Filopodia are finger-like protrusions at the leading edge of migrating cells that play a crucial antennal function during cell motility. It is known that actin filaments are bundled hexagonally and provide rigidity to filopodia by virtue of fascin, which plays a central role in actin filament bundling. However, the molecular mechanisms underlying their formation remain unclear. Here, we observed the filopodia of intact whole cells fixed by rapid freezing and revealed their three-dimensional structure by cryo-electron tomography and image processing; the actin filament bundling structure by fascin was clarified at high resolution under physiological conditions. It was found that actin filaments in vivo were more numerous than in bundles reconstructed in vitro, and each filopodial actin filament had limited variability in helical twisting. In addition, statistical analysis of actin filament bundles unveiled their detailed architecture. In filopodia, actin filaments had highly ordered structures, and the shift between cross-links of each adjacent actin filament was approximately 2.7 nm, similar to the monomer repeat of actin filaments. We then proposed a plausible actin-fascin cross-link model at the amino acid level and identified three fascin binding sites on two adjacent actin filaments: one filament bound fascin at two discrete, widely separated regions and the other bound fascin in a single small region. We propose that these two different binding modalities should confer rigid bundles that retain flexibility and dynamic performance.
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