**2. Discovery of ERRs**

ERR was first identified during the efforts of searching for isoforms of estrogen receptors (ER) using a reduced stringency hybridization protocol to screen recombinant DNA libraries and discover novel receptors [7]. In this screening, the estrogen receptor DNA-binding domain (DBD) was used as the hybridization probe to screen the human testis cDNA library. This screening resulted in the identification of three positive clones of which two encoded for known ERs. The third clone demonstrated partial sequence similarity with ER. This sequence was later identified as a novel receptor and termed the ERRs [7]. Like other NRs, members of the ERR family contain six conserved functional domain structures (A-F) [8, 9]. The most highly conserved region is the C domain, which contains the DBD. This domain is composed of two highly conserved zinc finger motifs, CI and CII, which are DNA-binding motifs that allow ERR to bind to the estrogen related response element (ERRE), which contains the hexanucleotide DNA sequence 5′AGGTCA-3′ that is also recognized by ER [10] (**Figure 1**). The ERRs are able to bind to ERRE as either monomers, homodimers, or heterodimers. The A/B domain located at the N-terminal, is also referred to as the activation-function-1 (AF-1) domain that is ligand-independent. This domain is the least conserved region compared to other NRs, and its activation capacity varies between different NRs [11]. The D domain contains the hinge region that is needed for receptor dimerization and is also involved in the interaction of ERR

#### **Figure 1.**

*ERRE binding motif sequence for ERR. JASPAR2022 database was used to generate a logo of the DNA binding motif sequence for ERR. The total height of the letters C, G, T and A at each position was derived from the mean information content available from a collection of transcription factor binding sites in units of bits (y-axis). The height of each letter is representative of the nucleotide frequency in a specific position (x-axis) in the aligned promoter sequence. The three isoforms of human estrogen related receptors have the consensus DNA sequence 5*′*AGGTCA-3*′*, which is referred to as the estrogen related receptor response element (ERRE).*

*Transcriptional Regulation by ERR and Its Role in NAFLD Pathogenesis DOI: http://dx.doi.org/10.5772/intechopen.109089*

with co-regulatory proteins [12]. The E/F domain located at the C-terminal contains the ligand binding domain (LBD). Once a ligand binds to the receptor, ligand-induced conformational changes to the LBD occur, and the ligand becomes trapped in the hydrophobic-binding pocket from the hydrophobic core of the LBD [12–14]. The LBD is also part of the AF-2 domain that is ligand-dependent. The full transcriptional activity for a NR requires the synergistic cooperation between AF-1 and AF-2 [11].

The DBD of ERR shares 54–68% amino acid homology with other known NRs including ER, but with very little target gene similarities [8, 15]. While structurally similar to ER, ERRs preferentially bind to 5′-TCAAGGTCA-3′ rather than the direct repeat sequence of two hexanucleotide sequence of 5′-AGGTCA-3′ that is preferentially recognized by ER [10]. This ERRE sequence represents a 3 bp 5′ extension of the classical hexanucleotide repeat response elements for classical nuclear receptors. Such response elements have been observed for receptors that bind to DNA as monomers, including the rev-Erb and retinoid-related orphan receptors among others. Thus, ERR is expected to be capable of binding to ERRE as a monomer. In addition, ERRs also share high LBD sequence similarity with ERs. However, despite the high sequence and structure similarities of the LBD, studies have found that ERR is unable to bind to estrogen as endogenous ligands. Crystal structures of the coactivator bound LBD domains of ERR show that they adopt an active conformation without the binding of a ligand [16–18]. This conformation is similar in structure as the estrogen bound active ERα. The putative ligand binding pocket formed by the LBD domains of ERR appears to be the smallest observed among nuclear receptors and can only accommodate a structure that is half the size of estrogen. In addition, docking studies show that steric hindrance of D ring on estrogen with L345 and F435, precludes binding of estrogen to ERR in the active conformation.
