The Biology Of Cyclic Nucleotides In E Coli

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Structural and regulatory studies on protein components of the Escherichia coli (E. coli) sugar transport system known as the phosphoenolpyruvate:sugar phosphotransferase system (PTS) continued. The first component of the PTS (enzyme I, EI) is phosphorylated by phosphoenolpyruvate (PEP) on an active site histidine and that phosphoryl group can be transferred to the active site of the second component (HPr). EI contains two domains and its activity is regulated by a monomer-dimer equilibrium. New studies demonstrated that dimerization is strongly promoted by PEP and Mg, ligands which also stabilize and couple the unfolding of the two domains; phosphorylation of EI also promotes dimerization. P-HPr can interact with and transfer a phosphoryl group to IIAglc. The solution structure of the HPr-IIAglc complex was solved by NMR. A convex surface on HPr interacts with a complementary concave depression on IIAglc. Both binding surfaces comprise a central hydrophobic core region surrounded by a ring of polar and charged residues, positive for HPr and negative for IIAglc. Complex formation involves no change in backbone structures, but some conformational rearrangements of interfacial sidechains. While the N-terminal tail of IIAglc is not required for phosphoacceptance from HPr, it is necessary for phosphodonation to IICBglc. It was demonstrated that this tail can form an amphipathic helix in the presence of phospholipids, but is a random coil in the absence of lipid. IIAglc also regulates the activity of lactose permease (LP). Binding of IIAglc, promoted by substrate, to LP was demonstrated. Mutations in LP affect the binding; the study suggested that binding of various substrates to LP results in a collection of unique conformations, each of which presents a specific surface toward the inner face of the membrane that can interact to varying degrees with IIAglc. The global repressor, Mlc, binds to the dephospho-, but not the phospho-form of IICBglc. The binding results in transcription of Mlc-regulated genes by displacing Mlc from its target sequences. Therefore, the glucose induction of Mlc-regulated genes is caused by dephosphorylation of the membrane-bound transporter enzyme IICBglc, which directly recruits Mlc to derepress its regulon. A total of five review articles or book chapters were written describing the three dimensional structures of protein-protein complexes involving PTS proteins or the regulation by HPr of glycogen phosphorylase activity.