Abstract:

Protein phosphatase 2A (PP2A) is a major serine/threonine phosphatase in all eukaryotic cells and is involved in a variety of important aspects of cellular processes. PP2A has been identified as a tumor suppressor and the malfunction of PP2A is closely associated with various human diseases. PP2A is a multi-subunit enzyme. The PP2A core enzyme is composed of a scaffold subunit and a catalytic subunit, while the PP2A holoenzyme consists of PP2A core enzyme and a variable regulatory subunit. PP2A is regulated at different levels by different means, such as post-translational modification, small molecule association and protein-protein interaction. Both the PP2A core enzyme and the holoenzyme can be recognized and associated with other cellular interaction partners. The regulation of PP2A determines PP2A complex composition, subcellular localization, catalytic activity and/or substrate specificity.

This dissertation reports a detailed structural and biochemical characterization of four protein complexes: PP2A core enzyme bound to tumor-inducing toxins, PP2A holoenzyme involving B' family regulatory subunit, PP2A holoenzyme involving B family regulatory subunit and the PP2A scaffold subunit bound to the small tumor antigen of SV40.

The scaffold subunit associates with the catalytic subunit at the C-terminus and recognizes the regulatory subunits and small tumor antigen at its N-terminus. Although the B and B' regulatory subunit associates with the PP2A core enzyme in a slightly different way, both of them sit on the conserved ridge of the N-terminus of scaffold subunit with a highly acidic surface for substrate recruitment. Characterization of the PP2A holoenzyme involving the B family regulatory subunit identified at least two non-overlapping binding sites on the PP2A substrate Tau, which contains multiple phosphorylation sites. Small tumor antigen also interacts with the PP2A scaffold subunit at its N-terminus, overlapping with the binding site of the regulatory subunits. Biochemical analysis demonstrated that small tumor antigen primarily recognizes and inhibits the activity of the PP2A core enzyme.