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 sigma factors. The activity of the first of these sigma factors, sigmaF, is restricted to the forespore although sigmaF is present in the predivisional cell and partitions into both compartments following the asymmetric septation. For sigmaF to become active, it must escape from a complex with its cognate anti-sigma factor, SpoIIAB. This relief from SpoIIAB inhibition requires the dephosphorylation of the anti-sigma factor antagonist, SpoIIAA. The phosphorylation state of SpoIIAA is thus a key determinant of sigmaF 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 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, Ser57, is located at the N terminus of helix alpha2. The phosphoserine is exceptionally well defined in the 1.2 A 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.
Seavers
P R
PR
Structural Biology Laboratory, Department of Chemistry, University of York, YO10 5DD, York, United Kingdom.
Lewis
R J
RJ
Brannigan
J A
JA
Verschueren
K H
KH
Murshudov
G N
GN
Wilkinson
A J
AJ
D000595:Amino Acid Sequence; D001407:Bacillus / Q000737:chemistry* / Q000502:physiology; D001426:Bacterial Proteins / Q000737:chemistry* / Q000378:metabolism*; D018360:Crystallography, X-Ray; D008958:Models, Molecular; D008969:Molecular Sequence Data; D010766:Phosphorylation; D011487:Protein Conformation; D017386:Sequence Homology, Amino Acid; D012808:Sigma Factor; D013171:Spores, Bacterial / Q000502:physiology; D014157:Transcription Factors
D001426:Bacterial Proteins; D012808:Sigma Factor; D014157:Transcription Factors; C092468:spoIIR protein, Bacillus subtilis; C030069:spore-specific proteins, Bacillus