Interactions between single actin and vimentin filaments
The cytoskeleton determines cell shape, mechanical properties, and motility by interconnected networks of protein filaments - actin filaments, microtubules and intermediate filaments. Their collective function relies on crosstalk between these filament systems, yet the physical basis of interactions between the filaments remains insufficiently understood. Actin and vimentin filaments and networks frequently co-localize within cells and jointly regulate contractility, force transmission and mechanical resilience, indicating functional cooperation. However, it remains unclear whether these interactions arise from direct filament-filament interactions or are mediated exclusively by accessory crosslinking proteins. Studies of reconstituted composite networks probing direct interactions by rheology have yielded inconsistent results. Here, we show that single actin filaments and vimentin intermediate filaments do indeed interact directly, forming force-bearing contacts in the absence of crosslinking proteins, with interaction strengths comparable to other previously reported cytoskeletal filament pairs. Using quadruple optical tweezers combined with microfluidics and confocal microscopy, we systematically probe these interactions under controlled conditions across a range of ionic environments. We find that, in contrast to other filament pairs, variations in ionic strength do not appreciably affect the interaction breaking forces between actin and vimentin filaments. However, the interaction geometry determines the achievable interaction strengths, because the limited stretchability of the actin filaments sets an upper bound to the measurable force range. This limit also imposes a restriction on direct quantification of interaction parameters, which we circumvent by a Bayesian unmasking strategy that allows us to infer bond parameters despite the breaking of actin filaments. Furthermore, actin bundling enhances the stability against breaking, enabling the detection of higher interaction forces. These findings demonstrate that actin and vimentin form a mechanically interacting system through direct filament bonds, and our work establishes a minimal, protein-linker-independent physical basis for actin-vimentin crosstalk and quantifies the interaction forces between the two filament types.