Tracking single myosin II filament dynamics during acto-myosin network remodeling to understand the role of mechanical feedback in the process of pattern formation

Darius Köster^1,2#,3*, Nikolas Hundt⁴, Lewis Mosby^2,5, Saravanan Palani^1,2, Gavin Young⁴, Adam Fineberg⁴,
Marco Polin^2,5, Philipp Kukura⁴, Satyajit Mayor^3,6

1 Division of Biomedical Sciences, Warwick Medical School, University of Warwick, Coventry CV4 7AL, UK
2 Centre for Mechanochemical Cell Biology, University of Warwick, Coventry CV4 7AL, UK
3 National Centre for Biological Sciences, Tata Institute for Fundamental Research, GKVK, Bellary Road, Bangalore 560065, India
4 Physical and Theoretical Chemistry Laboratory, Department of Chemistry, University of Oxford, Oxford OX1 QZ, UK
5 Physics Department, University of Warwick, Coventry CV4 7AL, UK
6 Institute for Stem Cell Biology and Regenerative Medicine, Bangalore 560065, India
# current address
* corresponding author: d.koester@warwick.ac.uk

Abstract
The plasma membrane and the underlying cytoskeletal cortex constitute active platforms for many
cellular processes. Recent work has shown that the remodeling acto-myosin network modifies local
membrane organization, but the molecular details are poorly understood due to difficulties with
experimentally accessing the relevant time and length scales. Here, we use interferometric scattering
(iSCAT) microscopy to investigate a minimal, membrane linked acto-myosin network. Using fast
acquisition and the magnitude of the interferometric contrast, proportional to molecular mass, we
detect and follow individual actin and myosin II filaments moving within the acto-myosin network. We
develop a method to track individual myosin II filaments, which allows to quantify dwell times,
orientation, bound state (weak/strong) and processivity as function of ATP concentration. Myosin II
filament dwell times, velocities and processivities increase with decreasing ATP concentrations and this
can be linked to global changes of the acto-myosin network from a remodeling to a contractile state.
This provides novel, experimental data to test recent theoretical models of myosin ensembles under
load. In addition, we identify conditions that lead to formation of acto-myosin spirals and vortices.
These were predicted by theory, but not observed experimentally yet, and underline the relevance of
acto-myosin generated torques for network dynamics.