Actin filament mechanics in the laser trap

Numerous biological processes, including muscular contraction, depend upon the mechanical properties of actin filaments. One such property is resistance to bending (flexural rigidity, EI). To estimate EI, we attached the ends of fluorescently labelled actin filaments to two microsphere 'handles' captured in independent laser traps. The positions of the traps were manipulated to apply a range of tensions (0-8 pN) to the filaments via the microsphere handles. With increasing filament tension, the displacement of the microspheres was inconsistent with a microsphere-filament system that is rigid. We maintain that this inconsistency is due to the microspheres rotating in the trap and the filaments bending near either attachments to accommodate this rotation. Fitting the experimental data to a simple model of this phenomena, we estimate actin's EI to be approximately 15 x 10(3) pNnm2, a value within the range of previously reported results, albeit using a novel method. These results both: support the idea that actin filaments are more compliant than historically assumed; and, indicate that without appropriately pretensioning the actin filament in similar laser traps, measurements of unitary molecular events (e.g. myosin displacement) may be significantly underestimated.