Two Distinct Jacalin-Related Lectins with a Different Specificity and Subcellular Location Are Major Vegetative Storage Proteins in the Bark of the Black Mulberry Tree

Els J.M. Van Damme ^*, Bettina Hause , Jialiang Hu , Annick Barre , Pierre Rougé , Paul Proost , and Willy J. Peumans

Laboratory for Phytopathology and Plant Protection, Katholieke Universiteit Leuven, 3001 Leuven, Belgium (E.J.M.V.D., J.H., W.J.P.); Institute of Plant Biochemistry, D-06018 Halle, Germany (B.H.); Institut de Pharmacologie et Biologie Structurale, Unité Mixte Recherche-Centre National de la Recherche Scientifique 5089, 31077 Toulouse cedex, France (A.B., P.R.); and Rega Institute, Laboratory of Molecular Immunology, Katholieke Universiteit Leuven, 3000 Leuven, Belgium (P.P.)

Using a combination of protein isolation/characterization and molecular cloning, we have demonstrated that the bark of the black mulberry tree (Morus nigra) accumulates large quantities of a galactose-specific (MornigaG) and a mannose (Man)-specific (MornigaM) jacalin-related lectin. MornigaG resembles jacalin with respect to its molecular structure, specificity, and co- and posttranslational processing indicating that it follows the secretory pathway and eventually accumulates in the vacuolar compartment. In contrast, MornigaM represents a novel type of highly active Man-specific jacalin-related lectin that is synthesized without signal peptide or other vacuolar targeting sequences, and accordingly, accumulates in the cytoplasm. The isolation and cloning, and immunocytochemical localization of MornigaG and MornigaM not only demonstrates that jacalin-related lectins act as vegetative storage proteins in bark, but also allows a detailed comparison of a vacuolar galactose-specific and a cytoplasmic Man-specific jacalin-related lectin from a single species. Moreover, the identification of MornigaM provides the first evidence, to our knowledge, that bark cells accumulate large quantities of a cytoplasmic storage protein. In addition, due to its high activity, abundance, and ease of preparation, MornigaM is of great potential value for practical applications as a tool and bioactive protein in biological and biomedical research.

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