Evolutionary relationships among proteins in the phytohemagglutinin-arcelin-?-amylase inhibitor family of the common bean and its relatives

Abstract

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.

Lectins as Plant Defense Proteins

“…Third, several legume species contain proteins that are clearly related to the lectins but are devoid of carbohydrate-binding activity. Well-known examples of this group of proteins are the Pkaseolus vulgaris arcelins and the a-amylase inhibitor (Mirkov et al, 1994).…”

Bean α-amylase inhibitor 1 in transgenic peas ( Pisum sativum ) provides complete protection from pea weevil ( Bruchus pisorum ) under field conditions

“…Seeds of certain wild accessions of P. vulgaris that are rich in the protein arcelin contain the homologue ␣AI-2, which shares 78% amino acid identity with ␣AI-1. ␣AI-2 does not inhibit mammalian amylases (7,8) but does inhibit the midgut ␣-amylase of Z. subfasciatus (7,9). The ␣AI-2-containing beans are resistant to the Mexican bean weevil.…”

Protective Mechanism of the Mexican Bean Weevil against High Levels of α--Amylase Inhibitor in the Common Bean

“…aAI is not just a single protein, but exists in at least two, and possibly more, isoforms (Mirkov et al, 1994;Ishimoto et al, 1995). The best-characterized a-amylase inhibitor of the common bean, now termed aAI-1, has been studied in many laboratories since 1945 (Bowman, 1945), and its cDNA has been cloned (Hoffman et al, 1982;Moreno and Chrispeels, 1989).…”

“…The best-characterized a-amylase inhibitor of the common bean, now termed aAI-1, has been studied in many laboratories since 1945 (Bowman, 1945), and its cDNA has been cloned (Hoffman et al, 1982;Moreno and Chrispeels, 1989). More recently, a new aAI, aAI-2, was purified from seeds of a wild accession of the common bean resistant to Z. subfasciatus and its cDNA has been cloned (Suzuki et al, 1993(Suzuki et al, , 1994Mirkov et al, 1994). aAI-1 and aAI-2 share 78% amino acid identity.…”