Structural and biochemical insights into the regulation of protein phosphatase 2A by small t antigen of SV40

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The small t antigen (ST) of DNA tumor virus SV40 facilitates cellular transformation by disrupting the functions of protein phosphatase 2A (PP2A) through a poorly defined mechanism. The crystal structure of the core domain of SV40 ST bound to the scaffolding subunit of human PP2A reveals that the ST core domain has a novel zinc-binding fold and interacts with the conserved ridge of HEAT repeats 3–6, which overlaps with the binding site for the B′ (also called PR61 or B56) regulatory subunit. ST has a lower binding affinity than B′ for the PP2A core enzyme. Consequently, ST does not efficiently displace B′ from PP2A holoenzymes in vitro. Notably, ST inhibits PP2A phosphatase activity through its N-terminal J domain. These findings suggest that ST may function mainly by inhibiting the phosphatase activity of the PP2A core enzyme, and to a lesser extent by modulating assembly of the PP2A holoenzymes.

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Figure 1: Structure of the scaffolding subunit of PP2A bound to SV40 ST.
Figure 2: Structural features of the core domain of ST.
Figure 3: Recognition of the A subunit of PP2A by ST.
Figure 4: The J domain of ST directly contributes to binding of the PP2A core enzyme.
Figure 5: ST inhibits the phosphatase activity of the PP2A core enzyme.
Figure 6: Binding of ST and of B′ to the PP2A core enzyme are mutually exclusive.
Figure 7: A proposed mechanistic model of ST-mediated inhibition of PP2A.

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We thank T. Roberts at Harvard Medical School for the complementary DNA encoding SV40 ST, and A. Saxena at the beamlines of the National Synchrotron Light Source, Brookhaven National Laboratory for help. This work was supported by grant R01-CA123155 from the US National Institutes of Health (Y.S.).

Author information

Y.C. and Y. Xu designed, performed and analyzed most of the experiments. Q.B. contributed to PP2A enzymology. Y. Xing, Z. Li and Z. Lin provided technical assistance. J.B.S. contributed to discussions. P.D.J. refined the structure. Y.S. led the team and wrote the paper.

Correspondence to Yigong Shi.

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