CA2-INDUCED TROPOMYOSIN MOVEMENT IN LIMULUS THIN-FILAMENTS REVEALED BY 3-DIMENSIONAL RECONSTRUCTION()

THE steric model of muscle regulation holds that tropomyosin strands r unning along thin filaments move away from myosin-binding sites on act in when muscle is activated. Exposing these sites would permit actomyo sin interaction and contraction to proceed. This compelling and widely cited model is based on changes observed in X-ray diffraction pattern s of skeletal muscle following activation(1-3) Although analysis of X- ray patterns can suggest models of filament structure, unambiguous int erpretation is not possible. In contrast, three-dimensional reconstruc tion 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' state(4). Thin filaments from the arthropod Limulus have a similar composition to those from vertebrate skeletal muscle(5), and troponin-tropomyosin is distributed in both sp ecies with the same characteristic 38-nm periodicity(6) Limulus thin f ilaments activate skeletal muscle myosin ATPase at micromolar Ca2+ con centrations and confer a high calcium dependence on the enzyme. Arthro pod and vertebrate troponin subunits form functional hybrids in vitro( 7) and the respective tropomyosins are functionally interchangeable(8, 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.