Motor proteins, myosin, and kinesin have γ-phosphate sensors in the switch II loop that play key roles in conformational changes that support motility. Here we report that a rotary motor, F1-ATPase, also changes its conformations upon phosphate release. The tryptophan mutation was introduced into Arg-333 in the β subunit of F1-ATPase from thermophilic Bacillus PS3 as a probe of conformational changes. This residue interacts with the switch II loop (residues 308-315) of the β subunit in a nucleotide-bound conformation. The addition of ATP to the mutant F1 subcomplex α3β(R333W)3γ caused transient increase and subsequent decay of the Trp fluorescence. The increase was caused by conformational changes on ATP binding. The rate of decay agreed well with that of phosphate release monitored by phosphate-binding protein assays. This is the first evidence that the β subunit changes its conformation upon phosphate release, which may share a common mechanism of exerting motility with other motor proteins.
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