Cross-correlated TIRF/AFM shows Self-assembled Synthetic Myosin Filaments are Asymmetric - Implications for Motile Filaments

TL;DR

This study uses combined AFM and TIRF imaging to reveal that self-assembled synthetic myosin filaments are inherently asymmetric, which has significant implications for understanding force generation and motility in cellular systems.

Contribution

It introduces a novel cross-correlation imaging approach to determine the asymmetric structure of myosin filaments and models how this asymmetry influences cellular force dynamics.

Findings

Myosin filaments are asymmetric with more motors on one half.

Fluorescence intensity correlates with filament height, indicating a shell of myosin heads.

Estimated motor density is approximately 50-100 heads per micron.

Abstract

Myosin-II's rod-like tail drives filament assembly with a head arrangement that should generate equal and opposite contractile forces on actin--if one assumes that the filament is a symmetric bipole. Self-assembled myosin filaments are shown here to be asymmetric in physiological buffer based on cross-correlated images from both atomic force microscopy (AFM) and total internal reflection fluorescence (TIRF). Quantitative cross-correlation of these orthogonal methods produces structural information unavailable to either method alone in showing that fluorescence intensity along the filament length is proportional to height. This implies that myosin heads form a shell around the filament axis, consistent with F-actin binding. A motor density of ~50 - 100 heads/micron is further estimated but with an average of 32% more motors on one half of any given filament compared to the other, regardless of length. A purely entropic pyramidal lattice model is developed that qualitatively captures this lack of length dependence and the distribution of filament asymmetries. Such strongly asymmetric bipoles are likely to produce an imbalanced contractile force in cells and in actin-myosin gels, and thereby contribute to motility as well as cytoskeletal tension.