The ubiquitous FIC domain catalyzes post-translational modifications (PTMs) of target proteins; i.e.adenylylation (=AMPylation) and, more rarely, uridylylation and phosphocholination. Fic proteins arethought to play critical roles in intrinsic signaling processes of prokaryotes and eukaryotes; however, asubset encoded by bacterial pathogens is translocated via dedicated secretion systems into the cytoplasm ofmammalian host cells. Some of these host-targeted Fic proteins modify small GTPases leading to collapse ofthe actin cytoskeleton and other drastic cellular changes. Recently, we described a large set of functionallydiverse homologues in pathogens of the genus Bartonella that are required for their “stealth attack” strategyand persistent course of infection [1, 2]. Our preliminary functional analysis of some of these host-targetedFic proteins of Bartonella demonstrated adenylylation activity towards novel host targets (e.g. tubulin andvimentin). Moreover, in addition to the canonical adenylylation activity they may also display a competingkinase activity resulting from altered ATP binding to the FIC active site. Finally, we described a conservedmechanism of FIC active site auto- inhibition that is relieved by a single amino acid exchange [1], thusfacilitating functional analysis of any Fic protein of interest. Despite this recent progress only a few Ficproteins have been functionally characterized to date; our understanding of the functional plasticity of theFIC domain in mediating diverse target PTMs and their specific roles in infection thus remains limited.In this project, we aim to study the vast repertoire of host-targeted Fic proteins of Bartonella to: 1)identify novel target proteins and types of PTMs; 2) study their physiological consequences and molecularmechanisms of action; and 3) analyze structure-function relationships critical for FIC-mediated PTMs and infer from these data determinants of target specificity, type of PTM and mode of regulation. At the forefront of infection biology research, this project is ground-breaking as (i) we will identify aplethora of novel host target PTMs that are critical for a “stealth attack” infection strategy and thus will opennew avenues for investigating fundamental mechanisms of persistent infection; and (ii), we will unveil themolecular basis of the remarkable functional versatility of the structurally conserved FIC domain.