Structure of the Bacillus cell fate determinant SpoIIAA in phosphorylated and unphosphorylated forms

Background: The asymmetric cell division during sporulation in Bacillus subtilis gives rise to two compartments: the mother cell and the forespore. Each follow different programs of gene expression coordinated by a succession of alternate RNA polymerase σ factors. The activity of the first of these σ factors, σF, is restricted to the forespore although σF is present in the predivisional cell and partitions into both compartments following the asymmetric septation. For σF to become active, it must escape from a complex with its cognate anti-σ factor, SpoIIAB. This relief from SpoIIAB inhibition requires the dephosphorylation of the anti-σ factor antagonist, SpoIIAA. The phosphorylation state of SpoIIAA is thus a key determinant of σF activity and cell fate.

Results: We have solved the crystal structures of SpoIIAA from Bacillus sphaericus in its phosphorylated and unphosphorylated forms. The overall structure consists of a central β-pleated sheet, one face of which is buried by a pair of α helices, while the other is largely exposed to solvent. The site of phosphorylation, Ser57, is located at the N terminus of helix α2. The phosphoserine is exceptionally well defined in the 1.2 Å electron density maps, revealing that the structural changes accompanying phosphorylation are slight.

Conclusions: Comparison of unphosphorylated and phosphorylated SpoIIAA shows that covalent modification has no significant effect on the global structure of the protein. The phosphoryl group has a passive role as a negatively charged flag rather than the active role it plays as a nucleus of structural reorganization in many eukaryotic signaling systems.