Figure 2.

Protein evolution entails the incorporation of a small structural module into a larger, functional tertiary fold, which can then associate with other functional domains to form quaternary assemblies with diverse, multi-component functions. This process is exemplified here with the βαβ motif-containing Rossmann fold, which is part of protein complexes with functions spanning histone deacetylation (sirtuin), DNA repair (photolyase), and dinitrogen reduction (nitrogenase). In all three quaternary assemblies, the Rossmann fold functions as the nucleotide-binding domain.