Characterization of an extracellular chymostatin-sensitive serine protease preferentially expressed in young plant tissues.

Intercellular washing fluids from leaves of all tested higher plant species contained a serine-type protease which efficiently cleaved the artificial fluorogenic substrate MCA-Pro-Leu-Gly-Leu-Dnp-Ala-Arg (MCA). The activity varied between the species. The classification as serine protease was based on the sensitivity towards chymostatin and phenylmethylsulfonyl fluoride. MCA protease activity strongly declined with leaf age and was also detected in stems, roots and flower petals. In tobacco, specific activity of the chymostatin-sensitive MCA protease was about 40-fold higher in intercellular washing fluids than in whole leaf homogenate confirming the extracellular location of the MCA protease. The same enzyme activity was detected in developing tomato fruits; it showed a correlation with fruit growth and was not detectable in ripe fruits. The tobacco protease was sensitive to temperatures above 50 degrees C, had an isoelectric point of 5.8+/-0.1 and an apparent molecular mass of 68 kDa. Its pH optimum was very broad with little difference in activity between pH 5 and 9. Conversely, a casein-cleaving protease also present in intercellular washing fluids was insensitive towards chymostatin and revealed a pronounced pH optimum around 6.0. The data biochemically characterize a new type of extracellular proteolytic activity which may be particularly important during tissue expansion.