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PLANTS INTERACTING WITH OTHER ORGANISMS

The Polygalacturonase-Inhibiting Protein PGIP2 of Phaseolus vulgaris Has Evolved a Mixed Mode of Inhibition of Endopolygalacturonase PG1 of Botrytis cinerea¹

Dipartimento di Biologia Vegetale, Università di Roma La Sapienza, 00185 Rome, Italy (F.S., C.C., G.D.L., F.C.); Molecular Modeling and Bioinformatics Unit, Parc Cientific de Barcelona, 08028 Barcelona, Spain (J.F.-R.); Dipartimento di Scienze Biochimiche A. Rossi Fanelli, Università di Roma La Sapienza, 00185 Rome, Italy (D.T.); and Centro Studi sull'Invecchiamento, Dipartimento di Scienze Biomediche, Università di Chieti e Pescara G. D'Annunzio, 66013 Chieti, Italy (L.F.)

Botrytis cinerea is a phytopathogenic fungus that causes gray mold in >1,000 plant species. During infection, it secretes several endopolygalacturonases (PGs) to degrade cell wall pectin, and among them, BcPG1 is constitutively expressed and is an important virulence factor. To counteract the action of PGs, plants express polygalacturonase-inhibiting proteins (PGIPs) that have been shown to inhibit a variety of PGs with different inhibition kinetics, both competitive and noncompetitive. The PG-PGIP interaction promotes the accumulation of oligogalacturonides, fragments of the plant cell wall that are general elicitors of plant defense responses. Here, we characterize the enzymatic activity of BcPG1 and investigate its interaction with PGIP isoform 2 from Phaseolus vulgaris (PvPGIP2) by means of inhibition assays, homology modeling, and molecular docking simulations. Our results indicate a mixed mode of inhibition. This is compatible with a model for the interaction where PvPGIP2 binds the N-terminal portion of BcPG1, partially covering its active site and decreasing the enzyme affinity for the substrate. The structural framework provided by the docking model is confirmed by site-directed mutagenesis of the residues that distinguish PvPGIP2 from the isoform PvPGIP1. The finding that PvPGIP2 inhibits BcPG1 with a mixed-type kinetics further indicates the versatility of PGIPs to evolve different recognition specificities.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Luca Federici (lfederici{at}unich.it).

Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.105.067546.

^* Corresponding author; e-mail lfederici{at}unich.it; fax 0039–0871–541598.

Received June 23, 2005; returned for revision September 8, 2005; accepted September 8, 2005.

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