Progestin-inducible EDD E3 ubiquitin ligase binds to α4 phosphoprotein to regulate ubiquitination and degradation of protein phosphatase PP2Ac

Mammalian α4 phosphoprotein binds to the protein phosphatase 2A catalytic subunit (PP2Ac) to regulate PP2A activity, and to poly(A)-binding protein (PABP) and progestin-inducible EDD E3 ubiquitin ligase. This study showed induction of the EDD protein by progesterone, 17β-estradiol and prolactin in breast cancer cells. Co-immunoprecipitation analyses, using lysates of COS-1 cells transfected with α4-deletion constructs, showed the α4 N-terminus binding to endogenous PP2Ac and PABP, and the C-terminus to EDD. Monoubiquitinated α4 in MCF-7 cells was unaffected by EDD-targeting siRNA (siEDD) nor by non-targetting siNT, thus, EDD does not ubiquitinate α4. PP2Ac is polyubiquitinated, and 36-kDa PP2Ac only was detected in siEDD- or siNT-transfected cells. However, treatment with proteasomal inhibitor MG132 showed polyubiquitinated-PP2Ac molecules (∼65-250 kDa) abundantly in siNT controls but low in siEDD-transfectants, implicating PP2Ac as an EDD substrate. Finally, progesterone induction of EDD in MCF-7 cells correlated with decreased PP2Ac levels, further implicating hormone-inducible EDD in PP2Ac turnover.