ORIENTATION OF ACTIN-FILAMENTS DURING MOTION IN IN-VITRO MOTILITY ASSAY

Rhodamine-phalloidin was added to F-actin, and the orientation of tran sition dipoles of the dye was measured single actin filaments by polar ization of fluorescence. Rhodamine-phalloidin was well immobilized on the surface of actin, indicating that changes in orientation of the dy e reported changes in orientation of actin monomers. In stationary fil aments the dipoles were inclined at 49.3 degrees with respect to the f ilament axis. The disorganization of dipoles in stationary filaments w as insignificant. When the filaments were made to translate, the avera ge orientation of the dye did not change, but disorganization slightly increased. Disorganization increased significantly when filaments wer e free in solution. We concluded that, within the accuracy of our meas urements (approximate to 18%), actin monomers did not undergo major re orientations during motion, but that binding of myosin heads deformed the structure of filaments.