The nuclear receptor (NR) superfamily is comprised of transcription factors that are ligand-activated in their majority and play a pivotal role in biological functions that are essential for life, such as metabolism and homeostasis. Following activation, they undertake the regulation of the transcription of their target genes with the help of co-regulator proteins, rendering them very promising pharmacological targets. In total, 59 NRs have been discovered in several species of the Animalia kingdom, 20 of which are still orphan receptors. The present study aimed to further enlighten the evolutionary scenario that reveals the association between members of the NR superfamily. An updated evolutionary analysis was performed for the NR protein superfamily with the aim of clustering all the NRs, and discovering conserved regions and motifs that play major roles in their signaling pathway; the mechanisms of action were also investigated. The findings of the present study demonstrate a clear separation of the NR family in three majors monophyletic branches, the steroid hormone-related, the thyroid hormone-related and the retinoid X receptor-related clusters, from which, through evolution, may correspond to three ancestral NRs that were differentiated from a common ancestral NR.

Nuclear receptors (NRs) are one of the most diverse and well-reported family of proteins. They are involved in numerous cellular processes as they play pivotal roles in cell signaling and the cell cycle. The participation of NRs in various applications in medicine and biology has greatly attracted the interest of the pharmaceutical industry for the discovery of novel and/or improved drugs for the treatment of several diseases, including cancer, diabetes or infertility. In the present study, in an effort to elucidate the molecular function of this superfamily and to identify novel pharmacological targets, a comprehensive sequence and structural analysis was performed using all available information from a repertoire of depositories. Functional conserved motifs were identified and analyzed with regards to their potential roles and implications in a number of biological processes. The essential differences among them were also addressed and discussed. In addition, these motifs were characterized in the main groups of the NRs, such as that of the steroid hormone receptors.