Preliminary crystallographic characterization of ricin agglutinin.

The quaternary structure of ricin agglutinin (RCA) has been determined by x-ray crystallography. The refined structure of ricin proved to be a successful search model using the molecular replacement method of phase determination. RCA forms an elongated molecule of dimensions 120 A x 60 A x 40 A with two A chains at the center and a B chain at each end. The A chains are covalently associated via a disulfide bridge between Cys 156 of both chains. Additional contacts at residues 114-5 stabilize the dimer interface. The covalent association of RCAA chains was confirmed by gel filtration under reducing and nonreducing conditions.