Inhibitory mechanism of the Qβ lysis protein A2

The lysis protein A(2), present as a single copy on the surface of Q beta virion particles, was previously shown to inhibit the activity of MurA, an enzyme that catalyses the first committed step of murein biosynthesis. Here we report experiments with a two-hybrid study that indicates A2 and MurA interact directly. Moreover, experiments with a soluble MBP-A(2) fusion indicate that the interaction between MurA and A(2) is dependent on a substrate-induced conformational change featured in the UDP-NAG-liganded state of MurA but not the tetrahedral intermediate state. Moreover, based on the location of L138Q, the original dominant A(2)-resistant mutant that identified MurA as the target, a directed mutagenesis strategy has identified a continuous surface required for A(2) binding. This surface spans the catalytic loop/cleft and encompasses both the catalytic and C-terminal domains. These data support a model in which A(2) preferentially binds MurA liganded with UDP-NAG, thereby preventing catalysis by occluding PEP from accessing the active site.