Structural and biochemical studies of human PP2A(B55) holoenzyme and ENSA protein complex

The serine/threonine protein phosphatase 2A (B55) holoenzyme (PP2A(B55)), a key cell cycle regulator, is tightly regulated by ENSA, whose phosphorylation at Ser67 inhibits PP2A(B55). The structural basis for how pENSA orchestrates this inhibition to govern mitotic progression remains unclear. In this study, we disapproved the previous hypothesis of direct ENSA-PP2A A subunit interaction, and showed that the ENSA-PP2A(B55) interaction requires the PP2A holoenzyme. We then determined cryo-EM structure of the PP2A(B55)-ENSA S67D complex at 3.03 [A] resolution, which reveals four distinct regions of ENSA, each engaging specific sites on the B55 or C subunits. The structure reveals an extended, inhibited conformation of the PP2A(B55) holoenzyme, which contrasts sharply with the contracted, active state of apo PP2A(B55). pENSA binding induces the C subunit to shift away from the B55 subunit, with the pS67-containing motif inserting into the catalytic site of C subunit and locking the PP2A(B55) holoenzyme in an inactive state. Our work also revealed novel ligand-binding surfaces on B55, and expanded definition of the short linear motif (SliM). Overall, our work provided novel insights into PP2A regulation during cell cycle, and offers new avenues for the discovery of specific PP2A(B55) inhibitors and modulators.