A NEW LOOK AT THIN FILAMENT REGULATION IN VERTEBRATE SKELETAL-MUSCLE

It is 30 years since Ebashi and colleagues showed that Ca2+ ions direc tly affect regulation of the myosin-actin interaction in muscle throug h the action of tropomyosin and troponin on muscle thin filaments. It is more than 20 years since the idea was put forward that tropomyosin mig ht act, at least in part, by changing its position on actin, thus uncovering or modifying the myosin binding site on actin when troponin molecules take up Ca2+. Since that time, a great deal of evidence for and against this steric blocking mechanism has been published: a stru cture for actin filaments at close to atomic resolution has been propo sed, and the whole regulation story has become both more complicated a nd more subtle. Here we review structural and biochemical aspects of r egulation in vertebrate skeletal muscle. We show that some basic ideas of the steric blocking mechanism remain valid. We also show that addi tional factors, such as troponin movements and structural changes with in the actin monomers themselves, may be crucial, A number of the resu lting regulation scenarios need to be distinguished.-Squire, J. M., Mo rris, E. P. A new look at thin filament regulation in vertebrate skele tal muscle.