Abstract
Transcriptional regulation by members of the nuclear hormone
receptor superfamily is a modular process requiring the mediation of
distinct subclasses of coregulators. These subclasses include members
of the steroid receptor coactivator-1 (SRC-1) coactivator family,
p300/CBP and their associated proteins, such as p300/CBP-associated
factor, human homologs of SWI/SNF proteins such as BRG-1, and the
less well-characterized E3 ubiquitin-protein ligases E6 papillomavirus
protein-associated protein and receptor-potentiating factor-1. Because
functional studies indicate that these coregulators may form higher
order complexes, we analyzed steady–state complexes of different
coregulator subclasses
in vivo
. T47D and HeLa cell
lysates were subjected to biochemical fractionation and screened by
immunoblotting using coregulator-specific antibodies. We show that
different subclasses of nuclear receptor coregulators exhibit distinct
fractionation profiles. Furthermore, evidence is provided that SRC-1
family members may exist
in vivo
in heteromultimeric
forms with each other. In addition, we demonstrate that liganded PR is
present in stable complexes containing SRC-1 and transcription
intermediary factor 2 (TIF2)
in vivo
. Our results
suggest that the assembly of large, modular transcriptional complexes
by recruitment of distinct subclasses of preformed coregulator
subcomplexes may be involved in transcriptional regulation by activated
nuclear receptors.
