Dimerization with retinoid X receptors promotes nuclear localization and subnuclear targeting of vitamin D receptors

The vitamin D receptor (VDR) acts as heterodimer with the retinoid X recept or alpha (RXR) to control transcriptional activity of target genes. To expl ore the influence of heterodimerization on the subcellular distribution of these receptors in living cells, we developed a series of fluorescent-prote in chimeras. The steady-state distribution of the yellow fluorescent protei n-RXR was more nuclear than the unliganded green fluorescent protein (GFP)- VDR. Coexpression of RXR-blue fluorescent protein (BFP) promoted nuclear ac cumulation of GFP-VDR by influencing both nuclear import and retention. Flu orescence resonance energy transfer microscopy (FRET) demonstrated that the unliganded GFP-VDR and RXR-BFP form heterodimers. The increase in nuclear heterodimer content correlated with an increase in basal transcriptional ac tivity. FRET also revealed that calcitriol induces formation of multiple nu clear foci of heterodimers. Mutational analysis showed a correlation; betwe en hormone-dependent nuclear VDR foci formation and DNA binding. RXR-BFP al so promoted hormone-dependent nuclear accumulation and intranuclear foci fo rmation of a nuclear localization signal mutant receptor (nlsGFP-VDR) and r escued its transcriptional activity. Heterodimerization mutant RXR failed t o alter GFP-VDR and nlsGFP-VDR distribution or activity. These experiments suggest that RXR has a profound effect on VDR distribution. This effect of RXR to promote nuclear accumulation and intranuclear targeting contributes to the regulation of VDR activity and probably the activity of other hetero dimerization partners.