Fig. 3.

A nonuniform actin polarity in pseudopodia. (A) Four 2D class averages of the in situ actin segments (SI Appendix, Fig. S2). A 14-Å resolved actin structure (shown in gray) allows identification of actin polarity, barbed (+) and the pointed (−) ends are indicated. (Scale bar, 10 nm.) (B) Back-mapping of the actin structure into the tomogram revealed a mixed actin polarity in each tomogram. Actin filaments pointing to the pseudopodia tip (blue) were found to compose 74% ± 10% of the total filament numbers, while actin filaments pointing toward the cell body (red) represent 26% ± 10%. (C) The pseudopodia shown in Fig. 2 were used to overlay the polarity of the total actin filament content within protrusions. Actin filaments and segments pointing to the plasma membrane are colored in blue while the actin pointing toward the cell is colored in red. The total segmented actin filaments are depicted in light brown. Additionally, microtubules (purple), membrane (light turquoise) and receptors (green) were segmented as well. (Scale bar, 100 nm.)