Functional characterization of the secondary actin binding site of myosin II

Actomyosin Complex

Formation of cross-bridges between actin and myosin occurs ubiquitously in eukaryotic cells and mediates muscle contraction, intracellular cargo transport, and cytoskeletal remodeling. Myosin motors repeatedly bind to and dissociate from actin filaments in a cycle that transduces the chemical energy from ATP hydrolysis into mechanical force generation. While the general layout of surface elements within the actin-binding interface is conserved among myosin classes, sequence divergence within these motifs alters the specific contacts involved in the actomyosin interaction as well as the kinetics of mechanochemical cycle phases. Additionally, diverse lever arm structures influence the motility and force production of myosin molecules during their actin interactions. The structural differences generated by myosin's molecular evolution have fine-tuned the kinetics of its isoforms and adapted them for their individual cellular roles. In this chapter, we will characterize the structural and biochemical basis of the actin-myosin interaction and explain its relationship with myosin's cellular roles, with emphasis on the structural variation among myosin isoforms that enables their functional specialization. We will also discuss the impact of accessory proteins, such as the troponin-tropomyosin complex and myosin-binding protein C, on the formation and regulation of actomyosin cross-bridges.

The Central Role of the F-Actin Surface in Myosin Force Generation

Actin is one of the most abundant and versatile proteins in eukaryotic cells. As discussed in many contributions to this Special Issue, its transition from a monomeric G-actin to a filamentous F-actin form plays a critical role in a variety of cellular processes, including control of cell shape and cell motility. Once polymerized from G-actin, F-actin forms the central core of muscle-thin filaments and acts as molecular tracks for myosin-based motor activity. The ATP-dependent cross-bridge cycle of myosin attachment and detachment drives the sliding of myosin thick filaments past thin filaments in muscle and the translocation of cargo in somatic cells. The variation in actin function is dependent on the variation in muscle and non-muscle myosin isoform behavior as well as interactions with a plethora of additional actin-binding proteins. Extensive work has been devoted to defining the kinetics of actin-based force generation powered by the ATPase activity of myosin. In addition, over the past decade, cryo-electron microscopy has revealed the atomic-evel details of the binding of myosin isoforms on the F-actin surface. Most accounts of the structural interactions between myosin and actin are described from the perspective of the myosin molecule. Here, we discuss myosin-binding to actin as viewed from the actin surface. We then describe conserved structural features of actin required for the binding of all or most myosin isoforms while also noting specific interactions unique to myosin isoforms.

Cryo-EM structures reveal specialization at the myosin VI-actin interface and a mechanism of force sensitivity

Despite extensive scrutiny of the myosin superfamily, the lack of high-resolution structures of actin-bound states has prevented a complete description of its mechanochemical cycle and limited insight into how sequence and structural diversification of the motor domain gives rise to specialized functional properties. Here we present cryo-EM structures of the unique minus-end directed myosin VI motor domain in rigor (4.6 Å) and Mg-ADP (5.5 Å) states bound to F-actin. Comparison to the myosin IIC-F-actin rigor complex reveals an almost complete lack of conservation of residues at the actin-myosin interface despite preservation of the primary sequence regions composing it, suggesting an evolutionary path for motor specialization. Additionally, analysis of the transition from ADP to rigor provides a structural rationale for force sensitivity in this step of the mechanochemical cycle. Finally, we observe reciprocal rearrangements in actin and myosin accompanying the transition between these states, supporting a role for actin structural plasticity during force generation by myosin VI.

Structure of actomyosin rigour complex at 5.2 Å resolution and insights into the ATPase cycle mechanism

Muscle contraction is driven by cyclic association and dissociation of myosin head of the thick filament with thin actin filament coupled with ATP binding and hydrolysis by myosin. However, because of the absence of actomyosin rigour structure at high resolution, it still remains unclear how the strong binding of myosin to actin filament triggers the release of hydrolysis products and how ATP binding causes their dissociation. Here we report the structure of mammalian skeletal muscle actomyosin rigour complex at 5.2 Å resolution by electron cryomicroscopy. Comparison with the structures of myosin in various states shows a distinctly large conformational change, providing insights into the ATPase-coupled reaction cycle of actomyosin. Based on our observations, we hypothesize that asymmetric binding along the actin filament could function as a Brownian ratchet by favouring directionally biased thermal motions of myosin and actin.

