Structural basis of stable PP2A latency by α4‐protein

Abstract

Phosphoprotein α4 is essential for cell survival and is known as an inhibitor of protein phosphatase 2A (PP2A). α4 binds to PP2A catalytic subunit (PP2Ac) and stabilizes it in an inactive, latent form. Strikingly, we show that α4 binds preferentially to the partially folded PP2Ac. The crystal structure of α4 bound to a folding sub‐domain of PP2Ac (PP2AFD) reveals that α4 binding entails a distorted PP2A active site and displacement of an internal packing near the phosphatase active site. Measurement of the H‐D exchange of the α4‐bound PP2Ac versus active enzyme reveals conformational changes in PP2Ac associated with α4 binding that correlates with the crystal structure. Our structural observations imply that binding of α4 to the inner surface of PP2Ac may serve dual function: 1) establish PP2A latency; 2) stabilize PP2Ac by preventing aggregation of the partially folded or newly synthesized PP2Ac and protecting PP2Ac from proteasomal degradation by shielding a PP2A polyubiquitination site. In addition, disruption of PP2A‐α4 interaction in vivo is sufficient to cause cell death and complete loss of PP2Ac in cells. Our data reveal molecular details of α4‐PP2A interaction and offer structural mechanism of α4 function and stable PP2A latency.