A helical RGD motif promoting cell adhesion: crystal structures of the Helicobacter pylori type IV secretion system pilus protein CagL.
RGD tripeptide motifs frequently mediate ligand binding to integrins. The type IV secretion system (T4SS) protein CagL of the gastric pathogen Helicobacter pylori also contains an RGD motif. CagL decorates the T4SS pilus and may function as an adhesin for host cells. Whether CagL binds integrins via its RGD motif is under debate. Here, we present crystal structures of CagL revealing an elongated four-helix bundle that appears evolutionarily unrelated to the proposed VirB5 orthologs. The RGD motif is surface-exposed but located within a long α helix. This is unprecedented as previously characterized integrin-binding RGD motifs are located within extended or flexible loops. Yet, adhesion of gastric epithelial cells to CagL was strictly RGD-dependent. Comparison of seven crystallographically independent molecules reveals substantial structural flexibility. Intramolecular disulfide bonds engineered to reduce CagL flexibility resulted in more stable protein, but unable to support cell adhesion. CagL may thus partly unfold during receptor binding.
Structural Biochemistry, Department of Chemistry, Bielefeld University, 33501 Bielefeld, Germany.
Stephan
S
Barden
Stefanie
S
Lange
Nicole
N
Tegtmeyer
Jens
J
Conradi
Norbert
N
Sewald
Steffen
S
Backert
Hartmut H
HH
Niemann
D001422:Bacterial Adhesion; D001426:Bacterial Proteins / Q000737:chemistry*; D058947:Bacterial Secretion Systems; D002478:Cells, Cultured; D018360:Crystallography, X-Ray; D003553:Cystine / Q000737:chemistry; D004847:Epithelial Cells; D016480:Helicobacter pylori; D006801:Humans; D006860:Hydrogen Bonding; D057927:Hydrophobic and Hydrophilic Interactions; D008958:Models, Molecular; D054730:Protein Interaction Domains and Motifs; D055550:Protein Stability; D017433:Protein Structure, Secondary
D001426:Bacterial Proteins; D058947:Bacterial Secretion Systems; C568315:cagL protein, Helicobacter pylori; D003553:Cystine