Department of Theoretical Biophysics, Max Planck Institute of Biophysics, Frankfurt, Germany
Ricardo Sánchez
Sofja Kovalevskaja Group, Max Planck Institute of Biophysics, Frankfurt, Germany
Ilona Rose
Molecular Microbiology and Bioenergetics, Institute of Molecular Biosciences, Goethe University Frankfurt, Frankfurt, Germany
Lennart Kirchner
Molecular Microbiology and Bioenergetics, Institute of Molecular Biosciences, Goethe University Frankfurt, Frankfurt, Germany
Gerhard Hummer
Department of Theoretical Biophysics, Max Planck Institute of Biophysics, Frankfurt, Germany; Institute of Biophysics, Goethe University Frankfurt, Frankfurt, Germany
Secretins form multimeric channels across the outer membrane of Gram-negative bacteria that mediate the import or export of substrates and/or extrusion of type IV pili. The secretin complex of Thermus thermophilus is an oligomer of the 757-residue PilQ protein, essential for DNA uptake and pilus extrusion. Here, we present the cryo-EM structure of this bifunctional complex at a resolution of ~7 Å using a new reconstruction protocol. Thirteen protomers form a large periplasmic domain of six stacked rings and a secretin domain in the outer membrane. A homology model of the PilQ protein was fitted into the cryo-EM map. A crown-like structure outside the outer membrane capping the secretin was found not to be part of PilQ. Mutations in the secretin domain disrupted the crown and abolished DNA uptake, suggesting a central role of the crown in natural transformation.
