A putative precursor protein in the evolution of the bean alpha-amylase inhibitor

Seeds of the common bean Phaseolus vulgaris and the tepary bean (P. acutifolius) contain a family of plant defence proteins that includes phytohaemagglutinin (PHA), arcelin and alpha-amylase inhibitor (alpha AI). These homologous proteins differ by the absence of short loops at the surface of the protein and by the presence of a proteolytic processing site (Asn77) that allows alpha AI to be post-translationally cleaved and activated. We now report the derived amino acid sequence of two amylase inhibitor-like (AIL) proteins that are not proteolytically processed, although they have the typical processing site. One protein is from the common bean, and the other from the tepary bean. On a dendrogram, these proteins are grouped with alpha AIs rather than with the arcelins or lectins. alpha AI differs from AIL primarily by the deletion of a 15-amino-acid segment from the middle of the AIL sequence. When alpha AI is expressed in tobacco, it is proteolytically processed to form an active molecule. However, AIL sequences are not processed. We suggest that the AIL proteins may be an intermediate in the evolution of an active alpha AI.