Organization of the AAA+ adaptor protein PspA is an oligomeric ring

The 25.3 kDa adaptor protein, PspA (dphage shock protein A), is found in the cytoplasm and in association with the inner membrane of certain bacteria. PspA plays critical roles in negatively regulating the phage shock response and maintaining membrane integrity, especially during the export of proteins such as virulence factors. Homologues of PspA function exist for thylakoid biogenesis. Here we report the first three-dimensional reconstruction of a PspA assembly from Escherichia coli, visualized by electron microscopy and single particle analysis to a resolution of 30 Angstrom. The assembly forms a 9-fold rotationally symmetric ring with an outer diameter of 200 Angstrom, an inner diameter of 95 Angstrom, and a height of similar to85 Angstrom. The molecular mass of the complex was calculated to be 1023 kDa by size exclusion chromatography, suggesting that each of the nine domains is likely to be composed of four PspA subunits. The functional implications of this PspA structure are discussed in terms of its interaction with the protein export machinery of the bacterial cell and its AAA(+) protein partner, PspF.