Mutagenesis of the Agrobacterium VirE2 single-stranded DNA-binding protein identifies regions required for self-association and interaction with VirE1 and a permissive site for hybrid protein construction.

The VirE2 single-stranded DNA-binding protein (SSB) of Agrobacterium tumefaciens is required for delivery of T-DNA to the nuclei of susceptible plant cells. By yeast two-hybrid and immunoprecipitation analyses, VirE2 was shown to self-associate and to interact with VirE1. VirE2 mutants with small deletions or insertions of a 31-residue oligopeptide (i31) at the N or C terminus or with an i31 peptide insertion at Leu236 retained the capacity to form homomultimers. By contrast, VirE2 mutants with modifications outside a central region located between residues 320 and 390 retained the capacity to interact with VirE1. These findings suggest the tertiary structure of VirE2 is important for homomultimer formation whereas a central domain mediates formation of a complex with VirE1. The capacity of VirE2 mutants to interact with full-length VirE2 in the yeast Saccharomyces cerevisiae correlated with the abundance of the mutant proteins in A. tumefaciens, suggesting that VirE2 is stabilized by homomultimerization in the bacterium. We further characterized the promoter and N- and C-terminal sequence requirements for synthesis of functional VirE2. A PvirB::virE2 construct yielded functional VirE2 protein as defined by complementation of a virE2 null mutation. By contrast, PvirE or Plac promoter constructs yielded functional VirE2 only if virE1 was coexpressed with virE2. Deletion of 10 or 9 residues from the N or C terminus of VirE2, respectively, or addition of heterologous peptides or proteins to either terminus resulted in a loss of protein function. However, an i31 peptide insertion at Tyr39 had no effect on protein function as defined by the capacity of the mutant protein to (i) interact with native VirE2, (ii) interact with VirE1, (iii) accumulate at abundant levels in A. tumefaciens, and (iv) restore wild-type virulence to a virE2 null mutant. We propose that Tyr39 of VirE2 corresponds to a permissive site for insertion of heterologous peptides or proteins of interest for delivery across kingdom boundaries.