Structure of the Bacillus cell fate determinant SpollAA 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 a factors. The activity of the first of these sigma factors, sigma (F), is restricted to the forespore although sigma (F) is present in the predivisional cell and partitions into both compartments following the asymmetric septation. For sigma (F) to become active, it must escape from a complex with its cognate anti-sigma factor, SpollAB. This relief from SpollAB inhibition requires the dephosphorylation of the anti-sigma factor antagonist, SpollAA. The phosphorylation state of SpollAA is thus a key determinant of sigma (F) activity and cell fate. Results: We have solved the crystal structures of SpollAA from Bacillus sphaericus in its phosphorylated and unphosphorylated forms. The overall structure consists of a central beta -pleated sheet, one face of which is buried by a pair of alpha helices, while the other is largely exposed to solvent. The site of phosphorylation, Ser(57), is located at the N terminus of helix alpha2. The phosphoserine is exceptionally well defined in the 1.2 Angstrom electron density maps, revealing that the structural changes accompanying phosphorylation are slight. Conclusions: Comparison of unphosphorylated and phosphorylated SpollAA 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.