FIGURE 2.

α-Actinin is a flexible cross-linker that cross-links actin in all orientations examined. A–C, gallery shows the formation of a single cross-link by α-actinin at a 90° cross of two actin filaments on three beads (supplemental Movie S1). d--H, α-actinin bundles aligned filaments. The filament marked in red is strung between the two beads and wraps around the top bead. The blue filament is strung between the two beads. All three filament segments bind into a tight bundle. This arrangement necessitates the formation of anti-parallel cross-links and could include parallel cross-links as well (supplemental Movie S2). I–P, α-actinin cross-link is stable, remaining bound while the link is rotated and pulled in various directions. Gallery shows a 48-s range. This link was observed for more than 3 min before the complex fell out of the optical traps (supplemental Movie S3). Scale bar in all figures equals 1 μm.