Electron microscopy of an oligomeric protein stabilized by polyfunctional cross-linking.

Oligomeric proteins can be intramolecularly cross-linked with polylysine in a reaction in which a water soluble carbodiimide mediates an amide linkage between the protein carboxyl groups and the epsilon-amino groups of polylysine. Studies carried out with a cytochrome p-450 indicate that a small number of molecules in a population which has been cross-linked in this way retain important features of their tertiary and quaternary structure when negatively stained and examined in the electron microscope. Use of the method in determining the subunit geometry of oligomeric proteins is discussed.