MOLECULAR MECHANISM ON THE REGULATION OF SMOOTH MUSCLE CONTRACTION STUDIED BY NEW EXPERIMENTAL TECHNIQUES

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Caldesmon is a fibrous protein existing specifically in smooth muscle cells. I have previously reported about the functional domain organization in the primary amino-acid sequence of that molecule ; that its C-terminal and N-terminal portions specifically bind to F-actin/tropomyosin and S2 moiety of myosin, respectively, and that the molecule might fold into half a length under low ionic strength. I examined by electron microscopy, the structure of reconstituted and natural thin filaments after brief chemical fixation, using negative staining and mica-flake technique coupled with quick-freeze deep-etch replica method. Reconstituted thin-filaments showed the fibrous molecules protruding from actin with the periodicity common to trepomyosin. Heavy meromyosin (HMM) heads do not usually associate with actin in the presence of ATP,if unphosphorylated. Possible clusters of unphosphorylated-HMM molecules, however, were found to bind to reconstituted thin-filaments with the same periodicity as … More that of caldesmon protrusion. Unphosphorylated-myosin also bound to thin-filaments under a similar condition. Though natural thin-filament showed no protrusions under ionic conditions close to physiological, actin helices were not clear suggesting the presence of some additional molecules on the surface of thin-filaments. When they were fixed under lower ionic strength, a similar image to that of reconstituted thin-filaments was obsereved. These structural results well coincide with the above biochemical data.Filaments of unphosphorylated myosin are known to be unstable under in vitro conditions and easily dissolve upon addition of ATP.It was found, however, that the co-existence of caldesmon stabilizes such filaments, and that they were tethered in a periodic manner to actin-filaments further added. Reported evidence for the existence of unphosphorylated myosin-filaments in relaxd smooth muscle tissue had been in conflict with the above in vitro biochemical results. The physiological in vivo situation might be now reasonably explained by such a characteristic property of caldesmon molecule. Less