title: The BID Domain of Type IV Secretion Substrates Forms a Conserved Four-Helix Bundle Topped with a Hook.
abstract: The BID (Bep intracellular delivery) domain functions as secretion signal in a subfamily of protein substrates of bacterial type IV secretion (T4S) systems. It mediates transfer of (1) relaxases and the attached DNA during bacterial conjugation, and (2) numerous Bartonella effector proteins (Beps) during protein transfer into host cells infected by pathogenic Bartonella species. Furthermore, BID domains of Beps have often evolved secondary effector functions within host cells. Here, we provide crystal structures for three representative BID domains and describe a novel conserved fold characterized by a compact, antiparallel four-helix bundle topped with a hook. The conserved hydrophobic core provides a rigid scaffold to a surface that, despite a few conserved exposed residues and similarities in charge distribution, displays significant variability. We propose that the genuine function of BID domains as T4S signal may primarily depend on their rigid structure, while the plasticity of their surface may facilitate adaptation to secondary effector functions.
authors.affiliation: Focal Area Infection Biology, Biozentrum University of Basel, 4056 Basel, Switzerland; Focal Area Structural Biology and Biophysics, Biozentrum University of Basel, 4056 Basel, Switzerland.
authors.forename: Frédéric V
authors.initials: FV
authors.lastname: Stanger
authors.affiliation: Focal Area Infection Biology, Biozentrum University of Basel, 4056 Basel, Switzerland.
authors.forename: Tjaart A P
authors.initials: TAP
authors.lastname: de Beer
authors.affiliation: Seattle Structural Genomics Center for Infectious Disease, The Center for Infectious Disease Research, Seattle, WA 98109, USA; Beryllium Discovery Corp., Bainbridge Island, WA 98110, USA.
authors.forename: David M
authors.initials: DM
authors.lastname: Dranow
authors.affiliation: Focal Area Structural Biology and Biophysics, Biozentrum University of Basel, 4056 Basel, Switzerland. Electronic address: tilman.schirmer@unibas.ch.
authors.forename: Tilman
authors.initials: T
authors.lastname: Schirmer
authors.affiliation: Seattle Structural Genomics Center for Infectious Disease, The Center for Infectious Disease Research, Seattle, WA 98109, USA.
authors.forename: Isabelle
authors.initials: I
authors.lastname: Phan
authors.affiliation: Focal Area Infection Biology, Biozentrum University of Basel, 4056 Basel, Switzerland. Electronic address: christoph.dehio@unibas.ch.
authors.forename: Christoph
authors.initials: C
authors.lastname: Dehio
mesh_terms: D001473:Bartonella / Q000737:chemistry / Q000378:metabolism*; D001665:Binding Sites; D017124:Conserved Sequence; D018360:Crystallography, X-Ray; D008958:Models, Molecular; D000072417:Protein Domains; D017433:Protein Structure, Secondary; D000069376:Type VI Secretion Systems / Q000737:chemistry*
chemical_list: D000069376:Type VI Secretion Systems
keywords: BID domain; Bartonella effector proteins (Beps); bacterial conjugation; four-helix bundle; novel fold; relaxases; secretion signal; secretion system; type IV secretion system (T4SS)