Purification of an endoproteinase that digests the wheat 'Em' protein in vitro, and determination of its cleavage sites.

Germinating wheat embryos contain two endoproteolytic activities which digest the prominent 'Em' polypeptide. These are easily assayed in clarified embryonic homogenates and are distinguishable by the pattern of their peptide products and by their different pH optima. One activity has a pH optimum of 4.0; the second activity is a cysteine endoproteinase with a preference for the 'Em' protein as its substrate. It is maximally active between pH 5.5 and 6 at 25 degrees C. Analysis of the early cleavage products of the cysteine proteinase indicates scissile bonds between residues Glu32-Ala33 and Asn36-Leu37 in the 'Em' polypeptide. This endoproteinase has been purified and identified as a single polypeptide species of ca. 38,000 kDa.