Fig. 4. (A) Illustration of multispecific oligomerization between two families of homologous proteins (A1–A3 and B1–B3). The lines between the proteins denote interactions with differing affinities. Protein contacts are exchanged between homologous members of a family (e.g. A1–B1 and A1–B2), depending on protomer co-localization and concentration, and their mutual affinities. Examples are given for multispecific oligomerization within one protein family (i.e. where A∼B): (B) Bacillus stearothermophilus HU homodimer and Escherichia coli IHF heterodimer (the α and β chain are depicted in red and orange, respectively), including the target DNA; (C) mouse NFκB P50–P50 homodimer and P50–P65 heterodimer (the P50 and P65 protomers are depicted in red and orange, respectively) of the dimerization and DNA-binding domain, including the bound operator DNA.