| The steroid receptors in the nuclear receptor superfamily are responsible for regulating many different functions including transcription. However, in order to completely and correctly activate transcription, the steroid receptors must recruit not only the general initiation factors but also other coregulatory factors, such chromatin remodelers and coactivators. Tip60, a 60 kD protein with an N-terminal chromodomain, a central intrinsic histone acetyltransferase (HAT) activity, and a C-terminal nuclear receptor binding domain (NR box) was previously reported to be a coactivator for androgen receptor, in addition to being able to bind to the estrogen receptor in vivo. Subsequently, Tip60 was identified to be recruited very early on to the pS2 estrogen response element1 (ERE1) site, hormone binding enhancer elements involved in transactivation, upon hormone treatment. Here, it is shown that the interaction between Tip60 and the estrogen receptor, both in vivo and in vitro , requires ligand binding to estrogen receptor and also interacts via the C-terminal NR box, specifically the LXXLL motif. Furthermore, the abolishment of the LXXLL motif compromises Tip60's recruitment to certain estrogen response elements (EREs) of ER target genes. In addition, the N-terminal chromodomain is shown to preferentially bind to histone H3 mono-methylated and di-methylated at lysine4. Finally, it is shown that a competition for methylated histones between the wildtype Tip60 and the exogenously expressed Tip60 chromodomain fragment also compromises its recruitment to certain EREs of ER target genes. Thus, the stable recruitment and association of Tip60 at the EREs is dependent upon the normal function of its N-terminal chromodomain and C-terminal NR box. |
