PspF‐binding domain PspA1–144 and the PspA·F complex: New insights into the coiled–coil‐dependent regulation of AAA+ proteins

Summary Phage shock protein A (PspA) belongs to the highy conserved PspA/IM30 family and is a key component of the stress inducible Psp system in Escherichia coli. One of its central roles is the regulatory interaction with the transcriptional activator of this system, the σ54 enhancer‐binding protein PspF, a member of the AAA+ protein family. The PspA/F regulatory system has been intensively studied and serves as a paradigm for AAA+ enzyme regulation by trans‐acting factors. However, the molecular mechanism of how exactly PspA controls the activity of PspF and hence σ54‐dependent expression of the psp genes is still unclear. To approach this question, we identified the minimal PspF‐interacting domain of PspA, solved its structure, determined its affinity to PspF and the dissociation kinetics, identified residues that are potentially important for PspF regulation and analyzed effects of their mutation on PspF in vivo and in vitro. Our data indicate that several characteristics of AAA+ regulation in the PspA·F complex resemble those of the AAA+ unfoldase ClpB, with both proteins being regulated by a structurally highly conserved coiled‐coil domain. The convergent evolution of both regulatory domains points to a general mechanism to control AAA+ activity for divergent physiologic tasks via coiled‐coil domains. PspA is the regulator of the AAA+ protein PspF, the transcriptional activator of the psp system in Escherichia coli. The crystal structure of the PspF‐binding domain of PspA indicates a conserved architecture of PspA‐like proteins, and data from mutational analyses suggest that PspF regulation might be more complex than previously thought. Similarities to the M‐domain of the AAA+ disaggregase ClpB point to convergently evolved characteristics in AAA+ regulation.