A New Methionine-Rich Seed Storage Protein from Maize

Abstract

The endosperm of maize (Zea mays) seeds contains severa1 classes of alcohol-soluble storage proteins called zeins, which together make up nearly 50% of the total seed protein content. The abundance of the zeins and their low content of the essential amino acid Lys is responsible for the relatively poor nutritional quality of maize protein. However, zeins rich in other nutritionally important amino acids, e.g. the sulfur amino acids Met and Cys, have been identified. Zeins can be separated by SDS-PAGE into polypeptides with apparent molecular masses of 27,22,19, 16, 14, and 10 kD (Larkins et al., 1984). The 19- and 22-kD zeins constitute the major zein classes and are encoded by multigene families (Marks et al., 1984), whereas the others are encoded by single or small gene families (Pedersen et al., 1986; Gallardo et al., 1988; Kirihara et al., 1988a). Among the latter are genes for the sulfur-rich zeins, the 16-kD zein containing 21% by weight Met plus Cys (Pedersen et al., 1986) and the 10-kD zein containing 33% by weight Met plus Cys (Kirihara et al., 1988a, 1988b). We report the identification and characterization of a maize gene encoding a new Met-rich zein that is distinct from but closely related to the 10-kD zein gene (Table I). It has been designated the high-sulfur zein gene. The DNA fragment sequenced is 2123 bp and contains an open reading frame of 633 nucleotides, compared with the 453 nucleotides of the 10-kD zein gene. A TATA box and polyadenylation signal (AATAAA), well-known eukaryotic gene expression signals, were found at similar positions in the high-sulfur zein gene and 10-kD zein gene, and the highsulfur zein gene is expressed in maize endosperm at the same developmental stage as the 10-kD zein gene. A putative 21-amino acid signal sequence, similar to that of the 10-kD gene, is encoded by the high-sulfur zein gene at the amino terminus of the precursor polypeptide. The high-sulfur zein gene codes for a protein that is the highest in Met content (about 37% by weight Met and 40% total sulfur amino acids) of any currently known. The mature high-sulfur zein protein is composed of a central Met-rich region (approximately 50% Met residues) flanked by amino-terminal and carboxy-terminal regions with lower Met content (10 and 7% Met, respectively). The central region is composed of variations of the repeating motif Met-Met-Met-Pro. The related 10-kD zein protein (76% amino acid sequence identity) has a similar structure, but the central region of the high-sulfur zein protein is about twice as large as the corresponding region in the 10-kD zein.