Translocation of YopJ family effector proteins through the VirB/VirD4 T4SS of <i>Bartonella</i>.

Abstract

The evolutionary conserved YopJ family comprises numerous type-III-secretion system (T3SS) effectors of diverse mammalian and plant pathogens that acetylate host proteins to dampen immune responses. Acetylation is mediated by a central acetyltransferase domain that is flanked by conserved regulatory sequences, while a nonconserved N-terminal extension encodes the T3SS-specific translocation signal. <i>Bartonella</i> spp. are facultative-intracellular pathogens causing intraerythrocytic bacteremia in their mammalian reservoirs and diverse disease manifestations in incidentally infected humans. Bartonellae do not encode a T3SS, but most species possess a type-IV-secretion system (T4SS) to translocate <i>Bartonella</i> effector proteins (Beps) into host cells. Here we report that the YopJ homologs present in Bartonellae species represent genuine T4SS effectors. Like YopJ family T3SS effectors of mammalian pathogens, the "<i>Bartonella</i> YopJ-like effector A" (ByeA) of <i>Bartonella taylorii</i> also targets MAP kinase signaling to dampen proinflammatory responses, however, translocation depends on a functional T4SS. A split NanoLuc luciferase-based translocation assay identified sequences required for T4SS-dependent translocation in conserved regulatory regions at the C-terminus and proximal to the N-terminus of ByeA. The T3SS effectors YopP from <i>Yersinia enterocolitica</i> and AvrA from <i>Salmonella</i> Typhimurium were also translocated via the <i>Bartonella</i> T4SS, while ByeA was not translocated via the <i>Yersinia</i> T3SS. Our data suggest that YopJ family T3SS effectors may have evolved from an ancestral T4SS effector, such as ByeA of <i>Bartonella</i>. In this evolutionary scenario, the signal for T4SS-dependent translocation encoded by N- and C-terminal sequences remained functional in the derived T3SS effectors due to the essential role these sequences coincidentally play in regulating acetyltransferase activity.