EVOLUTIONARY RELATIONSHIPS AMONG PROTEINS IN THE PHYTOHEMAGGLUTININ-ARCELIN-ALPHA-AMYLASE INHIBITOR FAMILY OF THE COMMON BEAN AND ITS RELATIVES

The common bean, Phaseolus vulgaris, contains a family of defense prot eins that comprises phytohemagglutinin (PHA), arcelin, and alpha-amyla se inhibitor (alpha AI). Here we report eight new derived amino acid s equences of genes in this family obtained with either the polymerase c hain reaction using genomic DNA, or by screening cDNA libraries made w ith RNA from developing beans. These new sequences are: two alpha AI s equences and arcelin-4 obtained from a wild accession of P. vulgaris t hat is resistant to the Mexican bean weevil (Zabrotes subfasciatus) an d the bean weevil (Acanthoscelides obtectus); an alpha AI sequence fro m the related species P. acutifolius (tepary bean); a PHA and an arcel in-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 identif ied groups: phytohemagglutinins, arcelins and alpha AIs. A comparison of these derived amino acid sequences indicates that one of the four a mino acid residues that is conserved in all legume lectins and is requ ired for carbohydrate binding is absent from all the arcelins; two of the four conserved residues needed for carbohydrate binding are missin g 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 proc essed. The presence of this site is therefore not sufficient for proce ssing to occur.