The plant VirE2 interacting protein 1. A molecular link between the Agrobacterium T-complex and the host cell chromatin?

The microbe Agrobacterium tumefaciens is harmful to plants and useful to scientists for one and the same reason: It transfers DNA into plant genomes. Found in soil worldwide, Agrobacterium causes disease in plants by transferring its own DNA into plant cells. But in the laboratory, the ability to move foreign genes into plants has made the microbe a standard tool for investigating plant genetics and modifying crops. During genetic transformation, a single stranded copy (T-strand) of the bacterial transferred DNA (T-DNA) and several virulence (Vir) proteins are exported from Agrobacterium into the plant cell cytoplasm, within which a mature transport (T) complex is assembled that contains a T-strand molecule covalently attached at its 5'-end to a single molecule of the VirD2 protein and packaged by multiple molecules of VirE2 into a telephone cord-like coiled structure. This complex is then imported into the plant cell nucleus with the help of both VirD2 and VirE2. Once inside the nucleus, the T-complex is targeted to the plant chromatin, uncoated of its protein components, and integrated into the host DNA, which must also be exposed for integration (for review, see Zupan et al., 2000; Tzfira and Citovsky, 2002; Gelvin, 2003). The molecular pathways by which most of these diverse steps of the transformation process occur have been identified; for example, DNA and proteins are exported from Agrobacterium via the type IV secretion system (for review, see Christie, 2004), the T-complex is imported into the host cell nucleus by the karyopherin alpha-dependent pathway (for review, see Tzfira and Citovsky, 2002) and is likely uncoated by targeted proteolysis via the Skp1/Cdc53-cullin/ F-box pathway (Tzfira et al., 2004). But how T-complex is targeted to the host chromatin remains completely unknown. Here, we offer a hypothesis that the Arabidopsis (Arabidopsis thaliana) VirE2 Interacting Protein 1 (AtVIP1), which associates with the T-complexes by binding to their VirE2 component (Tzfira et al., 2001), acts as a molecular link between the T-complex and the histone constituents of the host chromatin. This model is based on our observations that AtVIP1 interacts with histones in vitro and in planta.