FIGURE 2:

Filament bundling enhances rate of processive elongation. Conditions as in Figure 1, except 1 or 1.2 μM actin as indicated and total MgCl₂ concentration of 1 mM (A–C) or 10 mM (D–F). (A) Processive association of actin filaments to GST-PP-WWCA–coated nanofibers (dotted outlines) in 1.2 μM Mg-ATP-actin and 1 mM Mg²⁺, showing buckling of rapidly elongating filaments (white arrowheads) from compression between barbed ends (black arrowheads) and NEM-myosin attachment points. (B, C) Elongation of barbed (black) and pointed (gray) ends of untethered filaments (B) or tethered buckles (C) in 1 mM Mg²⁺. (D) Buckling filament (white arrowheads) with barbed end (black arrowheads) overlapping a nonbuckling filament (white arrow) attached to the end of a nanofiber (dotted outlines) in 10 mM Mg²⁺. (E, F) Elongation of untethered filaments (E) or tethered filaments (F) in 10 mM Mg²⁺. Barbed-end growth rates shown in Table 1. Scale bars, 10 μm.