The plant pathogenic bacterium Xanthomonas campestris pv. vesicatoria possesses a type III secretion system (T3S-System), which is essential for pathogenicity. Essential for a functional T3S-System is the pathogenicity factor HrpB2. The substrate specificity of the T3S-System depends on the switch protein HpaC. In this study, HrpB2 and HpaC were functionally characterized. It was possible to identify the conserved VxTLxK-motif in the C-terminal region of HrpB2. The VxTLxK motif is essential for the protein function. HrpB2 probably promotes the assembly of the pilus in the periplasm. Analyses with HpaC derivatives revealed, that the N-and C-terminal region of HpaC is required for the switch in the substrate specificity. Additional studies showed also that the amino acids 112 to 212 represents a potential T3S4 (type III secretion substrate specificity switch)-domain. Additionally, a conserved phenylalanine could be identified, which is essential the protein function of HpaC.