On the mechanism of energy transduction in myosin subfragment 1.

Abstract

It is proposed that the myosin subfragment 1 moiety of the muscle contractile apparatus is a self-contained "engine" whose operational plan is based on the interactive nature of ATP (or degradation intermediate) binding and actin binding, made possible by an intersite communication system. It is suggested that the spatial information required for examining this engine can, at least provisionally, be derived from fluorescence resonance energy transfer measurements interpreted by the Forster equation and that the existence of an intersite communication system can be deduced from piecewise detection of interacting pairs of points. A Theory of How the Myosin Subfragment 1 (S-i) Engine WorksIt has been thought that myosin S-1 moieties in active muscle fibers, pivoting about their attachments to subfragment 2 (S-2) moieties, deliver mechanical impulses to adjacent actin filaments, thus producing the relative interfilament translation ("slide") that characterizes isotonic muscle contraction (see ref. 1 for a review). Since there is no evidence that the gross shape (as distinct from local conformation) of S-1 changes during interaction with nucleotides (2) or actin (3), it is plausible to think that in its "purposeful" motion, as in its thermal motion, the cross-bridge rotates about S-1/S-2 as a quasirigid body. Because S-1 is elongate, a natural positional parameter is the angle that its "long" axis makes with the filament axis (4). There are now investigations showing that during isometric activity (i) this angle fluctuates (5) (7). Furthermore, the motional frequencies observed in i and iii are what might be expected from independent biochemical (8) and mechanical (9) observations and are magnitudes slower than thermally activated frequencies. Thus points i-iii support the idea that during activity cross-bridges move. They also suggest that the motion is describable by angles that position S-1 relative to the fiber axis (10), but in what follows it is necessary only that positional parameters relating S-1 to actin change with time repetitively.It is known that each S-1 bears an ATPase (N) site separate from the site (A) at which it binds actin (11) and that binding at these sites is interactive (12, 13). So it is tempting to think that the observed motions of S-1 are not those of an appendage passively moved by another agency, but rather that S-1 is a self-contained "unitary engine" strategically placed in the muscle apparatus. Previously we have elaborated on this hypothesis (14). We have noted that to examine this hypothesis we must look at common knowledge about S-1 in an uncommon way. For example, instead of considering S-1 motion in a fiber-based coordinate system we should position ligands (actin, nucleotides) in an S-1-based system, and we should think of enzymatic intermediates (15) as sequentially bound N-site ligands. These simple reinterpretations suggest a plan for the S-1 "engine": Because of the intersite communication system, binding of a particular intermediate at the N site determines a parti...

Effect of nucleotides, actin and temperature on thermolysin digestion of myosin subfragment‐1

“…The binding of the intermediates of the ATP cycle decreases the affinity of actin for its binding site and vice versa. Altogether, the results imply the existence of inter-site communication in S1 [13,141. On the basis of stability, the actin-S1 complexes can be resolved into three classes: (a) rigor complex, no occupant at the nucleotide site of S1, with highest stability; (b) strongly attached state, nucleotide-binding site of S1 is occupied by ADP, with intermediate stability; (c) weakly attached state, nucleotide site of Sl is occupied by ATP or ADP plus P,, with low stability.…”

Immunochemical Probing of the Functional Role of the 238–246 and 567–574 Sequences of Myosin Heavy Chain

“…Actin binding also produces structural changes in S l which affect ATP binding and hydrolysis. Thus, there is coupling between the two binding sites (Botts et al, 1984). It is, therefore, obvious that the characterization of the actin and ATP binding sites of S1 and of the conformational changes which take place in the S1 structure during the myosin -actin interaction, coupled with ATP hydrolysis, has a great significance.…”

Conformational transitions in the myosin head induced by temperature, nucleotide and actin

“…The 50-kDa fragment seems also to be involved in the nucleotide binding [24-261. Fluorescence resonance energy transfer measurements have shown that the nucleotide and actin binding sites are not within chemical reaction distance of each other, which suggests that the functionally important communication between these sites involves ligand-induced conformational changes [27]. Nucleotide-induced structural changes in the myosin head have been detected in a number of spectroscopic [28 -371, chemical modification [38 -421, and proteolytic digestion studies [43 -481 on myosin and its subfragments, heavy meromyosin and S1.…”