Abstract
The crystal structure of vitelline membrane outer layer protein I (VMO-I), which was isolated from the outer layer of the vitelline membrane of hen's eggs, has been determined by X-ray analysis. VMO-I is composed of three homologous structures, each containing a β-sheet forming Greek key motif, which are in accordance with the three repeats in the sequence. These three homologous structures are related by a pseudo three-fold symmetry. This new folding motif, recently designated as the β-prism, has also been observed in the second domain of δ-endotoxin. Thus, VMO-I and δ-endotoxin constitute a new family with a novel folding motif. The VMO-I molecule has a groove-like cavity. This region contains invariant acidic residues in the three repeats. VMO-I is assumed to be an oligosaccharide binding protein like lysozyme, although details of its catalytic function remain to be solved. A docking model of an oligosaccharide to VMO-I has been constructed. This model shows that the cavity has a sufficient space to bind roughly pentamer saccharides. The structural features strongly suggest that the negatively-charged cavity in the top region of the protein may be involved in an unknown catalytic reaction.
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