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.

Cryo-EM structure of a human cytoplasmic actomyosin complex at near-atomic resolution

The interaction of myosin with actin filaments is the central feature of muscle contraction and cargo movement along actin filaments of the cytoskeleton. The energy for these movements is generated during a complex mechanochemical reaction cycle. Crystal structures of myosin in different states have provided important structural insights into the myosin motor cycle when myosin is detached from F-actin. The difficulty of obtaining diffracting crystals, however, has prevented structure determination by crystallography of actomyosin complexes. Thus, although structural models exist of F-actin in complex with various myosins, a high-resolution structure of the F-actin–myosin complex is missing. Here, using electron cryomicroscopy, we present the structure of a human rigor actomyosin complex at an average resolution of 3.9 Å. The structure reveals details of the actomyosin interface, which is mainly stabilized by hydrophobic interactions. The negatively charged amino (N) terminus of actin interacts with a conserved basic motif in loop 2 of myosin, promoting cleft closure in myosin. Surprisingly, the overall structure of myosin is similar to rigor-like myosin structures in the absence of F-actin, indicating that F-actin binding induces only minimal conformational changes in myosin. A comparison with pre-powerstroke and intermediate (Pi-release) states of myosin allows us to discuss the general mechanism of myosin binding to F-actin. Our results serve as a strong foundation for the molecular understanding of cytoskeletal diseases, such as autosomal dominant hearing loss and diseases affecting skeletal and cardiac muscles, in particular nemaline myopathy and hypertrophic cardiomyopathy.

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.

Crystal structure of the rigor-like human non-muscle myosin-2 motor domain

We determined the crystal structure of the motor domain of human non-muscle myosin 2B (NM-2B) in a nucleotide-free state and at a resolution of 2.8 Å. The structure shows the motor domain with an open active site and the large cleft that divides the 50 kDa domain in a closed state. Compared to other rigor-like myosin motor domain structures, our structure shows subtle but significant conformational changes in regions important for actin binding and mechanochemical coupling. Moreover, our crystal structure helps to rationalize the impact of myosin, heavy chain 9 (MYH9)-related disease mutations Arg709Cys and Arg709His on the kinetic and functional properties of NM-2B and of the closely related non-muscle myosin 2A (NM-2A).

Structure of the rigor actin-tropomyosin-myosin complex

Regulation of myosin and filamentous actin interaction by tropomyosin is a central feature of contractile events in muscle and nonmuscle cells. However, little is known about molecular interactions within the complex and the trajectory of tropomyosin movement between its "open" and "closed" positions on the actin filament. Here, we report the 8 Å resolution structure of the rigor (nucleotide-free) actin-tropomyosin-myosin complex determined by cryo-electron microscopy. The pseudoatomic model of the complex, obtained from fitting crystal structures into the map, defines the large interface involving two adjacent actin monomers and one tropomyosin pseudorepeat per myosin contact. Severe forms of hereditary myopathies are linked to mutations that critically perturb this interface. Myosin binding results in a 23 Å shift of tropomyosin along actin. Complex domain motions occur in myosin, but not in actin. Based on our results, we propose a structural model for the tropomyosin-dependent modulation of myosin binding to actin.

A closer look at energy transduction in muscle

Muscular force is the sum of unitary force interactions generated as filaments of myosins move forcibly along parallel filaments of actins, understanding that the free energy required comes from myosin-catalyzed ATP hydrolysis. Using results from conventional biochemistry, our own mutational studies, and diffraction images from others, we attempt, in molecular detail, an account of a unitary interaction, i.e., what happens after a traveling myosin head, bearing an ADP-P(i), reaches the next station of an actin filament in its path. We first construct a reasonable model of the myosin head and actin regions that meet to form the "weakly bound state". Separately, we consider Holmes' model of the rigor state [Holmes, K. C., Angert, I., Kull, F. J., Jahn, W. & Schröder, R. R. (2003) Nature 425, 423-427], supplemented with several heretofore missing residues, thus realizing the "strongly bound state." Comparing states suggests how influences initiated at the interface travel elsewhere in myosin to discharge various functions, including striking the actins. Overall, state change seems to occur by attachment of a hydrophobic triplet (Trp-546, Phe-547, and Pro-548) of myosin to an actin conduit with a hydrophobic guiding rail (Ile-341, Ile-345, Leu-349, and Phe-352) and the subsequent linear movement of the triplet along the rail.

