In the initial funding period of the SNSF grant 31003A-132979 we used Bartonella and Brucella – two related zoonotic pathogens engaging the widespread type IV secretion (T4S) mechanism for establishing chronic bacterial infection - as models to study the cellular, molecular and evolutionary basis of the subversion of host cell functions by T4S-translocated effector proteins. For the three year prolongation period the established multidisciplinary experimental approach will be extended by adopting yeast as surrogate model for studying conserved eukaryotic processes targeted by T4S effectors. Subproject A will focus on the structure/function analysis of Bartonella effector proteins (Beps) translocated by the VirB T4S system of Bartonella and their physiological consequences on the host, with particular emphasis on studying the subversion of immune signaling processes. Subproject B will focus on the functional analysis of effector proteins translocated by the distinct VirB T4S system of Brucella and their physiological consequences on host cell interaction, with particular emphasis on studying the intracellular trafficking events from a late endosomal compartment, where the VirB system is activated in response to acidification, to the endoplasmic reticulum (ER)-related replicative niche of Brucella. Together, these studies will contribute to establishing a molecular paradigm for the role of T4S effectors in triggering chronic bacterial infection.