Characterization of the Aalpha and Abeta subunit isoforms of protein phosphatase 2A: differences in expression, subunit interaction, and evolution

Abstract

Protein phosphatase 2A (PP2A) is very versatile owing to a large number of regulatory subunits and its ability to interact with numerous other proteins. The regulatory A subunit exists as two closely related isoforms designated Aα and Aβ. Mutations have been found in both isoforms in a variety of human cancers. Although Aα has been intensely studied, little is known about Aβ. We generated Aβ-specific antibodies and determined the cell cycle expression, subcellular distribution, and metabolic stability of Aβ in comparison with Aα. Both forms were expressed at constant levels throughout the cell cycle, but Aα was expressed at a much higher level than Aβ. Both forms were found predominantly in the cytoplasm, and both had a half-life of approx. 10h. However, Aα and Aβ differed substantially in their expression patterns in normal tissues and in tumour cell lines. Whereas Aα was expressed at similarly high levels in all tissues and cell lines, Aβ expression varied greatly. In addition, in vivo studies with epitope-tagged Aα and Aβ subunits demonstrated that Aβ is a markedly weaker binder of regulatory B and catalytic C subunits than Aα. Construction of phylogenetic trees revealed that the conservation of Aα during the evolution of mammals is extraordinarily high in comparison with both Aβ and cytochrome c, suggesting that Aα is involved in more protein—protein interactions than Aβ. We also measured the binding of polyoma virus middle tumour antigen and simian virus 40 (SV40) small tumour antigen to Aα and Aβ. Whereas both isoforms bound polyoma virus middle tumour antigen equally well, only Aα bound SV40 small tumour antigen.