Three-dimensional structure of beta-momorcharin at 2.55 angstrom resolution

beta-Momorcharin (M-r similar or equal to 29 kDa) is a single-chained ribos ome-inactivating protein (RIP) with a branched hexasaccharide bound to Asn5 1. The crystal structure of beta-momorcharin has been determined using the molecular-replacement method and refined to 2.55 Angstrom resolution. The f inal structural model gave an R factor of 17.2% and root-mean-square deviat ions of 0.016 Angstrom and 1.76 degrees from ideal bond lengths and bond an gles, respectively. beta-Momorcharin contains nine alpha-helices, two 3(10) helices and three beta-sheets, and its overall structure is similar to tho se of other single-chained RIPs. Residues Tyr70, Sr109, Glu158 and Arg161 a re expected to define the active site of beta-momorcharin as an rRNA N-glyc osidase. The oligosaccharide is linked to the protein through an N-glycosid ic bond, beta-GlcNAc-(1-N)-Asn51, and stretches from the surface of the N-t erminal domain far from the active site, which suggests that it should not play a role in enzymatic function. The oligosaccharide of each beta-momorch arin molecule interacts with the protein through hydrogen bonds, although i n the crystals most of these are intermolecular interactions with the prote in atoms in an adjacent unit cell. This is the first example of an RTP stru cture which provides information about the three-dimensional structure and binding site of the oligosaccharide In the active chains of RIPs.