Clayton Foundation Biochemical Institute, Department of Chemistry and Biochemistry, University of Texas, Austin 78712.
The heterodimeric plant toxin ricin has been refined to 2.5 A resolution. The B-chain lectin (RTB) is described in detail. The protein has two major domains, each of which has a galactose binding site. RTB has no regular secondary structure but displays several omega loops. Each RTB domain is made of three copies of a primitive 40 residue folding unit, which pack around a pseudo threefold axis. In each domain, galactose binds in a shallow cleft formed by a three residue peptide kink on the bottom and an aromatic ring on the top. At the back of the cleft, an aspartate forms hydrogen bonds to the C3 and C4 hydroxyls of galactose, whereas a glutamine bonds to the C4 alcohol, helping to define specific epimer binding. In addition to analyzing the sugar binding mechanism, the assembly of subdomain units around the pseudo threefold axis of each domain is described. The subdomains contribute conserved Trp, Leu, and Ile residues to a compact central hydrophobic core. This tight threefold binding probably drives the peptide folding and stabilizes the protein structure.