Purification and characterization of zein-degrading proteases from endosperm of germinating maize seeds

We have purified a group of four proteases (molecular mass 26-33 kilodalton) from germinating seeds of maize (Zea mays L. var W64A) using ammonium sulfate and isoelectric precipitations, anion exchange chromatography, and electroelution from preparative nondenaturing polyacrylamide gels. Their appearance in the endosperm of germinating seeds coincides with the onset of zein degradation. We have shown that these proteases degrade zeins dissolved in alcoholic solution as well as aggregated in protein bodies from developing maize kernels. The apparent molecular weights and net negative charges of each of these proteases are very similar. Additionally, they are inhibited by thiol-blocking agents and activated by reducing compounds. These characteristics suggest that they are a group of cysteine proteases involved in the first steps of storage protein degradation.