Abstract
Lectins or agglutinins have various effects in plant. For instance, they play an important role in translation of sugar or the process of sugar storage. Because of a strong binding affinity between lectin and sugar, lectin can also serve a storage protein. Lectin is related to the disease-resistance in plant and it can prevent plant from infection of bacillus. Our previous study showed the expression of rice lectin gene is up-regulated in the growing coleoptiles when the anaerobic stress persists. The purpose of this study is to determine the 3-D structure of the rice lectin, analyze the active site and compare it with the other lectins from other species. The rice lectin structure determined at resolution 2.4 Å comprises 12 β-sheets with the prism folding. The model contains 145 residues in total. Dimeric molecular packing in both crystal and solution implies that rice lectin functions as dimmers, which is different from Heltuba, a mannose-binding protein as a tetramer. Six functional important residues located at the active site, i.e., Gly14, Leu88, Gly133, Thr134, Leu135, and Asp137, forms a negatively charged packet which is larger than that of Heltuba. Three amino acids, Leu88, Thr134, and Leu135, which is different from those of Heltuba in the packet, may be related to the binding specificity to mannoses. The fluorescence study on specific binding with 2α-mannoside showed the intensities are pH-dependent, indicating the conformation of the lectin changes in varied pH environment. Moreover, the mannose binding effect of lecin in pH7.0 is greater than in pH3.5. The rice lectin structure provides the structural information essential to understand the functional significance of this enzyme during growth and development in rice.