DNA-binding domain of vitamin D receptors (VDR) is composed of two C4-type zinc fingers
DNA-binding domain of vitamin D receptors (VDR) is composed of two C4-type zinc fingers. Each zinc finger contains a group of four Cys residues which coordinates a single zinc atom. VDR interacts with a VDR response element, a direct repeat of GGTTCA DNA site with 3 bp spacer upstream of the target gene, and modulates the rate of transcriptional initiation. VDR is a member of the nuclear receptor (NR) superfamily that functions as classical endocrine receptors. VDR controls a wide range of biological activities including calcium metabolism, cell proliferation and differentiation, and immunomodulation. VDR is a high-affinity receptor for the biologically most active Vitamin D metabolite, 1alpha,25-dihydroxyvitamin D3 (1alpha,25(OH)2D3). The binding of the ligand to the receptor induces a conformational change of the ligand binding domain (LBD) with consequent dissociation of corepressors. Upon ligand binding, VDR forms a heterodimer with the retinoid X receptor (RXR) that binds to vitamin D response elements (VDREs), recruits coactivators. This leads to the expression of a large number of genes. Approximately 200 human genes are considered to be primary targets of VDR and even more genes are regulated indirectly. Like other members of the nuclear receptor (NR) superfamily of ligand-activated transcription factors, VDR has a central well conserved DNA binding domain (DBD), a variable N-terminal domain, a flexible hinge and a C-terminal ligand binding domain (LBD).