The starch-bound alpha-amylase/trypsin-inhibitors in Avena

Oat kernels exhibit an extra-soft texture, a trait recently demonstrated to be largely modulated by starch-bound tryptophan-rich 2S proteins, the vromindolines. In this study, fractionation by two-dimensional electrophoresis of starch-bound proteins in 25 oat (Avena sativa) cultivars and 11 diploid or tetraploid Avena species revealed novel 2S proteins called Avena alpha-amylase/trypsin-inhibitors (AATI) because of their sequence similarity with wheat alpha-amylase/trypsin inhibitors. Thirty-seven AATI polypeptides, about 14 kDa in size, were split into three families named AATI-1, AATI-2, and AATI-3 with different primary structures and isoelectric points. AATI-1 and AATI-2 proteins showed 55.5-60.0 % sequence similarity with wheat alpha-amylase inhibitors CM1, CM2, and CM16, which have been found to cause innate immunity responses in celiac disease and non-celiac gluten sensitivity. Diploid A-genome and tetraploid AC-genome oat species possess three and five genes encoding for the AATI proteins, respectively, whereas hexaploid A. sativa exhibits 12 genes dispersed over the A-, C-, and D-genomes. Some AATI proteins expressed in hexaploid oats were assigned to the A-genome based on similarity to their counterparts in diploid species, contributing to further clarify the genetic origin of hexaploid oats. Moreover, AATI may interact with starch-bound vromindolines in determining the extra-soft texture of oat kernels and, due to their balanced amino acid compositions, may contribute to the biological value of oat proteins in a positive manner.