Protein Phosphatase 2A Regulatory Subunit B56α Associates with c-Myc and Negatively Regulates c-Myc Accumulation

Abstract

Protein phosphatase 2A (PP2A) plays a prominent role in controlling accumulation of the proto-oncoprotein c-Myc. PP2A mediates its effects on c-Myc by dephosphorylating a conserved residue that normally stabilizes c-Myc, and in this way, PP2A enhances c-Myc ubiquitin-mediated degradation. Stringent regulation of c-Myc levels is essential for normal cell function, as c-Myc overexpression can lead to cell transformation. Conversely, PP2A has tumor suppressor activity. Uncovering relevant PP2A holoenzymes for a particular target has been limited by the fact that cellular PP2A represents a large heterogeneous population of trimeric holoenzymes, composed of a conserved catalytic subunit and a structural subunit along with a variable regulatory subunit which directs the holoenzyme to a specific target. We now report the identification of a specific PP2A regulatory subunit, B56α, that selectively associates with the N terminus of c-Myc. B56α directs intact PP2A holoenzymes to c-Myc, resulting in a dramatic reduction in c-Myc levels. Inhibition of PP2A-B56α holoenzymes, using small hairpin RNA to knock down B56α, results in c-Myc overexpression, elevated levels of c-Myc serine 62 phosphorylation, and increased c-Myc function. These results uncover a new protein involved in regulating c-Myc expression and reveal a critical interconnection between a potent oncoprotein, c-Myc, and a well-documented tumor suppressor, PP2A.

Supplemental material for this article may be found at http://mcb.asm.org/.

We thank David Virshup for the HA-tagged B56 family expression vectors and Clodagh O'Shea for the E4orf4 expression vector.

Funding for this work was provided by NIH Training Grant 5-T32-GM08617 and OHSU TL Tartar Trust Research Fellowship AMEDG0063 to Hugh K. Arnold and by NIH grants RO1-CA100855 and KO1-CA086957 to Rosalie C. Sears.