The human transcription factor IID subunit human TATA-binding protein-associated factor 28 interacts in a ligand-reversible manner with the vitamin D-3 and thyroid hormone receptors

Using coexpression in COS cells, we have identified novel interactions betw een the human TATA-binding protein-associated factor 28 (hTAF(II)28) compon ent of transcription factor LTD and the ligand binding domains (LBDs) of th e nuclear receptors for vitamin D3 NDR) and thyroid hormone (TR alpha). Int eraction between hTAF(II)28 and the VDR and TR LBDs was ligand-reversible, whereas no interactions between hTAF(II)28 and the retinoid X receptors (RX Rs) or other receptors were observed. TAF(II)28 interacted with two regions of the VDR, a 40-amino acid region spanning alpha-helices H3-H5 and alpha- helix H8. Interactions were also observed with the H3-H5 region of the TR a lpha but not with the equivalent highly related region of the RXR gamma. Fi ne mapping using RXR derivatives in which single amino acids of the RXR gam ma LED have been replaced with their VDR counterparts shows that the determ inants for interaction with hTAF(II)28 are located in alpha-helix H3 and ar e not identical to those previously identified for interactions with hTAF(I I)55. We also describe a mutation in the H3-H5 region of the VDR LED, which abolishes transactivation, and we show that interaction of hTAF(II)28 with this mutant is no longer ligand-reversible.