Two jasmonate-inducible myrosinase-binding proteins from Brassica napus L. seedlings with homology to jacalin

Abstract.

Two homologous but different cDNAs encoding a 97-kDa and a 70-kDa protein from Brassica napus L. seedlings have been characterized. Both proteins contain sequence motifs with high homology to the IgA binding lectin, jacalin, and the deduced 97-kDa protein contains the peptide sequences of myrosinase-binding proteins. The 70-kDa and the 97-kDa protein can both be isolated as a complex containing myrosinase, indicating they indeed are myrosinase-binding proteins. We provide evidence that the 70-kDa protein binds IgA in vitro, and therefore classify the protein as a jacalin-type lectin. Both the 97-kDa and the 70-kDa proteins are encoded by a small number of genes in the Brassica genome. The mRNA for the 97-kDa protein is detected in both light- and dark-grown seedlings, whereas the mRNA for the 70-kDa protein is mainly detected in etiolated seedlings. The transcript levels for both proteins are transient and are rapidly increased by methyl jasmonate. The 70-kDa protein is synthesized de novo during germination and accumulates mainly in the hypocotyl and in the root. By immunogold labeling we show that a few cells scattered in the cotyledons of young seedlings (approx. 5% of total cells), contain protein-body-like structures with the 70-kDa protein. These bodies are present in a 10 000 g pellet from which the 70-kDa protein can be extracted by sodium carbonate. In addition, the 70-kDa protein is detected in the amorphous structures of the vacuole in a few cells of the cotyledon, the hypocotyl and the root.