Table 1. Properties and occupancy of the three states in the McKillop and Geeves model as assayed by different methods
TF statesBlockedClosedOpen
Tpm position on actinOuter domainInner/Outer domainInner domain
ATPase/activityOff/lowOff/lowOn/high
Myosin bindingTransient kineticOffOnOn
EquilibriumOffOffOn
ActinTpm occupancy−Tn00.80.2
+Tn −Ca0.70.250.05
+Tn +Ca00.80.3
+Myosin (1:1 ratio of myosin:actin) (+/−Tn, +/−Ca)001.0

Tpm position as defined in EM and x-ray fiber diffraction data. Inner and outer refer to the position of Tpm on the inner subdomains (3 and 4) or outer subdomains (1 and 2) of actin. Myosin binding refers to measurements of myosin binding to actin following rapid mixing of myosin S1 with thin filaments (kinetic) or in titration studies where myosin S1 is titrated slowly into a solution of thin filaments (equilibrium). In both cases the binding can be followed using the fluorescence of a pyrene label attached to actin. Actin-Tpm occupancy is the interpretation of the occupancy of the three actin.Tpm.Tn states from the myosin binding studies.