Minimum and Maximum Limit to Number of Myosin II Motors Participating in an Ensemble Motility

Abstract

Myosin II is an ensemble motor that participates in muscle contraction, actin cortex remodeling and cytokinesis ring constriction. It has been an interesting puzzle for scientific community to understand the allosteric rules between motors that regulate/co-ordinate when they work in group. We have performed classical In Vitro motility assay at different heavy meromyosin (HMM) density and ATP concentrations. By changing immobilized HMM density and ATP concentration, we change the number of HMM molecules available for interaction/unit length of actin filament. Actin filaments of >10 micron length were added in flow cell and assay was initiated by addition of ATP. Actin filaments breaks down to smaller pieces within minutes after addition of ATP. Average length of sliding filaments correlates well with motors density and ATP concentrations in solution. At 633 head/µm2 density and 2 mM ATP concentration the average filament length was 1747.6±614 nm and further reduced to 1290.5±385.5 nm at 0.1 mM ATP. At 4000 heads/µm2 density and 2 mM ATP concentration the average filament length was 935.5±286 nm and reduced to 599.5±154.2 nm at 0.1 mM ATP. Any filament below this average length does not slide continuously, they detach from surface with time and any filament above this average length fragments to smaller pieces with time. Assuming Poisson distribution of immobilized heads, we calculated maximum number of molecules that can interact for a given actin length. Number of molecules required for continuous sliding is independent of motor density and depends only on ATP concentration. For 2 mM ATP concentration, 123.6±28.9 heads and for 0.1 mM ATP concentration 62.3 ±19.5 heads are required for continuous sliding of actin. All values are mean ± SD.Supported by DST, Government of India.