The sarcomeric myosin heavy chain gene family in the dog: analysis of isoform diversity and comparison with other mammalian species

Sarcomeric myosin heavy chains (MyHC) are the major contractile proteins of cardiac and skeletal muscles and belong to class II MyHC. In this study the sequences of nine sarcomeric MyHC isoforms were obtained by combining assembled contigs of the dog genome draft available in the NCBI database. With this information available the dog becomes the second species, after human, for which the sequences of all members of the sarcomeric MyHC gene family are identified. The newly determined sequences of canine MyHC isoforms were aligned with their orthologs in mammals, forming a set of 38 isoforms, to search for the molecular features that determine the structural and functional specificity of each type of isoform. In this way the structural motifs that allow identification of each isoform and are likely determinants of functional properties were identified in six specific regions (surface loop 1, loop 2, loop 3, converter, MLC binding region, and S2 proximal segment).

Effect of nucleotide on interaction of the 567-578 segment of myosin heavy chain with actin

To probe the effect of nucleotide on the formation of ionic contacts between actin and the 567-578 residue loop of the heavy chain of rabbit skeletal muscle myosin subfragment 1 (S1), the complexes between F-actin and proteolytic derivatives of S1 were submitted to chemical cross-linking with 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide. We have shown that in the absence of nucleotide both 45 kDa and 5 kDa tryptic derivatives of the central 50 kDa heavy chain fragment of S1 can be cross-linked to actin, whereas in the presence of MgADP.AlF4, only the 5 kDa fragment is involved in cross-linking reaction. By the identification of the N-terminal sequence of the 5-kDa fragment, we have found that trypsin splits the 50 kDa heavy chain fragment between Lys-572 and Gly-573, the residues located within the 567-578 loop. Using S1 preparations cleaved with elastase, we could show that the residue of 567-578 loop that can be cross-linked to actin in the presence of MgADP.AlF4 is Lys-574. The observed nucleotide-dependent changes of the actin-subfragment 1 interface indicate that the 567-578 residue loop of skeletal muscle myosin participates in the communication between the nucleotide and actin binding sites.

The regulatory effects of tropomyosin and troponin-I on the interaction of myosin loop regions with F-actin

The N terminus of skeletal myosin light chain 1 and the cardiomyopathy loop of human cardiac myosin have been shown previously to bind to actin in the presence and absence of tropomyosin (Patchell, V. B., Gallon, C. E., Hodgkin, M. A., Fattoum, A., Perry, S. V., and Levine, B. A. (2002) Eur. J. Biochem. 269, 5088-5100). We have extended this work and have shown that segments corresponding to other regions of human cardiac beta-myosin, presumed to be sites of interaction with F-actin (residues 554-584, 622-646, and 633-660), likewise bind independently to actin under similar conditions. The binding to F-actin of a peptide spanning the minimal inhibitory segment of human cardiac troponin I (residues 134-147) resulted in the dissociation from F-actin of all the myosin peptides bound to it either individually or in combination. Troponin C neutralized the effect of the inhibitory peptide on the binding of the myosin peptides to F-actin. We conclude that the binding of the inhibitory region of troponin I to actin, which occurs during relaxation in muscle when the calcium concentration is low, imposes conformational changes that are propagated to different locations on the surface of actin. We suggest that the role of tropomyosin is to facilitate the transmission of structural changes along the F-actin filament so that the monomers within a structural unit are able to interact with myosin.

