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.
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.
Ruediger
R
R
Department of Pathology, University of California San Diego, La Jolla 92093.
Roeckel
D
D
Fait
J
J
Bergqvist
A
A
Magnusson
G
G
Walter
G
G
D000595:Amino Acid Sequence; D000952:Antigens, Polyomavirus Transforming / Q000378:metabolism*; D001483:Base Sequence; D001665:Binding Sites; D002384:Catalysis; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D010749:Phosphoprotein Phosphatases / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D011120:Polyomavirus; D054648:Protein Phosphatase 2; D013539:Simian virus 40
D000952:Antigens, Polyomavirus Transforming; D010749:Phosphoprotein Phosphatases; D054648:Protein Phosphatase 2