Cryo-electron microscopy of S1-decorated actin filaments

The Structure of Acto-Myosin

After several decades studying different acto-myosin complexes at lower and intermediate resolution - limited by the electron microscope instrumentation available then - recent advances in imaging technology have been crucial for obtaining a number of excellent high-resolution 3D reconstructions from cryo electron microscopy. The resolution level reached now is about 3-4 Å, which allows unambiguous model building of filamentous actin on its own as well as that of actin filaments decorated with strongly bound myosin variants. The interface between actin and the myosin motor domain can now be described in detail, and the function of parts of the interface (such as, e.g., the cardiomyopathy loop) can be understood in a mechanistical way. Most recently, reconstructions of actin filaments decorated with different myosins, which show a strongly bound acto-myosin complex also in the presence of the nucleotide ADP, have become available. The comparison of these structures with the nucleotide-free Rigor state provide the first mechanistic description of force sensing. An open question is still the initial interaction of the motor domain of myosin with the actin filament. Such weakly interacting states have so far not been the subject of microscopical studies, even though high-resolution structures would be needed to shed light on the initial steps of phosphate release and power stroke initiation.

Thin Filament Structure and the Steric Blocking Model

By interacting with the troponin-tropomyosin complex on myofibrillar thin filaments, Ca2+ and myosin govern the regulatory switching processes influencing contractile activity of mammalian cardiac and skeletal muscles. A possible explanation of the roles played by Ca2+ and myosin emerged in the early 1970s when a compelling "steric model" began to gain traction as a likely mechanism accounting for muscle regulation. In its most simple form, the model holds that, under the control of Ca2+ binding to troponin and myosin binding to actin, tropomyosin strands running along thin filaments either block myosin-binding sites on actin when muscles are relaxed or move away from them when muscles are activated. Evidence for the steric model was initially based on interpretation of subtle changes observed in X-ray fiber diffraction patterns of intact skeletal muscle preparations. Over the past 25 years, electron microscopy coupled with three-dimensional reconstruction directly resolved thin filament organization under many experimental conditions and at increasingly higher resolution. At low-Ca2+, tropomyosin was shown to occupy a "blocked-state" position on the filament, and switched-on in a two-step process, involving first a movement of tropomyosin away from the majority of the myosin-binding site as Ca2+ binds to troponin and then a further movement to fully expose the site when small numbers of myosin heads bind to actin. In this contribution, basic information on Ca2+-regulation of muscle contraction is provided. A description is then given relating the voyage of discovery taken to arrive at the present understanding of the steric regulatory model.

Electron Microscopy: From 2D to 3D Images with Special Reference to Muscle

This is a brief and necessarily very sketchy presentation of the evolution in electron microscopy (EM) imaging that was driven by the necessity of extracting 3-D views from the essentially 2-D images produced by the electron beam. The lens design of standard transmission electron microscope has not been greatly altered since its inception. However, technical advances in specimen preparation, image collection and analysis gradually induced an astounding progression over a period of about 50 years. From the early images that redefined tissues, cell and cell organelles at the sub-micron level, to the current nano-resolution reconstructions of organelles and proteins the step is very large. The review is written by an investigator who has followed the field for many years, but often from the sidelines, and with great wonder. Her interest in muscle ultrastructure colors the writing. More specific detailed reviews are presented in this issue.

Structural dynamics of the actomyosin complex probed by a bifunctional spin label that cross-links SH1 and SH2

We have used a bifunctional spin label (BSL) to cross-link Cys(707) (SH1) and Cys(697) (SH2) in the catalytic domain of myosin subfragment 1 (S1). BSL induces the same weakened ATPase activity and actin-binding affinity that is observed when SH1 and SH2 are cross-linked with pPDM, which traps an analog of the post-hydrolysis state A.M.ADP.P. Electron paramagnetic resonance showed that BSL reports the global orientation and dynamics of S1. When bound to actin in oriented muscle fibers in the absence of ATP, BSL-S1 showed almost complete orientational disorder, as reported previously for the weakly bound A.M.ADP. In contrast, helical order is observed for the strongly bound state A.M. Saturation transfer electron paramagnetic resonance showed that the disorder of cross-linked S1 on actin is nearly static on the microsecond timescale, at least 30 times slower than that of A.M.ADP. We conclude that cross-linked S1 exhibits rotational disorder comparable to that of A.M.ADP, slow rotational mobility comparable to that of A.M, and intermediate actin affinity. These results support the hypothesis that the catalytic domain of myosin is orientationally disordered on actin in a post-hydrolysis state in the early stages of force generation.

Nucleotide-induced conformations in the neck region of dimeric kinesin

The neck region of kinesin constitutes a key component in the enzyme's walking mechanism. Here we applied cryoelectron microscopy and image reconstruction to investigate the location of the kinesin neck in dimeric and monomeric constructs complexed to microtubules. To this end we enhanced the visibility of this region by engineering an SH3 domain into the transition between neck linker and neck coiled coil. The resulting chimeric kinesin constructs remained functional as verified by physiology assays. In the presence of AMP-PNP the SH3 domains allowed us to identify the position of the neck in a well defined conformation and revealed its high flexibility in the absence of nucleotide. We show here the double-headed binding of dimeric kinesin along the same protofilament, which is characterized by the opposite directionality of neck linkers. In this configuration the neck coiled coil appears fully zipped. The position of the neck region in dimeric constructs is not affected by the presence of the tubulin C-termini as confirmed by subtilisin treatment of microtubules prior to motor decoration.

