| Observations . | . | . | . | . | Parameters needed to model M3 and M6 fully . | . |
|---|---|---|---|---|---|---|
| Relaxed muscle . | . | . | . | . | . | . |
| M3 | IM3i | 1 | Lattice of head array on myosin | Known | ||
| IM3o | 1 | Configuration of heads on myosin assuming random occupancy | (7–13a) × 2 | |||
| dM3i | ∼143.2 Å | Known | Weight of heads on myosin | Known | ||
| dM3o | Known | Radial position of head origin | 1 | |||
| M6 | IM6i | 1 | Weight of contribution from backbone | 1 | ||
| IM6o | 1 | Axial position of backbone contribution | 1 | |||
| dM6i | ∼7.16 Å | Known | Axial size of backbone diffractor | 1 | ||
| dM6o | Known | Bare zone length | 1 | |||
| IML1 | 1 | Ratio of heads in to heads out | 1 | |||
| Interference | L | Known | Interference distance L | Known | ||
| Total observations | 5 | Parameters | 20–32 | |||
| Active muscle | Assume resting parameters if possible | |||||
| Weight of heads | Known | |||||
| M3 | IM3i | 1 | Shape of heads on myosin | Assume as relaxed | ||
| IM3o | 1 | Population of heads on myosin | 1 | |||
| dM3i | ∼145.7 Å | Known | Distribution of head array on actin | 4b | ||
| dM3o | Known | Shape of heads on actin | (7–13b) × 2 | |||
| M6 | IM6i | 1 | Relative axial position of head array on actin | 1 | ||
| IM6o | 1 | Weight of heads on actin | 1 | |||
| dM6i | Known | Axial position of contribution from backbone | Assume as relaxed | |||
| dM6o | Known | Weight of contribution from backbone | Assume as relaxed | |||
| IML1 | 1 | Population of heads in each attached state | 1 | |||
| Interference | L | Distribution of head array on myosin | Assume random | |||
| Total observations | 10 | Parameters | 22–34 |
| Observations . | . | . | . | . | Parameters needed to model M3 and M6 fully . | . |
|---|---|---|---|---|---|---|
| Relaxed muscle . | . | . | . | . | . | . |
| M3 | IM3i | 1 | Lattice of head array on myosin | Known | ||
| IM3o | 1 | Configuration of heads on myosin assuming random occupancy | (7–13a) × 2 | |||
| dM3i | ∼143.2 Å | Known | Weight of heads on myosin | Known | ||
| dM3o | Known | Radial position of head origin | 1 | |||
| M6 | IM6i | 1 | Weight of contribution from backbone | 1 | ||
| IM6o | 1 | Axial position of backbone contribution | 1 | |||
| dM6i | ∼7.16 Å | Known | Axial size of backbone diffractor | 1 | ||
| dM6o | Known | Bare zone length | 1 | |||
| IML1 | 1 | Ratio of heads in to heads out | 1 | |||
| Interference | L | Known | Interference distance L | Known | ||
| Total observations | 5 | Parameters | 20–32 | |||
| Active muscle | Assume resting parameters if possible | |||||
| Weight of heads | Known | |||||
| M3 | IM3i | 1 | Shape of heads on myosin | Assume as relaxed | ||
| IM3o | 1 | Population of heads on myosin | 1 | |||
| dM3i | ∼145.7 Å | Known | Distribution of head array on actin | 4b | ||
| dM3o | Known | Shape of heads on actin | (7–13b) × 2 | |||
| M6 | IM6i | 1 | Relative axial position of head array on actin | 1 | ||
| IM6o | 1 | Weight of heads on actin | 1 | |||
| dM6i | Known | Axial position of contribution from backbone | Assume as relaxed | |||
| dM6o | Known | Weight of contribution from backbone | Assume as relaxed | |||
| IML1 | 1 | Population of heads in each attached state | 1 | |||
| Interference | L | Distribution of head array on myosin | Assume random | |||
| Total observations | 10 | Parameters | 22–34 |
Note that, to define head shape, one can assume a particular head structure that needs to be oriented in space with a tilt, a slew, and a rotation around its long axis (three parameters), or one could define it as a lever arm on which there is a mobile head domain: three parameters for the lever arm and three more parameters for the motor domain. In relaxed muscle, each of the two heads needs separate parameters, giving either 6 or 12 parameters, in addition to a parameter defining the separation of the head origins. In active muscle, there will be at least two populations of actin attached heads, each requiring 3 or 6 parameters to define the head shape (6–12 in all).
Finding head attachment sites on actin requires two search parameters for the head (axial and azimuthal ranges) and a target area size and offset on the actin (two parameters). The axial distribution of labeled actin sites has a major effect on the values of M3 and M6; there are no heads on actin spaced 145.7 Å apart (I, intensity; d, distance; “known” means that the parameter value has been obtained experimentally). The table assumes that parameters for resting muscle can be determined first, and some of these can be used to help model active muscle.