Structure and mechanism of an actin-dependent bacterial phosphoryl AMPylase.

The two effectors LnaB and MavL of Legionella pneumophila coordinate the conversion of phosphoribosyl ubiquitin (PR-Ub) released by reversal of ubiquitination induced by members of the SidE effector family into functional Ub. LnaB acts as an actin-dependent phosphoryl AMPylase that converts PR-Ub into ADP-ribosylated (ADPR)-Ub. Catalysis by LnaB requires the conserved SHE motif present in a large family of bacterial toxins. Here we describe a series of structures of LnaB in complex with the cofactor actin and the substrate PR-Ub and ATP. LnaB harbors both adenylyltransferase and ATPase activities, which reveal an adenylylation mechanism involved in a two-step catalytic process. Actin performs a unique activation mechanism that promotes the recruitment of PR-Ub by LnaB to activate LnaB's ATPase activity through interacting with LnaB and PR-Ub. Mechanisms derived from this series of structures covering the process of LnaB action establish an important biochemical basis for protein AMPylation.