SUGAR IN YOUR GARDEN/DIET: LECTIN/SUGAR COMPLEXES. P.J. Rizkallah (Daresbury), C.D. Reynolds (John Moores University, Liverpool), S.D. Wood (JMU), L.M. Wright (JMU), R. Kelly (JMU), Pei-Wen Lei (JMU), R. Loris (Free University, Brussels), A.K. Allen (Imperial College)

1. Flower Bulb Lectins

The mannose binding lectins from flower bulbs were found to have antiviral activity, most importantly against HIV, by recognising the glycoprotein GP120 on its surface. This property has been exploited in the purification of HIV virions. The structures of these lectins from amaryllis and bluebell bulbs were solved, in order to characterise their interaction with mannose, and also as a first step towards solving the structure of the complex with GP120. Molecular Replacement (MR) experiments showed that as little as 4% of the total scattering material was sufficient to solve the amaryllis lectin structure, using a model of the snowdrop lectin. A difference electron density map revealed a mannose bound to each of the two molecules in the asymmetric unit. The overall packing showed tetrameric clusters with pseudo 222 symmetry, where 2 asymmetric units provided a dimer each. Refinement was carried out at 2.3 Å. The bluebell lectin is highly similar, in affinity and other properties, to the amaryllis lectin. It was crystallised in an orthrhombic space group, with and without mannose, and both forms diffracted to 1.85 Å. Using the refined model of the amaryllis lectin, the structure was solved with MR. The packing was slightly different, possibly due to the extra protein sequence in this lectin. Refinement is currently underway.

2. Legume Lectins

Unbound and complexed Lentil lectin has been crystallised and the structure refined at high resolution in three different crystal forms (all at 1.5Å). Seeds of leguminous plants use lectins to store specific sugars. Although lectin structures are similar, their specificity varies according to species. The gene structure is common among all, with a high degree of homology. The binding site motif is also common, with side chain mutations determining the specificity. Binding is indirectly mediated by two metal binding sites close to the saccharide site, stabilising an unusual cis-peptide bond, important for sugar recognition. The unbound lectin crystallised in two different space groups which diffracted to a resolution higher than 1.4 Å. Data were collected up to 1.5 Å. The sucrose complex crystallised in a third space group, and also diffracted to 1.5 Å resoltuion, when cryo-cooled. The differences between the three structures were mainly due to the packing arrangement.