2 JASMONATE-INDUCIBLE MYROSINASE-BINDING PROTEINS FROM BRASSICA-NAPUSL. SEEDLINGS WITH HOMOLOGY TO JACALIN

Two homologous but different cDNAs encoding a 97-kDa and a 70-kDa prot ein from Brassica napus L. seedlings have been characterized. Both pro teins contain sequence motifs with high homology to the IgA binding le ctin, jacalin, and the deduced 97-kDa protein contains the peptide seq uences of myrosinase-binding proteins. The 70-kDa and the 97-kDa prote in 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 pro tein 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 mR NA for the 97-kDa protein is detected in both light-and dark-grown see dlings, whereas the mRNA for the 70-kDa protein is mainly detected in etiolated seedlings. The transcript levels for both proteins are trans ient and are rapidly increased by methyl jasmonate. The 70-kDa protein is synthesized de novo during germination and accumulates mainly in t he hypocotyl and in the root. By immunogold labeling we show that a fe w cells scattered in the cotyledons of young seedlings (approx. 5% of total cells), contain protein-body-like structures with the 70-kDa pro tein. 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.