Mechanism of Homotropic Control to Coordinate Hydrolysis in a Hexameric AAA+ Ring ATPase

AAA+ proteins are ubiquitous mechanochemical ATPases that use energy from ATP hydrolysis to remodel their versatile substrates. The AAA+ characteristic hexameric ring assemblies raise important questions about if and how six often identical subunits coordinate hydrolysis and associated motions. The PspF AAA+ domain, PspF1–275, remodels the bacterial σ54–RNA polymerase to activate transcription. Analysis of ATP substrate inhibition kinetics on ATP hydrolysis in hexameric PspF1–275 indicates negative homotropic effects between subunits. Functional determinants required for allosteric control identify: (i) an important link between the ATP bound ribose moiety and the SensorII motif that would allow nucleotide-dependent α-helical α/β subdomain dynamics; and (ii) establishes a novel regulatory role for the SensorII helix in PspF, which may apply to other AAA+ proteins. Consistent with functional data, homotropic control appears to depend on nucleotide state-dependent subdomain angles imposing dynamic symmetry constraints in the AAA+ ring. Homotropic coordination is functionally important to remodel the σ54 promoter. We propose a structural symmetry-based model for homotropic control in the AAA+ characteristic ring architecture.