Fatty-acylation target sequence in the ligand-binding domain of vertebrate steroid receptors demarcates evolution from estrogen-related receptors

Present-day nuclear receptors (NRs) responding to adrenal and sex steroids are key regulators of reproduction and growth in mammals, and are thought to have evolved from an ancestral NR most closely related to extant estrogen-related receptors (ERRs). The molecular events (and ligands) that distinguish steroid-activated NRs (SRs) from their inferred ancestor, that gave rise to both the ERRs and SRs, remain unknown. We report that target sequences for fatty-acylation (palmitoylation) at a key cysteine residue (corresponding to Cys447 in human estrogen receptor ERα) in helix 8 of the ligand-binding domain accurately demarcate SRs from ERRs. Docking studies are consistent with the hypothesis that palmitate embeds into a key groove in the receptor surface. The implications of lipidation, and of potential alternative ligands for the key cysteine residue, for receptor function and the evolution of SRs are discussed.