An a-amylase inhibitor that inhibits insect
and mammalian a-amylases, but not plant a-amylases, is
present in seeds of the common bean (Phaseolus vulgaris). We
have purified the a-amylase inhibitor by using a selective heat
treatment in acidic medium and affinity chromatography with
porcine pancreas a-amylase coupled to agarose. Under sodium
dodecyl sulfate gel electrophoresis, the purified inhibitor gave
rise to five bands with mobilities corresponding to molecular
masses ranging from 14 to 19 kDa. N-terminal sequencing (up
to 15 amino acids) of the polypeptides obtained from these
bands resulted in only two different sequences matching two
stretches of the amino acid sequence deduced from an already
described lectin gene [Hoffman, L. M. (1984) J. Mol. Appl.
Gen. 2, 447-453]. This gene is different from but closely related
to the genes that code for phytohemagglutinin, the major lectin
of bean. Further evidence based on amino acid composition,
identification of a precursor, and recognition of the product of
the gene (expressed in Escherichia coli) by an anti-a-amylase
inhibitor serum confirms that the inhibitor is encoded by this
or a closely related lectin gene. This finding assigns a biological
function, which has been described at the molecular level, to a
plant lectin gene product and supports the defense role postulated
for seed lectins. The lack of homology with other families
of enzyme inhibitors suggests that this may be the first member
of a new family of plant enzyme inhibitors