Heterogenous Population of Kinesin-Streptavidin Complex Revealed by Mass Photometry*

The kinesin-streptavidin complex is widely used to drive filament-filament sliding in microtubule-based active matter studies. Although the stoichiometry of the kinesin-streptavidin complex is generally assumed to be 2:1, this assumption has not been experimentally verified. Here we employ mass photometry, a label-free single-molecule technique, to determine the mass of individual kinesin-streptavidin complexes in solution. We found that the complex population is heterogenous, although the relative abundance of different complexes depends sensitively on the kinesin:streptavidin incubation ratio. We identify an incubation ratio that maximizes the 2:1 complex stoichiometry optimal for filament-filament sliding in active matter studies.