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A Metalloprotease from Xanthomonas campestris That Specifically Degrades Proline/Hydroxyproline-Rich Glycoproteins of the Plant Extracellular Matrix

Culture supernatants of Xanthomonas campestris pv. campestris contain an enzymic activity capable of degrading gp120, a proline-rich glycoprotein associated with the extracellular matrix of the vascular bundles in petioles of turnip (Brassica campestris). This activity did not reside in any of the three previously characterized proteases of X. campestris pv. campestris that were identified by their action against the model substrate β-casein. The novel enzyme was purified by ion-exchange and size-exclusion high-performance liquid chromatography (HPLC). The enzyme, which has no activity against β-casein, is active against some plant glycoproteins of the hydroxyproline-rich class such as extensin from potato and tomato and gpS-3, a glycoprotein induced in B. campestris petioles by wounding. Other hydroxyproline-rich glycoproteins, such as the solanaceous lectins, were not substrates however. Studies of the products released upon degradation of tomato extensin suggested that the degradative mechanism was proteolysis. Inhibitor studies suggested that the enzyme was a zinc-requiring metalloprotease. Extracellular matrix glycoproteins of the proline-rich and hydroxy-proline-rich classes have been implicated in plant resistance to microbial attack, hence their degradation by X. campestris pv. campestris may have considerable significance for black rot pathogenesis.