Turning on the Thin Filament

A series of striking observations, beginning in the early 1960s, led to the formulation of the so-called steric-blocking model, which has been the major paradigm for the mechanism of thin filament activation (reviewed by Cohen and Vibert, 1987). 

Steric blocking is a simple idea, which by its very nature, specifies certain spatial relations between proteins (Fig. 7C). To quote Cohen and Vibert (1987)  

At the time that this simple two-state mechanism was conceived—and, indeed, in most attempts to model unknown structures—the proteins were pictured as smooth and symmetrical. In keeping with this image, the motions associated with both regulation and contraction were pictured by many as being simple and machine-like (but see an alternative view in Squire, 1975). 

Why Does Tropomyosin Move to a Third Position in the Fully-Activated State  

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Analyses of the tropomyosin sequence have shown long-range periodicities of certain surface acidic and apolar residues that are likely to be recognition sites for actin. These features are discussed below in relation to their role in regulation (see Turning on the Thin Filament section). 

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