N-terminal amino acid sequence homology of storage protein components from barley and a diploid wheat

Abstract

Wild barley (Hordeum spontaneum) and the wild diploid wheat Triticum boeoticum were possibly the first plants cultivated by early man¹, giving rise to the domesticated forms Hordeum vulgare L. and Triticum monococcum L. In addition, T. boeoticum may have contributed the A genome to polyploid wheats, including common bread wheat (Triticum aestivum)² which is a hexaploid with genome composition ABD. Hordeum seems to be the older genus, having diverged from some common ancestor before the divergence of Triticum and other genera of the subtribe Triticinae³. Prolamins constitute the major storage protein fraction of both barley and wheat; they are located in the endosperm of the caryopsis and are soluble in alcohol–water solutions⁴. Barley and wheat prolamins (hordeins and gliadins, respectively) contain large amounts of glutamine and proline, which together make up 50–75 mol per cent of total amino acids^4,5. The hordeins and gliadins are complex mixtures of components^6–8 that seem to be encoded by clusters of duplicated genes that have diverged to produce many distinguishable protein components. Despite the complexity of the gliadin mixture, the components retain considerable homology in their N-terminal region^9,10 and this has been reported for zeins, the prolamins of maize (Zea mays)¹¹, as well. Here, we report that a purified C-hordein component from barley is homologous in amino acid sequence with a purified ω-gliadin component from T. monococcum at 23 of 27 residues at the N-terminus. This result is in accord with the close relationship between the two species and indicates that, despite the propensity of prolamin genes to tolerate mutations, a significant portion of their sequences can be conserved over a period of time, which, although not accurately known, probably amounts to millions of years.