Myosin head region

Affinity chromatography of light chain iso- 191 enzymes of myosin head region Investigation of binding of fatty acids, 192... 

Approximately 500 of the 820 amino acid residues of the myosin head are highly conserved between various species. One conserved region, located approximately at residues 170 to 214, constitutes part of the ATP-binding site. Whereas many ATP-binding proteins and enzymes employ a /3-sheet-a-helix-/3-sheet motif, this region of myosin forms a related a-f3-a structure, beginning with an Arg at (approximately) residue 192. The /3-sheet in this region of all myosins includes the amino acid sequence... 

Figure 49-4. Diagram of a myosin moiecuie showing the two intertwined a-heiices (fibrous portion), the giobuiar region or head (G),the iight chains (L), and the effects of proteoiytic cieavage by trypsin and papain. The giobuiar region (myosin head) contains an actin-binding site and an L chain-binding site and aiso attaches to the remainder of the myosin moiecuie.

In contrast, myosin I, which is not present in muscle, possesses only one head region and a short tail. Its role in cells may be involved in movement associated with membranes (endocytosis, phagocytosis). 

In thinking about X-ray diffraction from this assembly, a number of the sarcomere components contribute to the observed patterns in ways that have been the subject of detailed analysis. In the A-band, these include the myosin filament backbone, where the coiled-coil a-helical myosin rods pack together, the myosin head arrays in the bridge regions of the myosin filaments, the non-myosin A-band proteins titin and C-protein (MyBP-C), and the A-band parts of the actin filaments. Very little has been seen in X-ray patterns so far that appears to be related to the M-band, probably... 

Fig. 17. Sets of computed diffraction pattern simulations for different patterns of labeling of myosin heads on actin, defined by the head angular search range A9, the head axial search range AZ, and the actin target area angular size (twice the large number on each pattern), in each case with at least 98% of the available myosin heads bound to acdn. For details of parameters and regions A, B, and C, see text. (Based on Squire et at, 2005b.)...

The origin of these closely spaced peaks was very quickly shown to be the interference effects observed within the sarcomere because, in the case of C-protein, the diffraction patterns from the two C-zones in a single A-band would interfere, and, in the case of troponin, the diffraction patterns from the two troponin arrays across the Z-band would interfere. Also, in the case of the M3 multiple, the diffraction from the myosin heads in the two bridge regions of a single A-band would interfere (see summary in Squire, 1981 pages 576-582). In the case of the C-zone interference, illustrated in Fig. 21A—C), the diffraction intensity profile from a single C-zone (A) would have a prominent peak at 430 A, but the two C-zones in one A-band would be centered a distance L apart (Fig. 21C). The two C-zones could then be considered as... 

The myosin protomers can self-associate to form a thick filament by the assembly of some 400 myosin tails in a staggered side-by-side packing with the myosin heads projecting at regular intervals in a helical array (Fig. 5-32). The thick filament is bipolar with a 150 nm bare zone in the center where two oppositely oriented sets of myosin tails come together. The center of this region is called the M line. This thick filament forms part of a myofibril. 

How does this cycle apply to muscle contraction Myosin molecules self-assemble into thick bipolar structures with the myosin heads protruding at both ends of a bare region in the center (Figure 34.19). Approximately 500 head domains line the surface of each thick filament. These domains are paired in myosin dimers, but the two heads within each dimer act independently. Actin filaments associate with each head-rich region, with the barbed ends of actin toward the Z-line. In the presence of normal levels of ATP, most of the myosin heads are detached from actin. Each head can independently hydrolyze ATP, bind to actin, release Pj, and undergo its power stroke. Because few other heads are... 

Figure 34.19. Thick Filament. (A) An electron micrograph of a reconstituted thick filament reveals the presence of myosin head domains at each end and a relatively narrow central region. (B) A schematic view shows how myosin molecules come together to form the thick filament. [Part A courtesy of Dr. Hugh Huxley.]...

Two of these directions correspond to up and down along the myosin duead, and the connections in the two lateral directions are also directed upwards and downwards. It is natural then to relate dtese four-coordinated channel regions of the Q surface to the myosin heads. Thus the possible connections via the 3i-centred actin threads of myosin threads have only two directions, which fulfil the crystallographic symmetry of the surface. It is proposed here that these two directions correspond to the initial direction of the myosin head before movement and the end direction after movement, respectively. In this model of the contraction, the time-phase of mobility represents a transient disorder condition of adjacent structure elements, whereas the structure as a whole fulfils the required crystallographic symmetry. If the individual molecular conformational changes must be accommodated within the periodicity of the surface, the necessary perfect long-range synchronisation of mobility over the entire muscle seems a natural consequence. 

The head domain of myosin shown in its relation to the actin filament. The NH2-terminal end of the myosin heavy chain is in the globular head. The light chains bind to the neck region of the MHC. In this figure, the orientation of the myosin to the actin is that of the rigor bond, i.e., at the end of the power stroke. [From M. Irving and G. Piazzesi, Motions of myosin heads that drive muscle contraction. News Physiol. Sci. 12(6), 249-254 (December 1997).]... 

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