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

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Abstract
The common bean, Phaseolus vulgaris, contains a family of defense proteins that comprises phytohemagglutinin (PHA), arcelin, and α-amylase inhibitor (αAI). 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 αAI 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 αAI sequence from the related species P. acutifolius (tepary bean); a PHA and an arcelin-like sequence from P. acutifolius; an αAI-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 αAIs. 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 αAIs. Proteolytic processing at an Asn-Ser site is required for the activation of αAI, and this site is present in all αAI-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.