Abstract
WW-domain binding protein-2 (WBP-2) was cloned as an E6-associated protein (E6-AP) interacting protein and its role in steroid hormone receptor (SHR) function was investigated. We show that WBP-2 differs from other SHR coactivators, as it specifically enhanced the transactivation functions of progesterone receptor (PR) and estrogen receptor (ER alpha), whereas it had no significant effect on the androgen receptor, glucocorticoid receptor or the activation functions of p53 or VP-16. We also demonstrated that, like other well characterized coactivators, WBP-2 contains an intrinsic activation domain. Depletion of endogenous WBP-2 with small interfering RNAs indicated that normal physiological protein level of WBP-2 was required for the proper functioning of ER alpha and PR. Moreover, chromatin immunoprecipitation (ChIP) assays demonstrate the hormone-dependent recruitment of WBP-2 onto an estrogen-responsive promoter. As we initially identified WBP-2 as an E6-AP interacting protein, we investigated whether WBP-2 and E6-AP function in concert. Our data shows that WBP-2 and E6-AP each enhance PR function and when co-expressed they additively enhance the transactivation functions of PR. However, WBP-2 was also able to enhance the transactivation functions of ER alpha and PR in mouse embryonic fibroblast cells generated from E6-AP knockout mice lines, suggesting that the coactivation functions of WBP-2 was not dependent on E6-AP. The further elucidate the molecular mechanism of action of WBP-2; we dissected the functional importance of the polyproline (PY) motifs contained within the WBP-2 protein. Mutational analysis suggests that one of three PY motifs, PY3 of WBP-2 was essential for its coactivation and intrinsic activation functions. In this study, we also demonstrate that the WBP-2 binding protein, Yes-kinase associated protein 1 (YAP1) acts as a secondary coactivator of ER alpha and PR. However, the coactivation function of YAP1 is revealed only in the presence of wild-type WBP-2 and not with the PY motif 3 mutant WBP-2. This is consistent with our observations that, unlike the wild-type WBP-2, the PY motif 3 mutant WBP-2 does not interact with YAP1. Our quantitative reChIP assays demonstrates an estrogen-dependent recruitment and association of ER alpha with both WBP-2 and YAP1. The hormone-dependent recruitment of YAP1 to ER alpha responsive promoter is dependent on the physiological expression levels of WBP-2. This is consistent with, our observation that the coactivation functions of YAP1 is dependent on WBP-2, and is also in agreement with other known secondary coactivators that get recruited to SHR responsive promoter via their interaction with primary coactivators. Surprisingly, the association of WBP-2 with ER alpha and its recruitment to the ER alpha target promoter was abrogated by YAP1 knock-down, suggesting that WBP-2 and YAP1 may stabilize each other at the promoter, and consequently, are functionally interdependent. Taken together our data establish the role of WBP-2 and YAP1 as selective coactivators for ER alpha and PR transactivation pathways.