NMR structure of a complex between the VirB9/VirB7 interaction domains of the pKM101 type IV secretion system

doi:10.1073/pnas.0609535104

NMR structure of a complex between the VirB9/VirB7 interaction domains of the pKM101 type IV secretion system

*Institute of Structural Molecular Biology, University College London/Birkbeck, Malet Street, London WC1E 7HX, United Kingdom;
^‡Department of Biochemistry and Molecular Biology, University College London, Gower Street, London WC1E 6BT, United Kingdom; and
^§Department of Microbiology and Molecular Genetics, University of Texas Medical School, 6431 Fannin Street, Houston, TX 77030

Edited by Patricia C. Zambryski, University of California, Berkeley, CA, and approved November 29, 2006 (received for review October 26, 2006)

Abstract

Type IV secretion (T4S) systems translocate DNA and protein effectors through the double membrane of Gram-negative bacteria. The paradigmatic T4S system in Agrobacterium tumefaciens is assembled from 11 VirB subunits and VirD4. Two subunits, VirB9 and VirB7, form an important stabilizing complex in the outer membrane. We describe here the NMR structure of a complex between the C-terminal domain of the VirB9 homolog TraO (TraO^CT), bound to VirB7-like TraN from plasmid pKM101. TraO^CT forms a β-sandwich around which TraN winds. Structure-based mutations in VirB7 and VirB9 of A. tumefaciens show that the heterodimer interface is conserved. Opposite this interface, the TraO structure shows a protruding three-stranded β-appendage, and here, we supply evidence that the corresponding region of VirB9 of A. tumefaciens inserts in the membrane and protrudes extracellularly. This complex structure elucidates the molecular basis for the interaction between two essential components of a T4S system.

Footnotes

^¶To whom correspondence may be addressed. E-mail: g.waksman{at}bbk.ac.uk or g.waksman{at}ucl.ac.uk
↵ ^†Present address: Section of Structural Biology, Institute of Cancer Research, 237 Fulham Road, London SW3 6JB, United Kingdom.
Author contributions: R.B., P.C.D., P.J.C., and G.W. designed research; R.B., R.H., L.C., A.M., R.F., P.J.C., and P.C.D. performed research; R.B. contributed new reagents/analytic tools; R.B., R.H., L.C., A.M., R.F., P.J.C., P.C.D., and G.W. analyzed data; and R.B., R.H., P.J.C., P.C.D., and G.W. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS direct submission.
Data deposition: The atomic coordinates have been deposited in the Protein Data Bank, www.pdb.org (PDB ID code 2OFQ).
This article contains supporting information online at www.pnas.org/cgi/content/full/0609535104/DC1.
Abbreviations:

AMS,

4-acetamido-4′-maleimidylstilbene-2,2′-disulfonic acid;

MPB,

3-(N-maleimidylpropionyl) biocytin;

T4S,

type IV secretion;

TraOCT,

TraO C-terminal domain.