Pea (Pisum sativum L.) seed isolectins 1 and 2 and pea root lectin result from carboxypeptidase-like processing of a single gene product

Abstract

The complete amino acid sequences of the α-subunits of pea (Pisum sativum L.) seed and root lectin, the C-terminal amino acids of the β-subunits of pea seed lectin, and most of the sequence of the β-subunit of pea root lectin were determined. In contrast to earlier reports it was shown that the β-subunits of both seed isolectins end at Asn-181. The α1 subunits end at Gln-241 (major fraction) or Lys-240 (minor fraction), whereas the α2 subunits end at Ser-239, Ser-238, Ser-237 or Thr-236. psl cDNA clones from seed are identical to psl cDNA clones from root, and root PSL is identical to seed PSL2, ending at Ser-239, Ser-238 or Ser-237. It seems that the presence of Lys-240 is the sole determinant of the charge difference between pea isolectins. PSL1 can be converted into PSL2 by carboxypeptidase P from Penicillium janthinellum. These results confirm that PSL from roots is encoded by the same gene as PSL from seeds. Thus, it seems that, next to an Asn-X specific protease responsible for the processing at positions 181/182 and 187/188, a carboxypeptidase is responsible for the conversion of PSL1 into PSL2, which is probably the final processing product.