STRUCTURE AND EXPRESSION OF AN INHIBITOR OF FUNGAL POLYGALACTURONASESFROM TOMATO

A polygalacturonase inhibitor protein (PGIP) was characterized from to mato fruit. Differential glycosylation of a single polypeptide account ed for heterogeneity in concanavalin A binding and in molecular mass. Tomato PGIP had a native molecular mass of 35 to 41 kDa, a native isoe lectric point of 9.0, and a chemically deglycosylated molecular mass o f 34 kDa, suggesting shared structural similarities with pear fruit PG IP. When purified PGIPs from pear and tomato were compared, tomato PGI P was approximately twenty-fold less effective an inhibitor of polygal acturonase activity isolated from cultures of Botrytis cinerea. Based on partial amino acid sequence, polymerase chain reaction products and genomic clones were isolated and used to demonstrate the presence of PGIP mRNA in both immature and ripening fruit as well as cell suspensi on cultures. Nucleotide sequence analysis indicates that the gene, uni nterrupted by introns, encodes a predicted 36.5 kDa polypeptide contai ning amino acid sequences determined from the purified protein and sha ring 68% and 50% amino acid sequence identity with pear and bean PGIPs , respectively. Analysis of the PGIP sequences also revealed that they belong to a class of proteins which contain leucine-rich tandem repea ts. Because these sequence domains have been associated with protein-p rotein interactions, it is possible that they contribute to the intera ction between PGIP and fungal polygalacturonases.