Structural basis of Fic mediated adenylylation

The Fic family of adenylyltransferases, defined by a core HPFx(D/E)GN(G/K)R motif, consist of over 2700 proteins found from bacteria to humans. IbpA from the bacterial pathogen Histophilus somni contains two Fic domains that adenylylate the switch1 Tyr residue of Rho-family GTPases, allowing the bacteria to subvert host defenses. Here we present the structure of the second Fic domain of IbpA (IbpAFic2) in complex with its substrate, Cdc42. IbpAFic2-bound Cdc42 mimics the GDI-bound state of Rho GTPases, with both its switch1 and switch2 regions gripped by IbpAFic2. Mutations disrupting the IbpAFic2-Cdc42 interface impair adenylylation and cytotoxicity. Importantly, the switch1 Tyr of Cdc42 is adenylylated in the structure, providing the first structural view for this post-translational modification. We also demonstrate that the nucleotide-binding mechanism is conserved among Fic proteins, and propose a catalytic mechanism for this recently discovered family of enzymes.