Sequence homology between barley trypsin inhibitor and wheat α-amylase inhibitors

Abstract

Protein proteinase inhibitors are one of the most studied of all proteins from the viewpoint of evolution. They frequently exist as ‘multi-headed’ inhibitors capable of inhibiting more than one enzyme simultaneously. Structural analysis of the multi-headed inhibitors revealed the presence of 2-7 repeating sequences, each of which contains a reactive (binding) site for a proteinase [ 11. Furthermore, a single species of plants or animals usually contains a set of many inhibitor variants, which, in spite of their closely resembling structures, show a wide variety of inhibition spectra resulted from minor changes of amino acid sequence around the reactive sites. The above findings are accepted as a strong evidence for the divergent evolution of proteinase inhibitors from a limited number of ancestors by fused or separated gene multiplication and mutation [ 11. This divergent evolution of the inhibitors so far observed is limited within the inhibitors of proteolytic enzymes. During the course of sequence determination of barley trypsin inhibitor we have noted that the amino-terminal sequence of this inhibitor is homologous to the partially known sequence of a-amylase inhibitors. Here, we present the first evidence for a possible evolutionary relationship between two apparently unrelated inhibitors, a trypsin inhibitor anda-amylase inhibitors. gal a-amylase, Taka-amylase A (Aspergillus oryzae) was a generous gift of Drs Hase and Ikenaka, Osaka University.