Interaction of the virulence protein VirF of Agrobacterium tumefaciens with plant homologs of the yeast Skp1 protein

The infection of plants by Agrobacterium tumefaciens leads to the formation of crown gall tumors due to the transfer of a nucleoprotein complex into p lant cells that is mediated by the virulence (vir) region-encoded transport system (reviewed in [1-5]), In addition, A, tumefaciens secretes the Vir p roteins, VirE2 and VirF, directly into plant cells via the same VirB/VirD4 transport system [6], and both assist there in the transformation of normal cells into tumor cells. The function of the 22 kDa VirF protein is not cle ar. Deletion of the virF gene in A. tumefaciens leads to diminished virulen ce [7, 8] and can be complemented by the expression of the virF gene in the host plant. This finding indicates that VirF functions within the plant ce ll [8], Here, we report that the VirF protein is the first prokaryotic prot ein with an F box by which it can interact with plant homologs of the yeast Skp1 protein. The presence of the F box turned out to be essential for the biological function of VirF, F box proteins and Skp1p are both subunits of a class of E3 ubiquitin ligases referred to as SCF complexes. Thus, VirF m ay be involved in the targeted proteolysis of specific host proteins in ear ly stages of the transformation process.