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Letters to Nature
Nature 368, 65 - 67 (03 March 1994); doi:10.1038/368065a0

Ca2+-induced tropomyosin movement in Limulus thin filaments revealed by three-dimensional reconstruction

William Lehman*, Roger Craig & Peter Vibert

*Department of Physiology, Boston University School of Medicine, 80 East Concord Street, Boston, Massachusetts 02118, USA
Department of Cell Biology, University of Massachusetts Medical School, Worcester, Massachusetts 01655, USA
Rosenstiel Basic Medical Sciences Center, Brandeis University, Waltham, Massachusetts 02554, USA

THE steric model of muscle regulation holds that tropomyosin strands running along thin filaments move away from myosin-binding sites on actin when muscle is activated. Exposing these sites would permit actomyosin interaction and contraction to proceed. This compelling and widely cited model is based on changes observed in X-ray diffraction patterns of skeletal muscle following activation1–3. Although analysis of X-ray patterns can suggest models of filament structure, unambiguous interpretation is not possible. In contrast, three-dimensional reconstruction of thin-filament electron micrographs could, in principle, offer direct confirmation of the predicted tropomyosin movement, but so far tropomyosin in skeletal muscle has been resolved definitively only in the 'on' state but not in the 'off' state4. Thin filaments from the arthropod Limulus have a similar composition to those from vertebrate skeletal muscle5, and troponin–tropomyosin is distributed in both species with the same characteristic 38-nm periodicity6. Limulus thin filaments activate skeletal muscle myosin ATPase at micro-molar Ca2+ concentrations and confer a high calcium dependence on the enzyme. Arthropod and vertebrate troponin subunits form functional hybrids in vitro 7 and the respective tropomyosins are functionally interchangeable8,9, arguing for a common mechanism of thin-filament-linked regulation in the two phyla. Here we report that tropomyosin is readily resolved in native filaments of troponin-regulated Limulus muscle in both the 'on' and 'off' states, and demonstrate tropomyosin movement, providing support for the importance of steric effects in muscle activation.

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