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

Background: The asymmetric cell division during sporulation in Bacillus sub tilis gives rise to two compartments: the mother cell and the forespore. Ea ch 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 mus t escape from a complex with its cognate anti-sigma factor, SpollAB. This r elief from SpollAB inhibition requires the dephosphorylation of the anti-si gma factor antagonist, SpollAA. The phosphorylation state of SpollAA is thu s a key determinant of sigma (F) activity and cell fate. Results: We have solved the crystal structures of SpollAA from Bacillus sph aericus in its phosphorylated and unphosphorylated forms. The overall struc ture 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 heli x alpha2. The phosphoserine is exceptionally well defined in the 1.2 Angstr om electron density maps, revealing that the structural changes accompanyin g phosphorylation are slight. Conclusions: Comparison of unphosphorylated and phosphorylated SpollAA show s that covalent modification has no significant effect on the global struct ure 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 structur al reorganization in many eukaryotic signaling systems.