Effect of interactions between Mip and PrtA on the full extracellular protease activity of Xanthomonas campestris pathovar campestris

Mip (macrophage infectivity potentiator) and Mip-like proteins have been demonstrated to be involved in virulence of several animal pathogens, but as yet none of their native bacterial targets has been identified. Our previous work demonstrated that the Mip-like protein found in the plant pathogen Xanthomonas campestris pv. campestris (Xcc) (hereafter called Mip(Xcc)) is also involved in virulence. Inactivation of the mip(Xcc) gene leads to a significant reduction in exopolysaccharide production and extracellular protease activity via an unknown mechanism. The Xcc genome encodes six extracellular proteases, all of which are secreted via the type II secretion system. The serine protease PrtA makes the largest contribution to Xcc's total extracellular proteolytic activity. In this study, Western blotting analysis demonstrated that Mip(Xcc) was located in the periplasm. Bacterial two-hybrid and far-Western analysis indicated that Mip(Xcc) interacted with PrtA directly. Purified Mip(Xcc) was found to be able to rescue the protease activity of periplasmic proteins extracted from the mip(Xcc) mutant. These findings show that Mip(Xcc) plays a role in the maturation of PrtA, which is the novel native target for at least one Mip or Mip-like protein.