The cofactor-induced pre-active conformation in PhoB.
PhoB is an Escherichia coli transcription factor from a two-component signal transduction system that is sensitive to limiting environmental phosphate conditions. It consists of an N-terminal receiver domain (RD) and a C-terminal DNA-binding domain. The protein is activated upon phosphorylation at the RD, an event that depends on Mg(2+) binding. The structure of PhoB RD in complex with Mg(2+) is presented, which shows three protomers in the asymmetric unit that interact across two different surfaces. One association is symmetric and has been described as a non-active dimerization contact; the other involves the alpha4-beta5-alpha5 interface and recalls the contact found in activated PhoB. However, here this last interaction is not perfectly symmetric and helix alpha4, which in the activated molecule undergoes a helical shift, becomes strongly destabilized in one of the interacting monomers. All protomers bind the cation in a similar manner but, interestingly, at the prospective binding site for the phosphate moiety the side chains of either Glu88 (in helix alpha4) or Trp54 alternate and interact with active-site atoms. When Glu88 is inside the cavity, helix alpha4 is arranged similarly to the unliganded wild-type structure. However, if Trp54 is present, the helix loses its contacts with the active-site cavity and vanishes. Accordingly, the presence of Trp54 in the active site induces a flexible state in helix alpha4, potentially allowing a helical shift that phosphorylation would eventually stabilize.
Solà
Maria
M
Institut de Biologia Molecular de Barcelona, CSIC, Parc Científic de Barcelona, c/ Josep Samitier 1-5, 08028 Barcelona, Spain. msvcri@ibmb.csic.es
Drew
Devin L
DL
Blanco
Alexandre G
AG
Gomis-Rüth
F Xavier
FX
Coll
Miquel
M
D001426:Bacterial Proteins / Q000737:chemistry*; D001665:Binding Sites; D002412:Cations; D018360:Crystallography, X-Ray; D004926:Escherichia coli / Q000378:metabolism*; D008274:Magnesium / Q000737:chemistry; D008670:Metals / Q000737:chemistry; D008968:Molecular Conformation; D010766:Phosphorylation; D011487:Protein Conformation; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D013499:Surface Properties; D014364:Tryptophan / Q000737:chemistry
D001426:Bacterial Proteins; D002412:Cations; D008670:Metals; C061278:PhoB protein, Bacteria; D014364:Tryptophan; D008274:Magnesium