Impact of heterodimerization on intracellular localization of the ecdysteroid receptor (EcR)

Initially, nuclear import of the ecdysteroid receptor (EcR) in vertebrate cells (CHO-K1 and COS-7) does not afford a heterodimerization partner. Later on, EcR is retained in the nucleus only in the presence of a heterodimerization partner. Ultraspiracle (Usp) is more efficient compared to its vertebrate orthologue RXR and leads to an exclusively nuclear localization of EcR even in the absence of ligand. The DNA binding domain of the heterodimerization partner is important for retainment of EcR in the nucleus as shown by Usp4 (Usp(R130C)), which has lost its DNA binding capability. The C-terminal end of Usp (Usp(Delta205-508)) encompassing the C-terminal part of the D-domain and the E- and F-domains are essential for retainment of EcR in the nucleus. Nuclear localization is further influenced by cell-specific factors, since hormone and heterodimerization stabilizes the EcR protein in a cell-specific way.