Figure 9.

Treadmilling of transmembrane proteins recycled from rear to front by internal transport provides ubiquitous propulsion. (A) Schematics of heterogeneous membrane treadmilling mechanism. The perspective view displays a swimming cell on which actin-bound proteins (orange) are advected backward and entangled with nonactin-bound proteins. Swimming is promoted by advected proteins and hindered by diffusing proteins, which results in swimming significantly slower than treadmilling. The cross-sectional view displays the complete cycling of actin-binding proteins (orange), which comprises endocytosis at cell rear, internal advection by vesicles forward, exocytosis at cell front, and advection at cell membrane backward by linkage to retrograde actin flow (green). (B) Specific perturbation of paddling cycle by inhibitors of endocytosis (cartoon). Left: shown are representative images of a cell; middle: shown are representative kymographs of the swimming motion of the corresponding cells; and right: shown is a plot of the raw curvilinear swimming speeds for control conditions, with primaquine at 100 μM and with a cocktail of inhibitors (pitstop2 at 50 μM, dynasore at 200 μM, and secinH3 at 20 μM). N = 30 cells, ∗p < 0.05 and ∗∗∗∗p < 0.0001 (two-tailed Student’s t-test). (C) Heterogeneous treadmilling fosters ubiquitous motility without adhesion. (i) Swimming on a nonadherent surface, (ii) crawling between two surfaces separated by 3 μm, with adherent and nonadherent zones (see also Video S17), and (iii) in a nonadherent tube of cross section 4 × 4 μm² (see also Video S18). Data for cases (i) and (ii) correspond to projected images taken every 10 s for 3 min of a given area with several cells. The round shapes in (ii) correspond to 100-μm diameter glass pillars that sustain the structure. Data for case (iii) show a sequence of images of a single cell for 5 min. To see this figure in color, go online.