Structural interactions between actin, tropomyosin, caldesmon and calcium binding protein and the regulation of smooth muscle thin filaments

The basic structure and functional properties of smooth muscle thin filamen ts were established about 19 years ago. Since then we and others have been working on the details of how tropomyosin. caldesmon and the Ca2+-binding p rotein regulate actin interaction with myosin. Our work has tended to empha size the similarities between caldesmon and troponin function whilst others have been more concerned with the differences; The need to resolve the res ulting differences has stimulated us to find new and more direct ways of in vestigating the mechanism of thin filament regulation. In recent years an a pparent divergence has opened up between functional measurements, which ind icate an allosteric-cooperative regulatory mechanism in which caldesmon and Ca2+-binding protein control actin-tropomyosin state in the same way as tr oponin, and structural measurements which show thin filament structures unl ike striated muscle thin filaments. The challenge is to interpret function in terms of structure. We have combined functional studies with expression and mutagenesis of caldesmon and with structural methods including X-ray cr ystalography of tropomyosin-caldesmon crystals, electron microscopy and hel ical reconstruction of actin-tropomyosin-caldesmon complexes and high resol ution nuclear magnetic resonance spectroscopy of the C-terminus of caldesmo n in interaction with actin and calmodulin. We have used this information t o propose a structural mechanism for caldesmon regulation of the smooth mus cle thin filament.