- Type IV secretion systems are macromolecular assemblies in the cell envelopes of bacteria that function in macromolecular translocation. Structural biology approaches have provided insights into the interaction of core complex components, but information about proteins that undergo transient interactions with membrane components has not been forthcoming. We have pursued an unbiased approach using peptide arrays and phage display to identify interaction partners and interaction domains of type IV secretion system assembly factor VirB8. These approaches identified the globular domain from the VirB5 protein to interact with VirB8. This interaction was confirmed in cross-linking, pull-down, and fluorescence resonance energy transfer (FRET)-based interaction assays. In addition, using phage display analysis, we identified different regions of VirB6 as potential interaction partners of VirB8. Using a FRET-based interaction assay, we provide the first direct experimental evidence of the interaction of a VirB6 periplasmic domain with VirB8. These results will allow us to conduct directed structural biological work and structure-function analyses aimed at defining the molecular details and biological significance of these interactions with VirB8 in the future.