Actin filaments myosin heads

Actin (thin) filaments and myosin (thick) filaments myosin heads form bridges during contraction filaments are stable in sarcomeres and stereocilia, but often labile in cytoplasm. 

Figure 34.18 Myosin motion along actin. A myosin head (yellow) in the ADP form is bound to an actin filament (blue). The exchange of ADP for ATP results in (1) the release of myosin from actin and (2) substantial reorientation of the lever arm of myosin. The hydrolysis of ATP (3) allows the myosin head to rebind at a site displaced along the actin filament (4). The release of P, (5 accompanying this binding increases the strength of the interaction between myosin and actin and resets the orientation of the lever arm.

A EXPERIMENTAL FIGURE 19-4 Decoration demonstrates the polarity of an actin filament. Myosin SI head domains bind to actin subunits in a particuiar orientation. When bound to aii the subunits in a fiiament, SI appears to spirai around the fiiament. This coating of myosin heads produces a series of arrowhead-iike decorations, most easiiy seen at the wide views of the fiiament. The poiarity in decoration defines a pointed (-) end and a barbed (-f) end the former corresponds to the top of the model in Figure 19-3c. [Courtesy of R. Craig.]... 

Myosin heads form cross-bridges between the actin and myosin filaments... 

FIGURE 17.23 The mechanism of skeletal muscle contraction. The free energy of ATP hydrolysis drives a conformational change in the myosin head, resulting in net movement of the myosin heads along the actin filament. Inset) A ribbon and space-filling representation of the actin—myosin interaction. (SI myosin image courtesy of Ivan Rayment and Hazel M. Holden, University of Wiseonsin, Madison.)... 

The crossbridge cycle in muscle. Myosin crossbridges interact cyclically with binding sites on actin filaments. Note that the energy release step—when ATP is broken down to ADP—recocks the crossbridge head. 

The myosin head has long been shown to induce, even in low ionic strength buffers, polymerization of G-actin into decorated F-actin-S i filaments that exhibit the classical arrowhead structure (Miller et al., 1988 and older references therein). However, to date, the molecular mechanism of this polymerization process remains unknown. 

First, in the striated muscles, the cross-sectional organization of filaments is highly ordered in a hexagonal pattern commensurate with the ratio of actin to myosin filaments and the distribution of active myosin heads, S-1 segments, helically every 60 degrees around the myosin filament. In smooth muscle, with perhaps 13 actin filaments per myosin filament, many actin filaments appear to be ranked in layers around myosin filaments. It is not known how the more distant actin filaments participate in contraction. 

How can hydrolysis of ATP produce macroscopic movement Muscle contraction essentially consists of the cychc attachment and detachment of the S-1 head of myosin to the F-actin filaments. This process can also be referred to as the making and breaking of cross-bridges. The attachment of actin to myosin is followed by conformational changes which are of particular importance in the S-1 head and are dependent upon which nucleotide is present (ADP or ATP). These changes result... 

When the sarcolemma is excited by a nerve impulse, the signal is transmitted into the T tubule system and a release channel in the nearby sarcoplasmic reticulum opens, releasing Ca + from the sarcoplasmic reticulum into the sarcoplasm. The concentration of Ca in the sarcoplasm rises rapidly to 10 mol/L. The Ca -binding sites on TpC in the thin filament are quickly occupied by Ca +. The TpC-4Ca + interacts with Tpl and TpT to alter their interaction with tropomyosin. Accordingly, tropomyosin moves out of the way or alters the conformation of F-actin so that the myosin head-ADP-P (Figure 49-6) can interact with F-actin to start the contraction cycle. 

Walsh There s strong evidence that it is anchored to the actin filament through an N-terminal domain that Jim Stull s group has defined very precisely. The age-old question remains how does an anchored MLCK molecule gain access to an adequate number of myosin heads to account for the phosphorylation stoichiometry that can be achieved in muscle ... 

Hydrolysis of ATP provides the energy required for each stroke of myosin heads to pull on the actin filament so shortening the muscle fibre (Figure 7.3). In the resting... 

Thin actin filaments are pulled inwards by myosin heads so shortening the fibre. The Z disc is an anchorage point for actin... 

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