Beta-turns as structural motifs for the proteolytic processing of seed proteins.

Fifteen NH2- and COOH-terminal ends from both small and large chains of the most abundant 2 S albumins from Brassica napus seeds have been sequenced. This allows the determination of the exact proteolytic maturation sites of these proteins. Each one of these proteins arises from a polypeptide precursor which is cleaved during the post-translational processing at four sites, giving two different chains linked by disulphide bridges on the mature 2 S albumin. The hydrolyzed bonds involved in the processing are located in proline and glycine-rich regions, forming tetra-peptides with a very high beta-turn probability. Similar results have been found through the analysis of the 2 S albumin precursors from other seeds. These facts are interpreted in terms of the existence of a beta-turn specific endoprotease activity involved in the maturation process of 2 S albumins.