Crystal structure of a protein phosphatase 2A heterotrimeric holoenzyme

Abstract

Protein phosphatase 2A (PP2A) is a principal Ser/Thr phosphatase, the deregulation of which is associated with multiple human cancers, Alzheimer's disease and increased susceptibility to pathogen infections. How PP2A is structurally organized and functionally regulated remains unclear. Here we report the crystal structure of an AB'C heterotrimeric PP2A holoenzyme. The structure reveals that the HEAT repeats of the scaffold A subunit form a horseshoe-shaped fold, holding the catalytic C and regulatory B' subunits together on the same side. The regulatory B' subunit forms pseudo-HEAT repeats and interacts with the C subunit near the active site, thereby defining substrate specificity. The methylated carboxy-terminal tail of the C subunit interacts with a highly negatively charged region at the interface between A and B' subunits, suggesting that the C-terminal carboxyl methylation of the C subunit promotes B' subunit recruitment by neutralizing charge repulsion. Together, our structural results establish a crucial foundation for understanding PP2A assembly, substrate recruitment and regulation.

PP2A holoenzyme assembly: in cauda venenum (the sting is in the tail)

“…Three families of B-type subunits have been described -PR55/B, PR61/B 0 and PR72/B 00 -each of which exists in at least four different isoforms in humans (see Table 1 in main text), thereby generating $70 different PP2A heterotrimers (PP2A T'x' ; Table 1). Ribbon diagrams represent a modeled structure of the PR55/B subunit, and the only known crystal structures (thus far) of PP2A D and PP2A T61g1 (adapted from PDB accession codes 2IAE [36] and 2IE4 [37]), with the PR61/Bg1 subunit shown in green. In the PR55/B subunit, some of the charged amino acids that mediate the interaction with the A subunit are indicated (green and blue).…”

“…The active site contains two catalytic metal ions and points towards the end of the Aa subunit to which the regulatory B-type subunit probably binds [34]. The configuration of PR61/B 0 g within the holoenzyme is also solved: it binds the intra-repeat loops of HEAT-repeats 2-7 on the same apical side of the horseshoe as the C subunit [36,37] (Box 1, Figure Ia). The structure of PR61/B 0 g contains eight two-helix units, termed 'pseudo' HEAT repeats, that stack to form a solenoid shaped structure.…”

“…The structure of PR61/B 0 g contains eight two-helix units, termed 'pseudo' HEAT repeats, that stack to form a solenoid shaped structure. However, the interface between A and PR61/B 0 g is rather loose, and PR61/ B 0 g also forms extensive contacts with the b12-b13 loop, the helical subdomain and the C-terminal tail of the C subunit [36,37]. The active site within the C subunit remains open for substrate access, but the close proximity of the regulatory subunit controls substrate specificity by providing a docking surface for specific substrates and thus preventing access of non-specific substrates.…”

“…Structural data show that this tail recognizes an acidic surface groove at the Aa-PR61/B 0 g1 interface and is thus in a crucial position for influencing binding of PR61/B 0 g and possibly other B-type subunits [36,37] (Box 1, Figure I). …”

The 3D Structure of Protein Phosphatase 2A: New Insights into a Ubiquitous Regulator of Cell Signaling

“…A report by Shi and colleagues solved the structure of the AC core dimer bound to inhibitory toxins (4). Two additional papers, one by Cho and Xu (5) and a second from Shi and colleagues (6), coincidently report the structure of the same PP2A holoenzyme composed of the AC core dimer associated with the B56␥ regulatory subunit.…”