Protein modifications giving rise to homo-oligomers

After the structures of numerous proteins have been established at the atomic level and after a multitude of proteins can be produced with almost no restrictions, the time seems ripe to apply this knowledge for engineering purposes. An apparently simple task is the designed association of protein molecules to form homo-oligomers. A number of worked examples are presented. The associations split into flexible versus rigid designs and also into fixed versus switchable ones. It is shown that the practical work is tightly governed by the multiplicity concept, which in turn is interwoven with symmetry. The available symmetries and multiplicities are explained. Unfortunately, the most desirable contacts with a multiplicity of one, which lead to asymmetric assemblies with 5-50 nm spacings, are most difficult to achieve. Emerging rules for the required surface properties are put forward. Suitable mutations for changing such surfaces are discussed.