based on experiments using purified myosin motor domains, reconstituted actomyosin complexes and rat heart ventricular trabeculae, we demonstrate direct binding of rumenic acid, the cis-delta-9-trans-delta-11 isomer of conjugated linoleic acid, to an allosteric site located in motor domain of mammalian cardiac myosin-2 isoforms. In the case of porcine β-cardiac myosin, the EC₅₀ for rumenic acid varies from 10.5 μM in the absence of actin to 200 nM in the presence of saturating concentrations of actin. Saturating concentrations of rumenic acid increase the maximum turnover of basal and actin-activated ATPase activity of β-cardiac myosin approximately 2-fold but decrease the force output per motor by 23% during isometric contraction. The increase in ATP turnover is linked to an acceleration of the release of the hydrolysis product orthophosphate. In the presence of 5 μM rumenic acid, the difference in the rate of ATP turnover by the super-relaxed and disordered relaxed states of cardiac myosin increases from 4-fold to 20-fold. The equilibrium between the two functional myosin states is not affected by rumenic acid. Calcium responsiveness is increased under zero-load conditions but unchanged under load. Molecular docking studies provide information about the rumenic acid binding site, the mode of binding, and associated allosteric communication pathways. They show how the isoform-specific replacement of residues in the binding cleft induces a different mode of rumenic acid binding in the case of non-muscle myosin-2C and blocks binding to skeletal muscle and smooth muscle myosin-2 isoforms.

Myosin-18B Regulates Higher-Order Organization of the Cardiac Sarcomere through Thin Filament Cross-Linking and Thick Filament Dynamics

MYO18B loss-of-function mutations and depletion significantly compromise the structural integrity of striated muscle sarcomeres. The molecular function of the encoded protein, myosin-18B (M18B), within the developing muscle is unknown. Here, we demonstrate that recombinant M18B lacks motor ATPase activity and harbors previously uncharacterized N-terminal actin-binding domains, properties that make M18B an efficient actin cross-linker and molecular brake capable of regulating muscle myosin-2 contractile forces. Spatiotemporal analysis of M18B throughout cardiomyogenesis and myofibrillogenesis reveals that this structural myosin undergoes nuclear-cytoplasmic redistribution during myogenic differentiation, where its incorporation within muscle stress fibers coincides with actin striation onset. Furthermore, this analysis shows that M18B is directly integrated within the muscle myosin thick filament during myofibril maturation. Altogether, our data suggest that M18B has evolved specific biochemical properties that allow it to define and maintain sarcomeric organization from within the thick filament via its dual actin cross-linking and motor modulating capabilities.

Mechanochemical properties of human myosin-1C are modulated by isoform-specific differences in the N-terminal extension

Myosin-1C is a single-headed, short-tailed member of the myosin class I subfamily that supports a variety of actin-based functions in the cytosol and nucleus. In vertebrates, alternative splicing of the MYO1C gene leads to the production of three isoforms, myosin-1C⁰, myosin-1C¹⁶, and myosin-1C³⁵, that carry N-terminal extensions of different lengths. However, it is not clear how these extensions affect the chemomechanical coupling of human myosin-1C isoforms. Here, we report on the motor activity of the different myosin-1C isoforms measuring the unloaded velocities of constructs lacking the C-terminal lipid-binding domain on nitrocellulose-coated glass surfaces and full-length constructs on reconstituted, supported lipid bilayers. The higher yields of purified proteins obtained with constructs lacking the lipid-binding domain allowed a detailed characterization of the individual kinetic steps of human myosin-1C isoforms in their productive interaction with nucleotides and filamentous actin. Isoform-specific differences include 18-fold changes in the maximum power output per myosin-1C motor and 4-fold changes in the velocity and the resistive force at which maximum power output occurs. Our results support a model in which the isoform-specific N-terminal extensions affect chemomechanical coupling by combined steric and allosteric effects, thereby reducing both the length of the working stroke and the rate of ADP release in the absence of external loads by a factor of 2 for myosin-1C³⁵. As the large change in maximum power output shows, the functional differences between the isoforms are further amplified by the presence of external loads.

Structural and Computational Insights into a Blebbistatin-Bound Myosin•ADP Complex with Characteristics of an ADP-Release Conformation along the Two-Step Myosin Power Stoke

The motor protein myosin drives a wide range of cellular and muscular functions by generating directed movement and force, fueled through adenosine triphosphate (ATP) hydrolysis. Release of the hydrolysis product adenosine diphosphate (ADP) is a fundamental and regulatory process during force production. However, details about the molecular mechanism accompanying ADP release are scarce due to the lack of representative structures. Here we solved a novel blebbistatin-bound myosin conformation with critical structural elements in positions between the myosin pre-power stroke and rigor states. ADP in this structure is repositioned towards the surface by the phosphate-sensing P-loop, and stabilized in a partially unbound conformation via a salt-bridge between Arg131 and Glu187. A 5 Å rotation separates the mechanical converter in this conformation from the rigor position. The crystallized myosin structure thus resembles a conformation towards the end of the two-step power stroke, associated with ADP release. Computationally reconstructing ADP release from myosin by means of molecular dynamics simulations further supported the existence of an equivalent conformation along the power stroke that shows the same major characteristics in the myosin motor domain as the resolved blebbistatin-bound myosin-II·ADP crystal structure, and identified a communication hub centered on Arg232 that mediates chemomechanical energy transduction.

Undefeated-Changing the phenamacril scaffold is not enough to beat resistant Fusarium

Filamentous fungi belonging to the genus Fusarium are notorious plant-pathogens that infect, damage and contaminate a wide variety of important crops. Phenamacril is the first member of a novel class of single-site acting cyanoacrylate fungicides which has proven highly effective against important members of the genus Fusarium. However, the recent emergence of field-resistant strains exhibiting qualitative resistance poses a major obstacle for the continued use of phenamacril. In this study, we synthesized novel cyanoacrylate compounds based on the phenamacril-scaffold to test their growth-inhibitory potential against wild-type Fusarium and phenamacril-resistant strains. Our findings show that most chemical modifications to the phenamacril-scaffold are associated with almost complete loss of fungicidal activity and in vitro inhibition of myosin motor domain ATPase activity.

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.