In vivo synthesis and processing of cereal lectins

Wheat-germ agglutinin is synthesized as a glycosylated precursor

The biosynthesis and processing of wheat-germ agglutinin (WGA) were studied in developing wheat (Triticum aestivum L. cv. Marshall) embryos using pulse-chase labeling, subcellular fractionation and immunocytochemistry. A substantial amount of newly synthesized WGA was organelle-associated. Isolation of WGA on affinity columns of immobilized N-acetylglucosamine indicated that it was present in a dimeric form. When extracts from embryos pulse-labeled with [(35)S]cysteine were fractionated on an isopycnic sucrose gradient, radioactivity incorporated into WGA was detected at a position coincident with the endoplasmic reticulum (ER) marker enzyme NADH-cytochromec reductase. The WGA in the ER could be slowly chased into the soluble, vacuolar fraction, with a half-life of approx. 8 h. Immunolocalization studies demonstrated the accumulation and distribution of WGA throughout the vacuoles.Four forms of the WGA monomer were characterized using immunoaffinity purification and sodium dodecyl sulfate-polyacrylamide gel electrophoresis. In-vitro translation of polyadenylated RNA isolated from developing wheat embryos produced a polypeptide with Mr 21 000. In-vivo labeling of embryos with radioactive amino acids resulted in the formation of a polypeptide of Mr 23 000 and the mature monomer of Mr 18000. When [(3)H]mannose was used in labeling studies, only the polypeptide of Mr 23 000 was detected. In-vivo labeling in the presence of tunicamycin yielded an additional polypeptide of Mr 20 000. These results indicate that WGA is cotranslationally processed by the removal of a signal peptide and the addition of a glycan, presumably at the carboxy-terminus (N.V. Raikhel and T.A. Wilkins, 1987, Proc. Natl. Acad. Sci. USA 84, 6745-6749). The glycosylated precursor of WGA is post-translationally processed to the mature form by the removal of a carboxyl-terminal glycopeptide.

TANSLEY REVIEW No 3. THE STRUCTURE AND SYNTHESIS OF PLANT LECTINS

Lectins are a group of proteins and glycoproteins which possess specific binding sites for particular sugars. They are very abundant in certain plant tissues, for example the seeds of legumes. Although their biological role in plants has not been convincingly explained, details of their structure, synthesis and molecular biology are beginning to emerge. Contents Summary 351 I. Introduction 351 II. Structure of lectins 352 III. Synthesis of lectins 359 IV. Conclusion 363 References 363.

Appearance of wheat-germ agglutinin during maturation of field-grown wheat

Field-grown wheat (Triticum aestivum L.) has been used as a developmental system to study the appearance of wheat-germ agglutinin during grain maturation. The lectin appears at the mid-grain growth period (30-34 days post-anthesis) and continues to be synthesised throughout the late stages of maturation and desiccation. An acidic endopeptidase activity, inhibited by pepstatin-phenanthroline is present in extracts of embryo and endosperm throughout maturation. After in-vivo labelling of immature embryos with [(35)S]methionine for 3 h and extraction in the presence of proteinase inhibitors, immunoprecipitates with anti-wheat-germ agglutinin were analysed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and fluorography, and found to contain three (35)S-labelled polypeptides of Mr 46000, 18000 and 13000. Comparison of two-dimensional tryptic maps of (125)I-labelled peptides indicate the three polypeptides are closely related.

Biosynthesis of lectin in roots of germinating and adult cereal plants

Root tips of wheat, rye, barley and rice seedlings contain lectins which are identical to the respective embryo lectins with respect to their molecular weight, sugar-specificity and serological properties. Using in vivo labelling techniques, it could be demonstrated that lectin is synthesized de novo in these tissues. The presence of lectin mRNA in seedlings was confirmed by in-vitro synthesis of lectin in root-tip extracts. Lectin synthesis occurs both in primary and first adventitious roots and is confined to the apical part (2mm) of the root. As seedling development proceeds, lectin synthesis in root tips gradually decreases. Adventitious roots of adult (five to six months old) wheat, rye and barley, but not rice, plants also contain lectins which are indistinguisable from the embryo lectins by the above-mentioned criteria. These lectins are synthesized in vivo in isolated root tips (5 mm) with labelled cysteine and in vitro in cell-free extracts prepared from root tips. Synthesis of lectin in roots of adult plants is also confined to the apical (2 mm) tip of the roots. At the molecular level, root lectin synthesis is very similar to that in embryos. All root lectins are synthesized as 23 000-Mr precursors which are post-translationally converted into the mature 18 000-Mr polypeptides. The observation that seedling roots and adventitious roots of six-month-old plants actively synthesize lectins strongly indicates that lectin genes are expressed in these tissues. In addition, since the root lectins are indistinguishable from the embryo lectins, we postulate that the same lectin genes are expressed.

Lectin accumulation and synthesis in developing rice embryos

Rice (Oryza sativa, L.) lectin is specifically synthesized during seed formation. It accumulates very rapidly between 8 and 16 days post anthesis and reaches its final content 20 days post anthesis. The rate of synthesis of this protein dramatically increases during the developmental phase coinciding with the rapid lectin accumulation. When the lectin accumulation stops, its rate of synthesis declines rapidly and eventually stops in fully mature embryos. It appears, therefore, that rice lectin is a maturation-specific protein.

Occurrence and immunological relationships of lectins in gramineous species

A survey of the occurrence of lectins in seeds from more than 100 grass species showed that all species belonging to the Triticeae tribe and the genera Brachypodium and Oryza contain lectins. All these lectins have the same sugar-binding specificity and are related to wheat-germ agglutinin, but to different degrees. Lectins from Triticeae species are immunologically indistinguishable from wheat lectin, whereas Brachypodium and rice lectins are only immunologically related to the wheat lectin. Attempts to detect lectin-deficient lines or varieties in wild and cultivated species of the three lectin-containing groups were unsuccessful. The possible use of lectins as a chemotaxonomic tool is discussed.