CATION-PROMOTED ASSOCIATION OF A REGULATORY AND TARGET PROTEIN IS CONTROLLED BY PROTEIN-PHOSPHORYLATION

A central question in molecular biology concerns the means by which a regulatory protein recognizes different targets. IIIGlc, the glucose-s pecific phosphocarrier protein of the bacterial phosphotransferase sys tem, is also the central regulatory element of the PTS. Binding of unp hosphorylated IIIGlc inhibits several non-PTS proteins, but there is l ittle or no sequence similarity between IIIGlc binding sites on differ ent target proteins. The crystal structure of Escherichia coli IIIGlc bound to one of its regulatory targets, glycerol kinase, has been refi ned at 2.6-Angstrom resolution in the presence of products, adenosine diphosphate and glycerol 3-phosphate. Structural and kinetic analyses show that the complex of IIIGlc with glycerol kinase creates an interm olecular Zn(II) binding site with ligation identical to that of the zi nc peptidase thermolysin. The zinc is coordinated by the two active si te histidines of IIIGlc, a glutamate of glycerol kinase, and a water m olecule. Zn(II) at 0.01 and 0.1 mM decreases the K-i of IIIGlc for gly cerol kinase by factors of about 15 and 60, respectively. The phosphor ylation of one of the histidines of IIIGlc, in its alternative role as phosphocarrier, provides an elegant means of controlling the cation-e nhanced protein-protein regulatory interaction. The need for the targe t protein to supply only one metal ligand may account for the lack of sequence similarity among the regulatory targets of IIIGlc.