REVIEW The Muscle Thin Filament as a Classical Cooperative/Allosteric Regulatory System Sherwin S. Lehrer 1 * and Michael A. Geeves 2 1 Muscle Research Group Boston Biomedical Research Institute, Boston, MA 01224, USA 2 Max Planck Institut f. Molekulare Physiologie D-44139, Dortmund Germany It is generally accepted that the regulation of muscle contraction involves cooperative and allosteric interactions among the protein components, actin, myosin, tropomyosin and troponin. But, as yet, the individual role of each component has not been clearly identi®ed. Here we compare the properties of the components of the muscle regulatory system with the corresponding components of two systems, hemoglobin and aspartate transcarbamylase, that are well described by the classical Monod, Wyman and Changeux (MWC) model. The analogy indicates that actin is the catalytic subunit, tropomyosin is the regulatory subunit and troponin in the absence and presence of Ca 2 is the allosteric inhibitor and activa- tor, respectively. The analogy additionally indicates that the substrate is myosin-ATP (or myosin-ADP-Pi) rather than ATP. Also, in contrast to other MWC systems, the activating ligand for actin-tropomyosin is a myosin-nucleotide intermediate or product that binds tightly to actin, rather than the substrate which binds weakly. This tightly bound inter- mediate switches the system from the off-state to the on-state (T to R-state in MWC nomenclature) in a concerted transition, affecting n actin subunits, allowing force to be developed. # 1998 Academic Press Limited Keywords: actin; tropomyosin; troponin; myosin, ATPase *Corresponding author Introduction Cooperative and allosteric interactions have been implicated in the regulation of muscle contraction beginning with studies in the laboratories of A. Weber (Bremel et al., 1972) and E. Eisenberg (Greene & Eisenberg, 1980). But, after 25 years, there is no general agreement about the character- istics or nomenclature of the several states of the thin ®lament involved in these interactions, or the roles of Ca 2 and myosin. The problem is, in part, the dif®culty of relating the physiological events that occur in muscle ®bers with relevant data obtained from reconstituted muscle protein sys- tems in solution. In muscle ®bers, where the effec- tive concentration of myosin heads is ®xed by the geometrical arrangement, contraction is triggered by Ca 2 . In solution, where the myosin head con- centration can be changed, ATPase activity measurements indicate that Ca 2 itself is not suf®- cient nor necessary to activate the thin ®lament. Instead, myosin heads (subfragment 1, S1) appear to be necessary. Muscle ®ber studies have indi- cated that in addition to the function of myosin heads as molecular motors, they are also involved in activation (Reuben et al., 1973; Swartz & Moss, 1992; Zot & Potter, 1989). The steric-blocking model introduced in the early 70's (Haselgrove, 1972; Huxley, 1972; Parry & Squire, 1973) and later structural studies, have suggested that the position of tropomyosin (Tm) on the actin ®lament changes in response to both Ca 2 and S1 binding (Vibert et al., 1997). The current challenge for understand- ing the regulation of muscle contraction is to relate ®ndings from solution studies with data from structural and physiological studies. Recent reviews have introduced the thin ®lament activity states (Lehrer, 1994) and discussed cooperativity from different points of view (Moss, 1992; Tobacman, 1996; Grabarek et al., 1992). The processes involved in the regulation of muscle contraction are simpler to understand when studied in solution, where the principal com- ponents and their concentrations can be varied and where binding and effects on the ATPase activity Abbreviations used: ATCase, aspartate transcarbamylase; Hb, hemoglobin; S1, myosin subfragment 1; NEM S1, N-ethyl maleimide modi®ed S1; PALA, N-phosphonacetyl-L-aspartate; Tm, tropomyosin; Tm*, pyrene-labeled Tm; Tn, troponin; MWC, Monod, Wyman and Changeux. J. Mol. Biol. (1998) 277, 1081±1089 0022±2836/98/151081±09 $25.00/0/mb981654 # 1998 Academic Press Limited