Molecular engineering of myosin

Protein engineering and design provide excellent tools to investigate the principles by which particular structural features relate to the mechanisms that underlie the biological function of a protein. In addition to studies aimed at dissecting the communication pathways within enzymes, recent advances in protein engineering approaches make it possible to generate enzymes with increased catalytic efficiency and specifically altered or newly introduced functions. Here, two approaches using state-of-the-art protein design and engineering are described in detail to demonstrate how key features of the myosin motor can be changed in a specific and predictable manner. First, it is shown how replacement of an actin-binding surface loop with synthetic sequences, whose flexibility and charge density is varied, can be employed to manipulate the actin affinity, the catalytic activity and the efficiency of coupling between actin- and nucleotide-binding sites of myosin motor constructs. Then the use of pre-existing molecular building blocks, which are derived from unrelated proteins, is described for manipulating the velocity and even the direction of movement of recombinant myosins.

Regulation of the actin-myosin interaction by titin

Titin is known to interact with actin thin filaments within the I-band region of striated muscle sarcomeres. In this study, we have used a titin fragment of 800 kDa (T800) purified from striated skeletal muscle to measure the effect of this interaction on the functional properties of the actin-myosin complex. MALDI-TOF MS revealed that T800 contains the entire titin PEVK (Pro, Glu, Val, Lys-rich) domain. In the presence of tropomyosin-troponin, T800 increased the sliding velocity (both average and maximum values) of actin filaments on heavy-meromyosin (HMM)-coated surfaces and dramatically decreased the number of stationary filaments. These results were correlated with a 30% reduction in actin-activated HMM ATPase activity and with an inhibition of HMM binding to actin N-terminal residues as shown by chemical cross-linking. At the same time, T800 did not affect the efficiency of the Ca(2+)-controlled on/off switch, nor did it alter the overall binding energetics of HMM to actin, as revealed by cosedimentation experiments. These data are consistent with a competitive effect of PEVK domain-containing T800 on the electrostatic contacts at the actin-HMM interface. They also suggest that titin may participate in the regulation of the active tension generated by the actin-myosin complex.

Dictyostelium myosin II as a model to study the actin-myosin interactions during force generation

During steady-state ATP hydrolysis by actomyosin, myosin cyclically passes through strong actin-binding states and weak actin-binding states, depending on the nature of a nucleotide in the ATPase site. This cyclic change of actin-myosin affinity is coupled with the lever-arm swing and is critical for the sliding motion and force generation of actomyosin. To understand the structure-function relationship of this ATPase-dependent actin-myosin interaction, Dictyostelium myosin II has been extensively used for site-directed mutagenesis. By generating a large number of mutant myosins, two hydrophobic actin-binding sites have been revealed, located at the tip of the upper and lower 50 K subdomains of Dictyostelium myosin, one of which is the 'cardiomyopathy loop'. Furthermore, the slight change in relative orientation of these two hydrophobic sites around the 'strut loop' has been shown to work as a switch to turn on and off the strong binding to actin. Once the switch is turned off, myosin enters in the weak-binding state, where ionic interactions between actin and the 'loop 2' of myosin become the dominant force to maintain the actin-myosin association. The details of actin-myosin interactions revealed by the Dictyostelium system can serve as a framework for further examinations of the myosin superfamily proteins.

Characterization of three regulatory states of the striated muscle thin filament

The troponin-tropomyosin-linked regulation of striated muscle contraction occurs through allosteric control by both Ca(2+) and myosin. The thin filament fluctuates between two extreme states: the inactive "off" state and the active "on" state. Intermediate states have been proposed from structural studies and transient kinetic measurements. However, in contrast to the well-characterised, on and off states, the mechanochemical properties of the intermediate states are much less well understood because of the instability of those states. In the present study, we have characterized a myosin-induced intermediate that is stabilized by cross-linking myosin motor domains (S1) to actin filaments (with a maximum of one S1 molecule for 50 actin monomers). A single S1 molecule is known to interact with two adjacent actin monomers. A detailed analysis revealed that thin filaments containing S1 molecules cross-linked to just one actin monomer (actin(1)-S1 complexes) are regulated with a 79% inhibition of the ATPase in the absence of Ca(2+). In contrast, filaments containing S1 molecules cross-linked at two positions, to two adjacent actin monomers (actin(2)-S1 complexes) totally lose their regulation in a highly cooperative manner. This loss of regulation was due both to an enhancement of the ATPase activity without calcium and an inhibition of the ATPase with calcium. Filaments containing actin(2)-S1 complexes, with significant ATPase activity in the absence of calcium (about 50%), did not move on a myosin-coated surface unless calcium was present. This partial uncoupling between the ATPase activity and in vitro motility in the absence of calcium demonstrates that the mechanical steps require actin-myosin contacts, which take place only in the on state and not in the off or intermediate states. These data provide new insights concerning the difference in cooperativity of Ca(2+) regulation that exists between the biochemical and mechanical cycles of the actin-myosin motor.

