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Journal of Bacteriology, December 1999, p. 7485-7492, Vol. 181, No. 24
0021-9193/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.

Vir Proteins Stabilize VirB5 and Mediate Its Association with the T Pilus of Agrobacterium tumefaciens

Heike Schmidt-Eisenlohr,1 Natalie Domke,1 Christina Angerer,1 Gerhard Wanner,2 Patricia C. Zambryski,3 and Christian Baron1,*

Institut für Genetik und Mikrobiologie der Universität München, Lehrstuhl für Mikrobiologie,1 and Botanisches Institut der Universität München,2 D-80638 Munich, Germany, and Department of Plant and Microbial Biology, University of California at Berkeley, Berkeley, California 947203

Received 26 July 1999/Accepted 29 September 1999

Three VirB proteins (VirB1*, VirB2, and VirB5) have been implicated as putative components of the T pilus from Agrobacterium tumefaciens, which likely mediates binding to plant cells followed by transfer of genetic material. Recently, VirB2 was indeed shown to be its major component (E.-M. Lai and C. I. Kado, J. Bacteriol. 180:2711-2717, 1998). Here, the influence of other Vir proteins on the stability and cellular localization of VirB1*, VirB2, and VirB5 was analyzed. Solubility of VirB1* and membrane association of VirB2 proved to be inherent features of these proteins, independent of virulence gene induction. In contrast, cellular levels of VirB5 were strongly reduced in the absence of other Vir proteins, indicating its stabilization by protein-protein interactions. The assembly and composition of the T pilus were analyzed in nopaline strain C58(pTiC58), its flagellum-free derivative NT1REB(pJK270), and octopine strain A348(pTiA6) following optimized virulence gene induction on solid agar medium. In all strains VirB2 was the major pilus component and VirB5 cofractionated during several purification steps, such as ultracentrifugation, gel filtration, and sucrose gradient centrifugation. VirB5 may therefore be directly involved in pilus assembly, possibly as minor component. In contrast, secreted VirB1* showed no association with the T pilus. In-frame deletions in genes virB1, virB2, virB5, and virB6 blocked the formation of virulence gene-dependent extracellular high-molecular-weight structures. Thus, an intact VirB machinery as well as VirB2 and VirB5 are required for T-pilus formation.

* Corresponding author. Mailing address: Institut für Genetik und Mikrobiologie der Universität München, Lehrstuhl für Mikrobiologie, Maria-Ward-Str. 1a, D-80638 München, Germany. Phone: 49-89-2180-2138. Fax: 49-89-2180-6122. E-mail: cbaron{at}lrz.uni-muenchen.de.

Journal of Bacteriology, December 1999, p. 7485-7492, Vol. 181, No. 24
0021-9193/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.


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