Structure of the native (unligated) mannose-specific bulb lectin from Scilla campanulata (bluebell) at 1.7 angstrom resolution

The X-ray crystal structure of native Scilla campanulata agglutinin, a mann ose-specific lectin from bluebell bulbs and a member of the Liliaceae famil y, has been determined by molecular replacement and refined to an R value o f 0.186 at 1.7 Angstrom resolution. The lectin crystallizes in space group P2(1)2(1)2 with unit-cell parameters a = 70.42, b = 92.95, c = 46.64 Angstr om. The unit cell contains eight protein molecules of M-r = 13143 Da (119 a mino-acid residues). The asymmetric unit comprises two chemically identical molecules, A and B, related by a noncrystallographic twofold axis perpendi cular to c. This dimer further associates by crystallographic twofold symme try to form a tetramer. The fold of the polypeptide backbone closely resemb les that found in the lectins from Galanthus nivalis (snowdrop) and Hippeas trum (amaryllis) and contains a threefold symmetric beta-prism made up of t hree antiparallel four-stranded beta-sheets. Each of the four-stranded beta -sheets (I, II and III) possesses a potential saccharide-binding site conta ining conserved residues; however, site II has two mutations relative to si tes I and III which may prevent ligation at this site. Our study provides t he first accurate and detailed description of a native (unligated) structur e from this superfamily of mannose-specific bulb lectins and will allow com parisons with a number of lectin-saccharide complexes which have already be en determined or are currently under investigation.