Crystal structure of the motor domain of a class-I myosin

The crystal structure of the motor domain of Dictyostelium discoideum myosin-IE, a monomeric unconventional myosin, was determined. The crystallographic asymmetric unit contains four independently resolved molecules, highlighting regions that undergo large conformational changes. Differences are particularly pronounced in the actin binding region and the converter domain. The changes in position of the converter domain reflect movements both parallel to and perpendicular to the actin axis. The orientation of the converter domain is approximately 30 degrees further up than in other myosin structures, indicating that MyoE can produce a larger power stroke by rotating its lever arm through a larger angle. The role of extended loops near the actin-binding site is discussed in the context of cellular localization. The core regions of the motor domain are similar, and the structure reveals how that core is stabilized in the absence of an N-terminal SH3-like domain.

Myosin motors: missing structures and hidden springs

High-resolution structures of the motor domain of myosin II and lower resolution actin-myosin structures have led to the "swinging lever arm" model for myosin force generation. The available kinetic data are not all easily reconciled with this model and understanding the final details of the myosin motor mechanism must await actin-myosin co-crystals. The observation that myosin can populate multiple states in the absence of actin has nonetheless led to significant insights. The currently known myosin structures correspond to defined kinetic states that bind weakly (K(d)>microM) to actin. It is possible that the myosin lever arm could complete its swing before strong binding to actin and force generation--a process that would correspond, in the absence of load, to a Brownian ratchet. We further suggest that, under load, internal springs within the myosin head could decouple force generation and lever arm movement.

Interaction of myosin with F-actin: time-dependent changes at the interface are not slow

The kinetics of formation of the actin-myosin complex have been reinvestigated on the minute and second time scales in sedimentation and chemical cross-linking experiments. With the sedimentation method, we found that the binding of the skeletal muscle myosin motor domain (S1) to actin filament always saturates at one S1 bound to one actin monomer (or two S1 per actin dimer), whether S1 was added slowly (17 min between additions) or rapidly (10 s between additions) to an excess of F-actin. The carbodiimide (1-ethyl-3-(3-dimethylaminopropyl) carbodiimide, EDC)-induced cross-linking of the actin-S1 complex was performed on the subsecond time scale by a new approach that combines a two-step cross-linking protocol with the rapid flow-quench technique. The results showed that the time courses of S1 cross-linking to either of the two actin monomers are identical: they are not dependent on the actin/S1 ratio in the 0.3-20-s time range. The overall data rule out a mechanism by which myosin rolls from one to the other actin monomer on the second or minute time scales. Rather, they suggest that more subtle changes occur at the actomyosin interface during the ATP cycle.

Functional diversity between orthologous myosins with minimal sequence diversity

To define the structural differences that are responsible for the functional diversity between orthologous sarcomeric myosins, we compared the rat and human beta/slow myosins. Functional comparison showed that rat beta/slow myosin has higher ATPase activity and moves actin filaments at higher speed in in vitro motility assay than human beta/slow myosin. Sequence analysis shows that the loop regions at the junctions of the 25 and 50 kDa domains (loop 1) and the 50 and 20 kDa domains (loop 2), which have been implicated in determining functional diversity of myosin heavy chains, are essentially identical in the two orthologs. There are only 14 non-conservative substitutions in the two myosin heavy chains, three of which are located in the secondary actin-binding loop and flanking regions and others correspond to residues so far not assigned a functional role, including two residues in the proximal S2 domain. Interestingly, in some of these positions the rat beta/slow myosin heavy chain has the same residues found in human cardiac alpha myosin, a fast-type myosin, and fast skeletal myosins. These observations indicate that functional and structural analysis of myosin orthologs with limited sequence diversity can provide useful clues to identify amino acid residues involved in modulating myosin function.