Identification of a new pathogen-induced member of the subtilisin-like processing protease family from plants

By using biochemical, immunological, and molecular strategies we have identified and cloned a cDNA encoding a protease from tomato (Lycopersicon esculentum) plants (P69B) that is part of a proteolytic system activated in the plant as a result of infection with citrus exocortis viroid. This new protease is closely related, in terms of amino acid sequence and structural organization, to the previously identified pathogenesis-related subtilisin-like protease (Tornero, P., Conejero, V., and Vera, P. (1996) Proc. Natl. Acad. Sci. U. S. A. 93, 6332-6337). The 745-residue amino acid sequence of P69B begins with a cleavable signal peptide, contains a prodomain and a 631-residue mature domain which is homologous to the catalytic modules of bacterial subtilisins and eukaryotic Kex2-like proteases. Within the catalytic domain, the essential Asp, His, and Ser residues that conform the catalytic triad of this family of proteases are conserved in P69B. Northern blot and reverse transcriptase-polymerase chain reaction analysis demonstrated widespread induced expression of the 2.5-kilobase hybridizing mRNA in plant tissues as a consequence of viroid infection. We propose that P69B is a member of a complex gene family of plant Kex2/subtilisin-like proteases presumably involved in a number of specific proteolytic events activated during pathogenesis in plants and that takes place in the extracellular matrix.