Purification and properties of an aspartic protease from potato tuber thatis inhibited by a basic chitinase

A protease was isolated from potato (Solanum tuberosum L. cv. Huinkul) tube r disks after 24 h of aeration when proteolysis is markedly increased. Puri fication was performed by ammonium sulfate precipitation, ion exchange chro matography, and affinity chromatography. A size of 40 kDa was estimated by sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) and g el filtration, it is monomeric and its properties are consistent with those of aspartic proteinases (EC 3.4.23): it had a pH optimum between 4 and 5 a nd it was inhibited by pepstatin. Partial homology with other plant asparti c proteinases was observed in two sequenced tryptic fragments. It binds to Sepharose-concanavalin A and can be eluted with alpha-methyl mannoside, ind icating that it is possibly glycosylated. Unlike other aspartic proteinases from Solanaceae that degrade pathogenesis-related proteins, it is unable t o cleave a basic chitinase from potato. Moreover, this aspartic protease is strongly inhibited by the basic chitinase; the 50% inhibition is obtained when the molar ratio approaches 1, the same as with pepstatin. The interact ion between this aspartic protease and a new type of endogenous inhibitor m ay be an interesting starting point to study the regulation of these aspart ic proteases during stress.