A major component of the endosperm albumin, this protein acts as a storage protein during grain filling, contributing a substantial part of the grain's lysine. May have an inhibitory function during filling or germination. Inhibits cathepsin G in vitro. Highly expressed in embryo and endosperm. Is accumulated and stored in the endosperm, where it exists in a free and a bound form. Expressed in roots, coleoptiles, shoots and leaves. Synthesized 10-25 days after fertilization (developing endosperm). Its expression is regulated by the 'high lysine' alleles Lys1 and Lys3a. The reactive center loop (RCL) extends out from the body of the protein and directs binding to the target protease. The protease cleaves the serpin at the reactive site within the RCL, establishing a covalent linkage between the carboxyl group of the serpin reactive site and the serine hydroxyl of the protease. The resulting inactive serpin-protease complex is highly stable (By similarity). Belongs to the serpin family.