Identification of Binding Sites on the Regulatory A Subunit of Protein Phosphatase 2A for the Catalytic C Subunit and for Tumor Antigens of Simian Virus 40 and Polyomavirus

Abstract

Protein phosphatase 2A is composed of three subunits: the catalytic subunit C and two regulatory subunits, A and B. The A subunit consists of 15 nonidentical repeats and has a rodlike shape. It is associated with the B and C subunits as well as with the simian virus 40 small T, polyomavirus small T, and polyomavirus medium T tumor antigens. We determined the binding sites on subunit A for subunit C and tumor antigens by site-directed mutagenesis of A. Twenty-four N- and C-terminal truncations and internal deletions of A were assayed by coimmunoprecipitation for their ability to bind C and tumor antigens. It was found that C binds to repeats 11 to 15 at the C terminus of A, whereas T antigens bind to overlapping but distinct regions of the N terminus. Simian virus 40 small T binds to repeats 3 to 6, and polyomavirus small T and medium T bind to repeats 2 to 8. The data suggest cooperativity between C and T antigens in binding to A. This is most apparent for medium T antigen, which can only bind to those A subunit molecules that provide the entire binding region for the C subunit. We infer from our results that B also binds to N-terminal repeats. A model of the small T/medium T/B-A-C complexes is presented.

ATM phosphorylates PP2A subunit A resulting in nuclear export and spatiotemporal regulation of the DNA damage response

“…The two PR65 isoforms, Aα (PPP2R1A) and Aβ (PPP2R1B) are 86% identical. Each PR65 isoform is shaped into a horseshoe composed of 15 nonidentical internal repeats of a 39 amino acid sequence, termed HEAT ( H untingtin, E longation factor 3, Protein phosphatase 2A-subunit A , and the yeast kinase T OR1) motif [ 7 – 9 ]. Almost 90% of the PP2A holoenzymes have the Aα isoform whereas Aβ is primarily associated with development and the CNS [ 10 ].…”