Structure of the Protein Phosphatase 2A Holoenzyme

Abstract

Protein Phosphatase 2A (PP2A) plays an essential role in many aspects of cellular physiology. The PP2A holoenzyme consists of a heterodimeric core enzyme, which comprises a scaffolding subunit and a catalytic subunit, and a variable regulatory subunit. Here we report the crystal structure of the heterotrimeric PP2A holoenzyme involving the regulatory subunit B'/B56/PR61. Surprisingly, the B'/PR61 subunit has a HEAT-like (huntingtin-elongation-A subunit-TOR-like) repeat structure, similar to that of the scaffolding subunit. The regulatory B'/B56/PR61 subunit simultaneously interacts with the catalytic subunit as well as the conserved ridge of the scaffolding subunit. The carboxyterminus of the catalytic subunit recognizes a surface groove at the interface between the B'/B56/PR61 subunit and the scaffolding subunit. Compared to the scaffolding subunit in the PP2A core enzyme, formation of the holoenzyme forces the scaffolding subunit to undergo pronounced conformational rearrangements. This structure reveals significant ramifications for understanding the function and regulation of PP2A.

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.…”

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.…”

“…The rearrangements are due to conformational changes within HEAT repeats 11 and 12 that occur in the AC core dimer and the holoenzyme. An important contribution to the rearrangement is made by the breakage of a hydrogen bond in HEAT repeat 11 that is present in the AC core dimer but not the holoenzyme (6). The structural flexibility of the PP2A scaffold subunit is probably an important feature of the enzyme.…”