Fig. P1.

Stepwise evolution of a gene regulatory complex. In ancestral amniotes, HoxA11 (blue protein) was unable to interact with Foxo1a (green protein) despite the presence of an interaction interface present on each protein (blue triangle, green antitriangle). We believe the interaction interface is masked by N-terminal regions of HoxA11 (blue lines) and that selection in the mammalian stem lineage for amino acid substitutions caused a structural change (orange lines) that unmasks the binding interface. Later, in the eutherian lineage, additional and as yet unidentified changes to the protein complex facilitate the transactivating potential of the complex. PPI, protein-protein interaction; Act, gene activation potential.