Fig. 1. (A) An actin filament moves over a surface coated with 3.6 myosin V molecules/µm² in 2 mM ATP. The pointed (green dots) and barbed (yellow dots) ends of the filament are marked when the ends are in the image plane. Panels 1-10 (left to right) show the time course of a filament before (panel 1) and after (panels 2-9) it has bound the surface. The apparent point of surface contact (crosshair) appears pronounced in the average fluorescent intensity throughout the time course of movement (panel 10). Panels 11-20 show the time course of a shorter filament moving over 5.4 molecules/µm². This filament encounters a second contact point before releasing its first. Nodal swiveling behavior reminiscent of seminal observations with kinesin (Howard et al., 1989; Hunt and Howard, 1993) provided the first hint of myosin V processivity (Wang et al., 2000; Mehta et al., 1999a). Bar, 5 µm. (B) The rate at which actin filaments land and move, as a function of myosin V surface density. A filament was considered ‘landed’ if it moved >0.5 µm and for >2 seconds. (C) The fraction of filaments that moved more than their length (as in panels a11-a20) before dissociating, as a function of surface density. The fit reflects the single molecule model prediction of P(n>1|n>0), where P(N) represents the density-dependent probability that N molecules populate an arbitrary area. Figure reproduced, with permission, from Mehta et al., 1999a (http.//www.nature.com).