Arrangement of subunits in peanut lectin. Rotation function and chemical cross-linking studies.

X-ray intensity data from the native orthorhombic crystals of peanut lectin have been collected using oscillation photography. Rotation function studies using data up to a resolution of 4.5 A indicate that the four subunits in the molecule, which constitute the asymmetric unit in the crystals, are related to one another by three mutually perpendicular noncrystallographic 2-fold axes. Chemical cross-linking experiments in solution followed by sodium dodecyl sulfate gel electrophoresis, carried out in parallel, suggest that there is more than one type of intersubunit approach in the molecule. Rotation function and cross-linking studies thus show that the tetrameric molecule of peanut lectin is a dimer of a dimer. The two monomers in a dimer are related by a 2-fold axis. The two dimers are in turn related by another 2-fold axis perpendicular to the one that relates the two monomers in the dimer, endowing the molecule with 222 (D2) symmetry.