THE MOLECULAR-STRUCTURE OF AGROBACTERIUM VIRE2-SINGLE STRANDED DNA COMPLEXES INVOLVED IN NUCLEAR IMPORT

Nuclear import of DNA is a central event in genetic transformation of plant cells by Agrobacterium tumefaciens. Agrobacterium elicits tumors on plant hosts by transporting a single-stranded (ss) copy of the bac terial transferred DNA (T-DNA) from its Ti (tumor-inducing) plasmid in to the plant tell nucleus. Presumably, the process of T-DNA nuclear im port is mediated by two agrobacterium proteins, VirD2 and VirE2, which are thought to directly associate with the transported T-DNA. Both pr oteins have been shown to contain functional nuclear localizations sig nals (NLS). Recently, VirE2 alone has been shown to actively transport ssDNA into the plant cell nucleus. To understand the process of DNA n uclear import, it is important to know the structure of the transport intermediate. To this end, complexes of VirE2 and ssDNA were analyzed by scanning transmission electron microscopy (STEM). This analysis sug gests that VirE2 packages ssDNA into semi-rigid, hollow cylindrical fi laments with a telephone cord-like coiled structure. The outer diamete r of these complexes is too large to enter the nucleus by diffusion bu t is within the size exclusion limits of the active nuclear import. De tailed mass analysis of VirE2-ssDNA filaments is presented and a struc tural model is proposed. (C) 1997 Academic Press Limited.