A key example is the divergence between the progesterone and androgen receptors (PR, AR), which display a high degree of sequence similarity and yet display differential ligand preferences. Less obvious, however, are the forces driving the selectivity of highly similar ligands. The most ancestral members of this family display promiscuous ligand binding due to their large ligand binding pockets, while more recently evolved SRs tend to have smaller cavities. Here, we use ancestral proteins to trace the evolutionary history of ligand recognition in a sub-class of steroid receptors (SRs), an ancient family of ligand-regulated transcription factors that enable long-range cellular communication central to multicellular life. Despite much speculation, we know little about how protein function evolves. The genetic and biophysical mechanisms by which new protein functions evolve are central concerns in evolutionary biology and molecular evolution.
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