STRUCTURAL STUDIES OF THE ESCHERICHIA-COLI SIGNAL-TRANSDUCING PROTEINIIA(GLC) - IMPLICATIONS FOR TARGET RECOGNITION

In Escherichia coli, the glucose-specific phosphocarrier protein of th e phosphotransferase system (PTS), IIA(Glc) (IIIGlc in older literatur e), is also the central regulatory protein of the PTS. Depending upon its state of phosphorylation, IIA(Glc) binds to a number of different proteins that display no apparent sequence homology. Previous structur al studies suggested that nonspecific hydrophobic interactions, specif ic salt bridges, and an intermolecular Zn(IT) binding site contribute to the wide latitude in IIA(Glc) binding sites. Two new crystal forms of IIA(Glc) have been solved at high resolution, and the models were c ompared to those previously studied. The major intermolecular contacts in the crystals differ in detail, but all involve the hydrophobic act ive site of IIA(Glc) interacting with a hydrophobic patch on a neighbo r and all are shown to be surprisingly similar to the physiologically relevant regulatory interaction of IIA(Glc) with glycerol kinase. In t wo crystal forms, a helix on one molecule interacts with the face of a nother, while in the other crystal form, the primary crystal contact c onsists of a strand of beta-sheet that contributes to an intermolecula r Zn(II) binding site with tetrahedral ligation identical to that of t he zinc peptidase thermolysin. Thus, relatively nonspecific hydrophobi c interactions combined with specific salt bridges and an intermolecul ar cation binding site (cation-promoted association) permit a regulato ry protein to bind to target proteins that have little or no sequence or structural homology with one another. It is suggested that signal t ransduction by IIA(Glc) is a binary switch in which phosphorylation at the active site directly controls binding to target molecules.