Abstract
Bacterial flagellar motor (BFM) is a large membrane-spanning molecular rotary machine for swimming motility. Torque is generated by the interaction between the rotor and multiple stator units powered by ion-motive force (IMF). The number of bound stator units is dynamically changed in response to the external load and the IMF. However, the detailed dynamics of stator unit exchange process remains unclear. Here, we directly measured the speed changes of sodium-driven chimeric BFMs under fast perfusion of different sodium concentration conditions using computer-controlled, high-throughput microfluidic devices. We found the sodium-driven chimeric BFMs maintained constant speed over a wide range of sodium concentrations by adjusting stator units in compensation to the sodium-motive force (SMF) changes. The BFM has the maximum number of stator units and is most stable at 5 mM sodium concentration rather than higher sodium concentration. Upon rapid exchange from high to low sodium concentration, the number of functional stator units shows a rapidly excessive reduction and then resurrection that is different from predictions of simple absorption model. This may imply the existence of a metastable hidden state of the stator unit during the sudden loss of sodium ions.
Highlights
The cell membrane is the barrier for life and the working place for many essential cellular functions
The stator-unit protein PomA is fused with enhanced green fluorescent protein (GFP), and the rotor protein FliN is fused with mCherry
We found that the chimeric Bacterial flagellar motor (BFM) has constant speed over a wide range of sodium ion concentrations, the stator occupancy does not saturate to its maximal capacity
Summary
The cell membrane is the barrier for life and the working place for many essential cellular functions. Membrane proteins show rich dynamics such as gating (Moreau et al, 2014), diffusing (Oswald et al, 2014; Lin et al, 2018), and exchange (Leake et al, 2006; Tusk et al, 2018; Armitage and Berry, 2020). Earlier investigations focused on the mechanical properties such as the gating mechanism or the diffusivity of membrane proteins. Very little is known for the membrane protein energetic dynamics. Sodium Dependent Stator Dynamics motor (BFM) as an example to study the energetic coupling dynamics of the protein complex with high-throughput and high-resolution optical measurements
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