The role of tropomyosin-troponin in the regulation of skeletal muscle contraction.

Steric blocking of actin-myosin interaction by tropomyosin has been a working hypothesis in the study of the regulation of skeletal muscle contraction, yet the simple movement of actin-associated tropomyosin from a myosin-blocking position (relaxation) to a nonblocking position (contraction) cannot adequately account for all of the biophysical and biochemical observations which have been made to date. Ambiguous assignment of tropomyosin positions on actin during contraction, due in part to the limited resolution of reconstruction techniques, may also hint at a real lack of clearcut 'on' and 'off' positioning of tropomyosin and tropomyosin-troponin complex. Recent biochemical evidence suggests processes relatively independent of tropomyosin-troponin may have a governing effect on contraction, involving kinetic constraints on actin-myosin interaction influenced by the binding of ATP and the intermediates of ATP hydrolysis. Based on our current understanding put forth in this review, it is clear that regulatory interactions in muscle contraction do not consist solely of steric effects but involve kinetic factors as well. Where the latter are being defined in systems reconstituted from purified proteins and their fragments, the steric components of regulation are most clearly observed in studies of structurally more intact physiologic systems (e.g. intact or skinned whole muscle fibres). The fine detail of the processes and their interplay remains an intriguing question. Likewise, the precise physical relationship of myosin with actin in the crossbridge cycle continues to elude definition. Refinement of several methodologies (X-ray crystallography, three-dimensional reconstruction, time-resolved X-ray diffraction) will increase the potential for detailing the molecular basis of the regulation of muscle contraction.