The D-galactose specific lectin of field bean (Dolichos lablab) seed binds sugars with extreme negative cooperativity and half-of-the-sites binding

Expression profiling of the Dolichos lablab lectin during germination and development of the seed

The temporal expression of the field bean (Dolichos lablab) galactose specific lectin, DLL-II, during germination, post-germination and seed development was evaluated using Native-PAGE followed by activity staining, immunodetection and quantitative Real Time PCR (qPCR). A rapid and steep decline in the polyphenol oxidase (PPO) and hemagglutinating activity during the initial stages of germination, which did not correlate with the slow decline in total protein was observed. During post germination period, PPO and hemagglutination activities were negligible, whereas a rapid resorption of the protein was evident. These results suggest that DLL-II is not a storage protein. The presence of mRNA in the quiescent seed and initial stages of germination are indicative of a very stable mRNA. DLL-II was expressed in high copies during seed development and increased dramatically between 10 and 20 days after flowering (DAF), suggesting a switch over stage in DLL-II expression. Transcript levels reached a maximum at the mature stage of seed development. Among the non-seed tissues examined, root showed the highest level. The high affinity binding to kinetin and indole acetic acid, the key hormones that regulate root development and its vascular differentiation add a new dimension to the physiological role of DLL-II in the seed. This finding, coupled with the PPO and hemagglutinating activity makes DLL-II, truly a multifunctional protein.

Chloroplast Glutamine Synthetase, the Key Regulator of Nitrogen Metabolism in Wheat, Performs Its Role by Fine Regulation of Enzyme Activity via Negative Cooperativity of Its Subunits

Glutamine synthetase (GS) is of central interest as the main route of ammonia assimilation in plants, and as a connection point between the organic and inorganic worlds. Even though GS activity is critical for producing high yields of crop plants, the autoregulation of substrate consumption of wheat GS remained unknown until now. Here we show kinetic evidence, that the chloroplast localized GS isoform (GS2) of wheat (Triticum aestivum L. cv. Jubilejnaja-50) takes place at the carbon-nitrogen metabolic branch point, where it is a mediator, and its enzymatic activity is regulated in a negatively cooperative allosteric manner. We have discovered that GS2 activity is described by a tetraphasic kinetic curve in response to increasing levels of glutamate supply. We constructed a model that explains the kinetic properties of glutamate consumption and this unique allosteric behavior. We also studied the subunit composition of both wheat leaf GS isoenzymes by a combination of two dimensional gel electrophoresis and protein blotting. Both leaf isozymes have homogeneous subunit composition. Glutamate is both a substrate, and an allosteric regulator of the biosynthetic reaction. We have concluded on the basis of our results and previous reports, that wheat GS2 is probably a homooctamer, and that it processes its substrate in a well-regulated, concentration dependent way, as a result of its negatively cooperative, allosteric activity. Thus, GS2 has a central role as a regulator between the nitrogen and the carbon cycles via maintaining glutamine-glutamate pool in the chloroplast on the level of substrates, in addition to its function in ammonia assimilation.

The enzymatic lectin of field bean (Dolichos lablab): salt assisted lectin-sugar interaction

Field bean seed contains a Gal/GalNAc lectin (DLL-II) that exhibits associated polyphenol oxidase (PPO) activity and does not bind to its sugar specific affinity matrix. The molecular basis for this lack of binding is not known. The DLL-II gene was therefore cloned and its sequence analyzed. A conserved aromatic residue in the sugar binding site required for a stacking interaction with the apolar backbone of Gal is replaced by His in DLL-II, which explains the lack of binding. However, specific sugar binding is achieved by including (NH₄)₂SO₄ in the buffer. Interestingly two other salts of the Hofmeister series, K₂HPO₄ and Na₂SO₄ also assist binding to immobilized galactose. In the presence of (NH₄)₂SO₄ the surface hydrophobicity of DLL-II and dissociation constant for 8-anilino 1-naphthalene sulfonic acid were enhanced three fold. This increased surface hydrophobicity in the presence of salt is probably the cause for assisted sugar binding in legume lectins that lack aromatic stacking interactions. Accordingly, two other lectins which lack the conserved aromatic residue show similar salt assisted binding. The salt concentrations required for Gal/GalNAc binding are not physiologically relevant in vivo, suggesting that the role of DLL-II per se in the seed is primarily that of a PPO purportedly for plant defense.