Characterization of HrpB2 from Xanthomonas campestris pv. vesicatoria identifies protein regions that are essential for type III secretion pilus formation.

The Gram-negative plant-pathogenic bacterium Xanthomonas campestris pv. vesicatoria employs a type III secretion (T3S) system to translocate effector proteins into plant cells. T3S depends on HrpB2, which is essential for assembly of the extracellular T3S pilus and is itself weakly secreted. To characterize the role of HrpB2, we used a transposon mutagenesis approach, which led to the insertion of pentapeptide-encoding sequences into hrpB2. Complementation studies with HrpB2 mutant derivatives revealed that the N-terminal region of HrpB2 tolerates pentapeptide insertions, whereas insertions in the regions spanning amino acids 60-74 and 93-130, respectively, resulted in a loss of bacterial pathogenicity and T3S, including secretion of HrpB2 itself. The C-terminal region (amino acids 93-130) of HrpB2 contains a conserved VxTLxK amino acid motif that is also present in predicted inner rod proteins from animal-pathogenic bacteria and is required for the contribution of HrpB2 to pilus assembly and T3S. Electron microscopy and fractionation studies revealed that HrpB2 is not a component of the extracellular pilus structure but localizes to the bacterial periplasm and the outer membrane. We therefore propose that the essential contribution of HrpB2 to T3S and pilus assembly is linked to its possible function as a periplasmic component of the T3S system at the base of the pilus.