ABSTRACT
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.
Subject(s)
Fabaceae/chemistry , Glycoproteins/chemistry , Plant Proteins/chemistry , Plants, Medicinal , Protein Precursors/chemistry , Amino Acid Sequence , Chromosome Mapping , DNA, Complementary , Fabaceae/genetics , Glycoproteins/genetics , Molecular Sequence Data , Plant Proteins/genetics , Sequence Homology, Amino Acid , Trypsin Inhibitors , alpha-Amylases/antagonists & inhibitorsABSTRACT
The common bean, Phaseolus vulgaris, contains a family of defense proteins that comprises phytohemagglutinin (PHA), arcelin, and alpha-amylase inhibitor (alpha AI). Here we report eight new derived amino acid sequences of genes in this family obtained with either the polymerase chain reaction using genomic DNA, or by screening cDNA libraries made with RNA from developing beans. These new sequences are: two alpha AI sequences and arcelin-4 obtained from a wild accession of P. vulgaris that is resistant to the Mexican bean weevil (Zabrotes subfasciatus) and the bean weevil (Acanthoscelides obtectus); an alpha AI sequence from the related species P. acutifolius (tepary bean); a PHA and an arcelin-like sequence from P. acutifolius; an alpha AI-like sequence from P. maculatus; and a PHA sequence from an arcelin-5 type P. vulgaris. A dendrogram of 16 sequences shows that they fall into the three identified groups: phytohemagglutinins, arcelins and alpha AIs. A comparison of these derived amino acid sequences indicates that one of the four amino acid residues that is conserved in all legume lectins and is required for carbohydrate binding is absent from all the arcelins; two of the four conserved residues needed for carbohydrate binding are missing from all the alpha AIs. Proteolytic processing at an Asn-Ser site is required for the activation of alpha AI, and this site is present in all alpha AI-like sequences; this processing site is also found at the same position in certain arcelins, which are not proteolytically processed. The presence of this site is therefore not sufficient for processing to occur.
