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Published on September 29, 2006; 10.1104/pp.106.086587

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Received July 12, 2006
Accepted September 22, 2006

A Novel Function for the Cathepsin D Inhibitor in Tomato

Purificación Lisón , Ismael Rodrigo *, and Vicente Conejero

Instituto de Biología Molecular y Celular de Plantas (IBMCP), UPV-CSIC, Camino de Vera, s/n, 46022 Valencia, Spain.

* Corresponding author; email: irodrig{at}ibmcp.upv.es.

Proteinaceous aspartic proteinase inhibitors are rare in nature and are described in only a few plant species. One of them corresponds to a family of cathepsin D inhibitors described in potato, involving up to 15 isoforms with a high sequence similarity. In this work, we describe a tomato wound-inducible protein called JIP21 (Jasmonic Induced Protein, 21 kDa). The sequence analysis of its cDNA predicted a putative function as a cathepsin D inhibitor. The JIP21 gene, whose protein has been demonstrated to be glycosylated, is constitutively expressed in flowers, stem and fruit, and is inducible to high levels by wounding and methyl-jasmonate in leaves of tomato plants. The genomic sequence of JIP21 shows that the gene is intronless and reveals the presence of both an MeJ box (TGACT) and G-box (CACGT) in the promoter. In contrast to the presumed role of JIP21 based on sequence analysis, a detailed biochemical characterization of the purified protein uncovers a different function as a strong chymotrypsin inhibitor, which questions the previously predicted inhibitory activity against aspartic proteinases. Moreover, Spodoptera littoralis larvae fed on transgenic tomato plants overexpressing JIP21, present an increase in mortality and a delay in growth when compared with larvae fed on wild-type plants. These larvae belong to the Lepidoptera family whose main digestive enzymes have been described as being serine proteases. All these results support that tomato JIP21 should be considered as a chymotrypsin inhibitor belonging to the serine proteinase inhibitors rather than a cathepsin D inhibitor. Therefore, we propose to name this protein as TCI21 (Tomato Chymotrypsin Inhibitor, 21 kDa).




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