Agrobacterium tumefaciens induces crown gall tumors on plants by transferring a 
nucleoprotein complex, the T-complex, from the bacterium to the plant cell. The 
T-complex consists of T-DNA, a single-stranded DNA segment of the tumor-inducing 
plasmid, VirD2, an endonuclease covalently bound to the 5' end of the T-DNA, and 
perhaps VirE2, a single-stranded DNA binding protein. The yeast two-hybrid 
system was used to screen for proteins interacting with VirD2 and VirE2 to 
identify components in Arabidopsis thaliana that interact with the T-complex. 
Three VirD2- and two VirE2-interacting proteins were identified. Here we 
characterize the interactions of VirD2 with two isoforms of Arabidopsis 
cyclophilins identified by using this analysis. The VirD2 domain interacting 
with the cyclophilins is distinct from the endonuclease, omega, and the nuclear 
localization signal domains. The VirD2-cyclophilin interaction is disrupted in 
vitro by cyclosporin A, which also inhibits Agrobacterium-mediated 
transformation of Arabidopsis and tobacco. These data strongly suggest that host 
cyclophilins play a role in T-DNA transfer.