A novel protein programmed by the mRNA conserved in dry wheat embryos. The principal site of cysteine incorporation during early germination

Abstract

If bulk mRNA from dry wheat embryos (wheat germ) is used to direct cell-free incorporation of [35S]cysteine into proteins, a striking proportion of the total radioactivity is channeled into a single protein. During early postimbibition development, when protein synthesis is directed by the mRNA conserved in dry embryos, incorporation of cysteine is preponderantly (20-25%) directed into synthesis of this one protein: the 'early' cysteine-labeled protein (Ec). When conserved mRNA from the dry embryos has been fully degraded, as when cellular or cell-free protein synthesis is directed by the mRNA in germinated embryos, synthesis of Ec is not detected. Reliable detection of Ec requires prior alkylation of wheat embryo proteins, and it was especially interesting to find that when wheat embryo proteins are alkylated by iodo[14C]acetamide, two proteins co-dominate the distribution of radioalkylated products in dodecylsulphate/polyacrylamide gels: Ec and wheat germ agglutinin. Using co-electrophoresis with the isotopically labeled protein to detect a dye-staining counterpart, Ec has been purified by combined cation-exchange and gel-filtration chromatography of alkylated wheat germ proteins. The purified protein can be recovered in milligram quantity (5-10 mg/100 g wheat germ) and compositional analysis shows that it is unusually rich in cysteine (approx. 15%) and glycine (approx. 17%), as is wheat germ agglutinin.