Isolation of the receptor for Amaranthus leucocarpus lectin from murine naive thymocytes.

From murine medullary thymocytes we purified the receptor for the Amaranthus leucocarpus lectin (ALL) using a complex with the biotin-labeled lectin and avidin-agarose as the affinity matrix. Most ALL(+)thymocytes (83%) are naive cells with the CD4(+)CD8(-)CD45RB(+)phenotype. The receptor for this lectin is a 70 kDa glycoprotein that contains 20% of sugar by mass. It is constituted mainly by aspartic and glutamic acids, serine, proline, and glycine; its glycosidic portion contains mainly O-glycosidically linked glycans with Gal, GalNAc and NeuAc residues as well as one N-glycosidically linked glycan per molecule. Ionic strength chromatography revealed that the ALL-thymocyte receptor (ALLTr) is made up by three isoforms, which possess similar amino acid composition but show slight differences in their sugar composition. The N-terminal amino acid residues are blocked both in the receptor and its purified isoforms. Analyses of the receptor's peptides, obtained by trypsin digestion with MALDI-TOF (matrix assisted laser desorption ionization-time of flight), were compared with the relative values obtained from the NCBInr (Swiss-Prot 10/01/99) database. Our results indicate that the peptides of ALLTr show low homology (<17%) with the human KIIA protein, the Fas-associated death domain protein, and the transforming growth factor-beta type II receptor. Our results suggest that the ALL thymocyte receptor could be considered a novel phenotypic marker specific for naive T cells.