UC Riverside

The heterotrimeric PP2A holoenzyme consists of a scaffolding A, a catalytic C, and a variable regulatory B subunit. Previous study has reported that B56gamma subunit mediates the tumor suppressive function of PP2A. B56gamma specific PP2A inhibits cell transformation by dephosphorylating and activating p53. In addition, B56gamma specific PP2A possesses the p53 independent tumor suppressive function although its mechanism is unclear. In the present study, we demonstrate the mechanism of B56gamma tumor suppressive function inactivation through characterizing B56gamma mutations identified in human cancer.

First, we demonstrate that the A383G and F395C mutations disrupt B56gamma from interacting and activating p53. The mutants are unable to dephosphorylate p53 and thus lack the p53 dependent tumor suppressive function. Moreover, we identify the region adjacent to these mutations as the p53 binding domain. This finding implies that the bridging interaction between B56gamma and p53 is required for p53 dephosphorylation by PP2A.

Second, we show that the C39R mutation within the HEAT repeat 1 disrupts B56gamma binding with the AC core and thus abolishes the B56gamma tumor suppressive function completely. We further demonstrate that the intact HEAT repeat 1 is required for B56gamma to bind the AC core, providing structural insights into the B56gamma specific PP2A holoenzyme assembly.

Further characterization of additional B56gamma mutations reveals two classes of mutations with different mechanisms of tumor suppressive function inactivation. Mutations in the first class disrupt the interaction between B56gamma and the AC core and thus fail to regulate all substrates and completely lose the tumor suppressive function. Mutations in the second class prevent specific substrates from binding B56gamma and partially reduce the tumor suppressive function. Although it remains to be investigated how frequently these mutations occur, our results underline the importance of B56gamma tumor suppressive function in human cancer.

Finally, we identified the S220N mutation that specifically disrupts the p53 independent tumor suppressive function of B56gamma, suggesting that it may disrupt the binding of unknown substrates. Identification of these proteins would contribute to the further understanding of the tumor suppressor role of B56gamma in cells.