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 (Lycopers icon esculentum) plants (P69B) that is part of a proteolytic system ac tivated in the plant as a result of infection with citrus exocortis vi roid, This new protease is closely related, in terms of amino acid seq uence and structural organization, to the previously identified pathog enesis-related subtilisin-like protease (Tornero, P,, Conejero, V,, an d 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 i s homologous 60 the catalytic modules of bacterial subtilisins and euk aryotic Kex2-like proteases, Within the catalytic domain, the essentia l Asp, His, and Ser residues that conform the catalytic triad of this family of proteases are conserved in P69B, Northern blot and reverse t ranscriptase-polymerase chain reaction analysis demonstrated widesprea d induced expression of the 2.5-kilobase hybridizing mRNA in plant tis sues as a consequence of viroid infection, We propose that P69B is a m ember of a complex gene family of plant Kex2/subtilisin-like proteases presumably involved in a number of specific proteolytic events activa ted during pathogenesis in plants and that takes place in the extracel lular matrix.