Novel mannose-binding rice lectin composed of some isolectins and its relation to a stress-inducible salT gene.

The novel mannose-binding rice lectin (MRL) purified by Sephadex G-50 or maltamyl Sepharose 4B affinity chromatography was not homogeneous, but the components were separated clearly by two dimensional polyacrylamide gel electrophoresis (1st; isoelectric focusing with Immobiline, 2nd; SDS-PAGE). The major spots were located at pI 4.85 and 4.74, and minor spots at pI 4.66, 4.56, and 4.44; all spots were distributed at about MW 45,000. Other faint spots were sometimes detected just below the major spots. In the western blot analysis, all the spots reacted with the monoclonal antibodies specific to MRL, which bound to MRL and inhibited the lectin activity to agglutinate rabbit erythrocytes. The proteins of the spots at pI 4.85, 4.77, 4.66, and 4.56 had lectin activity. The major proteins at pI 4.85 and 4.77 also had the common amino acid sequence at N-terminus, TLVKIGPWGGNGGSAQDISV, which is almost identical to salt and drought stress-inducible salT gene products in rice plants. High homology was also conserved in both the cDNA and the genomic clones encoding the MRL component at pI 4.85, which were selected with MRL-specific antibodies and an oligonucleotide designed from the partial amino acid sequence. All results suggest that MRL is composed of several isolectins, if not, related proteins having a common epitope and may belong to a family of stress-inducible proteins.