Modulation of host cell function by bacterial type IV secretion: Molecular basis of the translocation process and the intracellular effector function of secreted virulence factors

Type IV secretion (T4S) is a virulence mechanism shared by several unrelated pathogenic bacteria. By studying T4S in Bartonella we anticipate to provide molecular mechanistic insights into this major virulence mechanism. Based on the subversion of vascular endothelial cell functions by the T4S system of Bartonella, we further expect to contribute new knowledge and tools for the field of vascular biology.

Background
The type IV secretion (T4S) systems of gram-negative bacteria mediate the transfer of macromolecular substrates into various target cells, e.g., conjugative DNA transfer into recipient bacteria and intracellular delivery of effector proteins into eukaryotic target cells. Virulence-associated T4S systems are widely distributed among human pathogenic bacteria, including members of the genus Bartonella. These widespread mammalian pathogens typically cause persistent infection of endothelial cells and erythrocytes, which may result in a broad range of clinical manifestations, including fever and hemolytic anemia as the result of erythrocyte invasion, and the formation of vascular tumors due to massive proliferation of endothelial cells. Bartonellae encode two distinct T4S systems which are both essential for pathogenesis. The Trw T4S system is required for erythrocyte infection. In contrast, the VirB/VirD4 T4S system mediates subversion of vascular endothelial cell functions, e.g. in relation to cytoskeleton, inflammation, apoptosis, and proliferation. The latter cellular changes are all dependent on the translocation of seven effector proteins (BepA-BepG). Translocation of these Beps depends on a C-terminal signal, while their N-termini are considered to contribute to effector functions within host cells.

Aims
We are studying the Bartonella VirB/VirD4 T4S system and the thereby translocated Bep effector proteins in regard to (i) the molecular interactions of the T4S apparatus with the C-terminal translocation signal of the Beps, and (ii) the effector functions of individual Beps within endothelial cells. We are also testing the suitability of this translocation system for intracellular delivery of heterologous protein and DNA substrates into host cells. In the frame of this project we apply a multidisciplinary approach including bacterial genetics, cell biology, molecular biology, biochemistry, structural biology, genomics, and animal experimentation. The focus of our studies is the structure/function analysis of the Beps, including an interaction analysis (i) with components of the VirB/VirD4 T4S apparatus during translocation into host cells and (ii) with their cellular targets within host cells, which modulate Bep effector functions.