Distribution of attachment events relative to actin binding sites as evidenced in a bidirectional actomyosin interaction model.

Optical trapping is one of the most evolving technologies that measures biophysical quantities and provides insights into some of the fundamental questions in the study of molecular motor proteins such as myosin. Several laboratories have successfully used this technique to observe and score nanometre-size displacements produced by myosin on interacting with actin. We have studied the distribution of attachment events for two myosin molecules with different orientations interacting with an actin filament within the framework of a Langevin-type bidirectional mathematical model. When myosin is detached from actin, our model predicts Brownian displacements centred at 0 +/- 8 nm (mean +/- SD, n = 251,058). When attached, the time-averaged displacements of the actin filament system produced step sizes with peaks of 8 +/- 6 nm (mean +/- SD, n = 22,174) (forward displacements) and -8 +/- 6 nm (mean +/- SD, n = 26,769) (reverse displacements). We infer from our results that the population distribution of attachment events is strongly dependent on (i) the magnitude of the Brownian displacements, (ii) the location of the actin binding sites relative to the myosin molecules, (iii) the orientation of the myosin molecules, and (iv) the relative kinetics (rate constants) for the forward and reverse displacement events.