Immunolocalization of PsNLEC-1, a lectin-like glycoprotein expressed in developing pea nodules.

The pea (Pisum sativum) nodule lectin gene PsNlec1 is a member of the legume lectin gene family that is strongly expressed in infected pea nodule tissue. A full-length cDNA sequence of PsNlec1 was expressed in Escherichia coli and a specific antiserum was generated from the purified protein. Immunoblotting of material from isolated symbiosomes revealed that the glycoprotein was present in two antigenic isoforms, PsNLEC-1A and PsNLEC-1B. The N-terminal sequence of isoform A showed homology to an eight-amino acid propeptide sequence previously identified from the cDNA sequence of isoform B. In nodule homogenates the antiserum recognized an additional fast-migrating band, PsNLEC-1C. Fractionation studies indicated that PsNLEC-1C was associated with a 100,000 g nodule membrane fraction, suggesting an association with cytoplasmic membrane or vesicles. Immunogold localization in pea nodule tissue sections demonstrated that the PsNLEC-1 antigen was present in the symbiosome compartment and also in the vacuole but revealed differences in distribution between infected host cells in different parts of the nodule. These data suggest that PsNLEC-1 is subject to posttranslational modification and that the various antigenic isoforms can be used to monitor membrane and vesicle targeting during symbiosome development.