Myosin complex structure

Rayment, 1., et al. Structure of the actin-myosin complex and its implications for muscle contraction. Science 261 58-65, 1996. 

Rayment, L, Holden, H.M., Whittaker, M., Yohn, C.B., Lorenz, M., Holmes, K.C., Milligan, R.A. (1993b). Structure of the actin-myosin complex and its implications for muscle contraction. Science 261, 58-65. 

In addition to its effects on enzymes and ion transport, Ca /calmodulin regulates the activity of many structural elements in cells. These include the actin-myosin complex of smooth muscle, which is under (3-adrenergic control, and various microfilament-medi-ated processes in noncontractile cells, including cell motility, cell conformation changes, mitosis, granule release, and endocytosis. 

N ow that some of the major types of protein structures have been described it is appropriate to turn to the question of how protein structure relates to protein function. To explore this question, two protein systems, hemoglobin and the actin-myosin complex, are examined in detail. 

M. Lorenz, K. C. Holmes, and R. A. Milligan, Structure of the Actin-Myosin Complex and Its Implications for Muscle Contraction. Science 261 58-65, 1993. 

Starting from the pre-powerstroke state myosin complex with ADP and Pi tightly bound (summarized structure in Table I), the M.D.Pi is in rapid equilibrium with an actin-bound state on the microsecond-millisecond time scale. This is very dependent on ionic strength (Furch et al., 2000 White and Taylor, 1976) and is therefore probably a non-stereo-specific weak binding state and is controlled by the ionic interactions between loop 2 and the N-terminus of actin. Other ionic interactions may also be involved. This loose association between actin and myosin probably does not alter the overall conformation of myosin. 

It has long been surmised that switch-2 movement and the concomitant swinging of the lever arm must be controlled by binding to and detachment from the actin filament to avoid futile consumption of ATP. However, direct evidence was lacking because near-atomic resolution crystal structures are necessarily obtained in the absence of the filament. Now, crystal structures of Dictyostelium myosin II (Reubold et at, 2003) and chicken myosin-V (Coureux et al., 2003) have revealed that the switch-1 motif can also exist in open and closed conformations. It has been inferred that switch-1 opening may be coupled to cleft closure and tight binding of the myosin head to the actin filament. This conclusion is supported by electron microscopy (Holmes et al., 2003) and fluorescence spectroscopy (Conibear et al., 2003) studies of the acto-myosin complex, which show that the concepts derived from crystal structures of isolated myosin heads are indeed valid for the functional complex. 

The description of structure in complex chemical systems necessarily involves a hierarchical approach we first analyse microstructure (at the atomic level), then mesostructure (the molecular level) and so on. This approach is essential in many biological systems, since self-assembly in the formation of biological structures often takes place at many levels. This phenomenon is particularly pronounced in the complex structures formed by amphiphilic proteins that spontaneously associate in water. For example myosin molecules associate into thick threads in an aqueous solution. Actin can be transformed in a similar way from a monomeric molecular solution into helical double strands by adjusting the pH and ionic strength of the aqueous medium. The superstructure in muscle represents a higher level of organisation of such threads into an arrangement of infinite two-dimensional periodicity. 

In general there is a set of criteria which can be used to demonstrate whether an observed step is, or can be, on the direct pathway of a complex process. The rate of the step and its dependence on changing conditions has to be compatible with the overall rate. It is of course easier to exclude a step from the direct pathway, because it is too slow, than it is to ascertain its inclusion. This applies in the exploration of enzyme reactions, as will be discussed in detail in section 5.1, as well as to physiological responses. An example of the latter, which will be used to illustrate points in different sections, is the relation between steps in the hydrolysis of ATP by myosin and structural and mechanical changes during muscle contraction. Similarly the time course of calcium release and removal has to be correlated with the stimulation and relaxation of contraction and other phenomena. 

Smith, C. A., Rayment, 1. X-ray structure of the magnesium (11). ADP-vanadate complex of the Dictyostelium discoideum myosin motor domain to 1.9 A resolution. Biochemistry 35 5404-5407, 1996. 

Fisher, A., Smith, C., Thoden, J., et al., 1995. Structural studies of myosin nncleotide complexes A revised model for die molecular basis of muscle contraction. Biophysical Journal... 

Smidi, C., and Rayment, I., 1995. X-ray structure of the magnesinm(II)-pyrophosphate complex of the truncated head in Dietyostelium discoideum myosin to 2.7 A resolndon. Biochemistry 34 8973-8981. 

In striated muscle, there are two other proteins that are minor in terms of their mass but important in terms of their function. Tropomyosin is a fibrous molecule that consists of two chains, alpha and beta, that attach to F-actin in the groove between its filaments (Figure 49-3). Tropomyosin is present in all muscular and muscle-fike structures. The troponin complex is unique to striated muscle and consists of three polypeptides. Troponin T (TpT) binds to tropomyosin as well as to the other two troponin components. Troponin I (Tpl) inhibits the F-actin-myosin interaction and also binds to the other components of troponin. Troponin C (TpC) is a calcium-binding polypeptide that is structurally and functionally analogous to calmodulin, an important calcium-binding protein widely distributed in nature. Four molecules of calcium ion are bound per molecule of troponin C or calmodulin, and both molecules have a molecular mass of 17 kDa. 

Upon entering the smooth muscle cell, Ca++ ions bind with calmodulin, an intracellular protein with a chemical structure similar to that of troponin. The resulting Ca++-calmodulin complex binds to and activates myosin kinase. This activated enzyme then phosphorylates myosin. Crossbridge cycling in smooth muscle may take place only when myosin has been phosphorylated. 

Tropomyosin and troponin are proteins located in the thin filaments, and together with Ca2+, they regulate the interaction of actin and myosin (Fig. 43-3) [5]. Tropomyosin is an a-helical protein consisting of two polypeptide chains its structure is similar to that of the rod portion of myosin. Troponin is a complex of three proteins. If the tropomyosin-troponin complex is present, actin cannot stimulate the ATPase activity of myosin unless the concentration of free Ca2+ increases substantially, while a system consisting solely of purified actin and myosin does not exhibit any Ca2+ dependence. Thus, the actin-myosin interaction is controlled by Ca2+ in the presence of the regulatory troponin-tropomyosin complex [6]. 

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