DIMERIZING THE ESTROGEN-RECEPTOR DNA-BINDING DOMAIN ENHANCES BINDING TO ESTROGEN RESPONSE ELEMENTS

In this work, we provide a rationale for the finding that the estrogen receptor (ER) binds to its DNA response element as a homodimer in viv o. Binding of the monomer estrogen receptor DNA binding domain (ER DBD ) to a palindromic, consensus estrogen response element (ERE) is incre ased 5-6-fold when the ER DBD is dimerized either by a monoclonal anti body that recognizes an attached epitope tag or by expressing the ER D BD as a single molecule in which the two monomers are joined by a pept ide linker, Most of the increase in binding is due to stabilization of the ER DBD ERE complex. We observed only an approximately 2.5-fold re duction in binding when a consensus ERE was replaced with widely space d ERE half-sites, suggesting that the interaction between ER DBDs on t he ERE is relatively weak, and that in full-length ER the DBDs can mov e independently of each other, To test binding to an imperfect palindr ome, typical of the imperfect EREs found in almost all natural estroge n receptor responsive genes, we used the pS2 ERE. Even at high concent rations of ER DBD, specific binding of the ER DBD to the imperfect pS2 ERE was undetectable, Both of the dimerized ER DBDs exhibited efficie nt binding to the imperfect pS2 ERE, with an affinity at least 25-fold greater than monomer ER DBD, These data support the View that steroid receptor dimerization provides an important mechanism facilitating th e recognition of naturally occurring, imperfect hormone response eleme nts.