Evidence for the Essential Role of Myosin Subfragment-2 in Muscle Contraction: Functional Communication between Myosin Head and Subfragment-2

Abstract

In 1971, Harrington et al. put forward a hypothesis, in which helix-coil transition in the hinge region of myosin subfragment-2 (S-2) contributes to muscle contraction. The helix-coil transition hypothesis has been, however, ignored by muscle investigators over many years. In 1992, we worked with him to examine the effect of polyclonal antibody to myosin subfragment-2 (anti-S-2 antibody), and found that the antibody eliminated Ca 2+ -activated isometric force generation of skinned vertebrate muscle fibers without affecting MgATPase activity. Further studies using the same antibody indicated functional communication between myosin head and myosin S-2, including regulation of binding strength between myosin head and actin filament during Ca 2+ -activated contraction in vertebrate muscle fibers. These findings indicate that the swinging lever arm hypothesis, in which muscle contraction results from active rotation of myosin head converter domain, is incomplete because it ignores of the role of myosin S-2. Much more experimental work is necessary to reach full understanding of muscle contraction mechanism at the molecular level. Highlights• Harrington's helix-coil transition theory of muscle contraction is explained.• Using the gas environmental chamber attached to electron microscope, we observed marked ATP-induced myosin head movement in hydrated myosin-paramyosin core complex filaments, which is only accounted for by the helix-coil transition taking place in myosin subfragment-2 region.• We obtained experimental evidence for the functional communication between myosin head (myosin subfragment-1) and subfragment-2, including the regulation of binding strength of myosin head with actin filament.• We emphasize the essential role of subfragment-2 in producing muscle contraction, which has been totally ignored in the current swinging lever arm hypothesis appearing in every textbooks. Theory Structural basis of muscle contractionIn 1954, Hugh E Huxley and Jean Hanson made a monumental discovery that muscle contraction results from relative sliding between actin and myosin filaments, which in turn is produced by cyclic attachment and detachment between myosin heads extending from myosin filaments and corresponding sites in actin filaments [1]. As shown in Figure 1A, a myosin molecule (MW, 450,000) consists of two pear-shaped heads (myosin subfragment-1) and a rod of 156 nm long, and is split enzymatically into two parts: (1) the rod of 113 nm long (light meromyosin, LMM) and (2) the rest of myosin molecule including two heads and a rod of 43 nm long (heavy meromyosin, HMM). The HMM can be further split into two separate heads (subfragment-1, S1) and a rod (subfragment-2, S-2). In myosin filaments, LMM aggregates to form filament backbone, which is polarized in opposite directions across the filament central region (bare region, as illustrated in Figure 1B). The S-2 rod serves as a hinge between the S-1 heads and the filament backbone, so that the S-1 heads can swing away from the filament to interact with acti...

Temperature Effects on Force and Actin–Myosin Interaction in Muscle: A Look Back on Some Experimental Findings

“…As presented in the review by Geeves & Holmes [ 101 ], a change in crossbridge structure pulling on the lever arm is considered to be a possible mechanism of active force development. Such a process is unlikely to be endothermic; also, whether such a process—without a change in its attachment to thick filament—can generate much force remains unclear [ 4 ]. Changes in attachments of crossbridge states (non-stereospecific to stereospecific, hydrophilic to hydrophobic, etc.)…”

“…Furthermore, they proposed an interesting idea that the forward rate of force generation step is increased, but the reverse rate is decreased with an increase of temperature. In a recent review, Sugi [ 4 ] has re-examined the possible importance of a change in crossbridge attachment (subfragment-2) during force generation. It seems that specific experimental details of an endothermic structural mechanism (with a volume increase) for crossbridge force generation in muscle that can account also for its coupling to acto-myosin cycle are still lacking.…”

“…This mechanics cycle is coupled to an enzymic reaction, hydrolysis of ATP by acto-myosin ATPase [ 3 ], so that energy liberated during release of the products of ATP hydrolysis (phosphate = Pi, and adenosine diphosphate = ADP) is converted into work (and heat); an active muscle is a machine converting chemical to mechanical energy. Despite many investigations and using different techniques, exactly how various steps in these two cyclic processes, chemical and mechanical, are coupled during an active muscle contraction, remains not fully understood [ 4 ].…”

“…This step is strain-sensitive, enhanced by negative strain and inhibited by positive strain, perhaps providing a basis for increased energy consumption in shortening contraction-producing power. A possible structural mechanism for endothermic force generation in an attached crossbridge may well be a “protein folding/unfolding-process” within an attached crossbridge (myosin head) [ 9 ], or outside [ 4 ], or both.…”