Structure and function of myosin filaments
Introduction
Myosin filaments (also called thick filaments) are key components of muscle and non-muscle cells. In striated muscle, they overlap with thin (actin-containing) filaments in an orderly array, making a repeating pattern of sarcomeres, the basic units of contraction [1] (Figure 1a). In smooth and non-muscle cells, myosin and actin filaments also form overlapping arrays, but their organization is less ordered and more labile. Myosin filaments play two key roles in muscle contraction and cell motility. The myosin heads (or crossbridges), which lie on the surface of the filaments, bring about contraction by cyclic interaction with actin subunits in the thin filaments. This ‘crossbridge cycle’ causes the thick and thin filaments to slide past each other, producing movement [2, 3••]. Thick filaments in many types of muscle and non-muscle cells also participate in regulating or modulating contractile activity. Understanding how thick filaments are assembled and carry out these functions requires a detailed knowledge of their structure.
The structure of thick filaments has been elucidated over the past 40 years by electron microscopy (EM), X-ray diffraction and other techniques. Although this has provided many insights into the principles of myosin assembly, critical molecular details have been missing and the relationship of filament structure to function has been unclear. Here, we review the key advances that have led to our current understanding of thick filament structure and its relation to function, with an emphasis on recent progress. We focus on myosin organization in striated muscle filaments in the relaxed state, in which the filament structure is stable and unperturbed by interaction with actin. We also discuss myosin filaments in smooth and non-muscle cells, and the role of myosin-binding proteins in thick filament assembly and function.
Section snippets
Myosin molecules and thick filaments
The major component of thick filaments is myosin II, a member of the myosin superfamily of motor proteins that produce motility by moving actin [4••]. Myosin II is an elongated, two-headed molecule consisting of two identical heavy chains and two pairs of light chains [5] (Figure 1d). The C-terminal half of each heavy chain is α helical, whereas its N-terminal half folds into a globular head region. The heads (also called subfragment 1 or S1) consist of a motor domain, which hydrolyzes ATP and
Striated muscle thick filaments
In striated muscle, thick filaments are bipolar structures, formed by antiparallel tail interactions at the filament center (creating a ‘bare zone’ free of myosin heads) and parallel interactions at either end (the head or ‘crossbridge’ region) [7] (Figure 1b,c). In most muscles, thick filaments are connected to each other at their bare zones via cytoskeletal bridges constituting the M-line (Figure 1a) [8•]. These connections help organize the filament lattice and maintain filament register.
Smooth muscle and non-muscle thick filaments
Myosin filaments are also present in smooth muscle and non-muscle cells, where they pull on actin to produce filament sliding, as in striated muscle. In vertebrates, smooth muscle and non-muscle filaments are more labile than those of striated muscle. Under in vitro relaxing conditions, dephosphorylated filaments disassemble into monomers with a folded structure and low ATPase activity. Upon phosphorylation, the molecules unfold and assemble into filaments that have high ATPase activity and are
Assembly of thick filaments
The correct assembly of myosin molecules into thick filaments in muscle and non-muscle cells has several key requirements, which have been well demonstrated by genetic and other analyses [54]. These requirements vary by system, but may include: the correct level of myosin expression; the presence of critical domains of myosin; expression and proper assembly of non-myosin proteins (e.g. paramyosin, titin, MyBP-C); regulation of assembly by phosphorylation of the heavy or light chains; and the
Conclusions
Myosin filaments comprise a diverse array of macromolecular assemblies. Although their basic structure has been understood for more than 40 years, progress towards an atomic-level description has been slow. Recent innovative studies have finally revealed details of the organization of the heads and tails in tarantula filaments at near-atomic level [20••], suggesting a structural model for myosin filament regulation. Sequence analysis and crystallographic studies of other systems are providing
References and recommended reading
Papers of particular interest, published within the annual period of review, have been highlighted as:
• of special interest
•• of outstanding interest
Acknowledgements
Our thick filament studies are supported by grant AR34711 from the National Institutes of Health. Rendering and atomic fitting of reconstructions was carried out using UCSF Chimera [63].
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