Domain motion between the regulatory light chain and the nucleotide site in skeletal myosin

Resonance energy transfer probes were attached to skeletal myosin's nucleotide site and regulatory light chain (RLC) to examine nucleotide analog-induced structural transitions. A novel chemical modification of the RLC was developed for specific labeling of the basic N-terminus without affecting myosin ATPase activity. The modification allows attachment of a terbium chelate to rabbit skeletal RLC and was mapped by tryptic digestion to an amino group on the six N-terminal RLC residues. The use of terbium as a resonance energy transfer donor allowed the determination of the efficiency of energy transfer by sensitized emission lifetime measurements that practically eliminate background from unlabeled donor and acceptor sites as well as potential orientation factor artifacts in the calculation of the critical transfer distance. The nucleotide site was labeled with a functional CY3-labeled nucleotide as an energy transfer acceptor. Of the nucleotide states examined, ADP, ADP. vanadate, ADP. A1F4, and ADP. BeFx, the difference between the ADP and ADP. vanadate states was greatest (0.4-nm change), but was not considered to be statistically significant. The binding of actin to ADP-myosin also failed to produce a statistically significant change (0.3-nm change). These results are not consistent with a number of versions of the swinging lever arm hypothesis.

Calcium modulates conformational changes in F-actin induced by smooth muscle heavy meromyosin

The effect of Ca2+ on conformational changes in rhodamine-phalloidin-labeled F-actin induced by binding of smooth muscle heavy meromyosin (HMM) with either phosphorylated or dephosphorylated regulatory light chains (LC20) was studied by polarized fluorimetry. LC20 phosphorylation caused alterations in the F-actin structure typical of the force-producing (strong-binding) state, while dephosphorylation of the chains led to alterations typical of the formation of non-force-producing (weak-binding) state of the actomyosin complex. The presence of Ca2+ enhanced the effect of LC20 phosphorylation and weakened the effect of LC20 dephosphorylation. These data suggest that Ca2+ modulates actin-myosin interaction in smooth muscle by promoting formation of the strong-binding state.

A new look at thin filament regulation in vertebrate skeletal muscle

It is 30 years since Ebashi and colleagues showed that Ca2+ ions directly affect regulation of the myosin-actin interaction in muscle through the action of tropomyosin and troponin on muscle thin filaments. It is more than 20 years since the idea was put forward that tropomyosin might act, at least in part, by changing its position on actin, thus uncovering or modifying the myosin binding site on actin when troponin molecules take up Ca2+. Since that time, a great deal of evidence for and against this steric blocking mechanism has been published: a structure for actin filaments at close to atomic resolution has been proposed, and the whole regulation story has become both more complicated and more subtle. Here we review structural and biochemical aspects of regulation in vertebrate skeletal muscle. We show that some basic ideas of the steric blocking mechanism remain valid. We also show that additional factors, such as troponin movements and structural changes within the actin monomers themselves, may be crucial. A number of the resulting regulation scenarios need to be distinguished.

Structural changes in muscle crossbridges accompanying force generation

We have investigated the structure of the crossbridges in muscles rapidly frozen while relaxed, in rigor, and at various times after activation from rigor by flash photolysis of caged ATP. We used Fourier analysis of images of cross sections to obtain an average view of the muscle structure, and correspondence analysis to extract information about individual crossbridge shapes. The crossbridge structure changes dramatically between relaxed, rigor, and with time after ATP release. In relaxed muscle, most crossbridges are detached. In rigor, all are attached and have a characteristic asymmetric shape that shows strong left-handed curvature when viewed from the M-line towards the Z-line. Immediately after ATP release, before significant force has developed (20 ms) the homogeneous rigor population is replaced by a much more diverse collection of crossbridge shapes. Over the next few hundred milliseconds, the proportion of attached crossbridges changes little, but the distribution of the crossbridges among different structural classes continues to evolve. Some forms of attached crossbridge (presumably weakly attached) increase at early times when tension is low. The proportion of several other attached non-rigor crossbridge shapes increases in parallel with the development of active tension. The results lend strong support to models of muscle contraction that have attributed force generation to structural changes in attached crossbridges.

Actin mediated regulation of muscle contraction

Striated and smooth muscles have different mechanisms of regulation of contraction which can be the basis for selective pharmacological alteration of the contractility of these muscle types. The progression in our understanding of the tropomyosin-troponin regulatory system of striated muscle from the early 1970s through the early 1990s is described along with key concepts required for understanding this complex system. This review also examines the recent history of the putative contractile regulatory proteins of smooth muscle, caldesmon and calponin. A contrast is made between the actin linked regulatory systems of striated and smooth muscle.