Simian Virus 40 Small t Antigen Mediates Conformation-Dependent Transfer of Protein Phosphatase 2A onto the Androgen Receptor

Abstract

The tumor antigens simian virus 40 small t antigen (ST) and polyomavirus small and medium T antigens mediate cell transformation in part by binding to the structural A subunit of protein phosphatase 2A (PP2A). The replacement of B subunits by tumor antigens inhibits PP2A activity and prolongs phosphorylationdependent signaling. Here we show that ST mediates PP2A A/C heterodimer transfer onto the ligand-activated androgen receptor (AR). Transfer by ST is strictly dependent on the agonist-activated conformation of AR, occurs within minutes of the addition of androgen to cells, and can occur in either the cytoplasm or the nucleus. The binding of ST changes the conformation of the A subunit, and ST rapidly dissociates from the complex upon PP2A A/C heterodimer binding to AR. PP2A is transferred onto the carboxyl-terminal half of AR, and the phosphatase activity is directed to five phosphoserines in the amino-terminal activation function region 1, with a corresponding reduction in transactivation. Thus, ST functions as a transfer factor to specify PP2A targeting in the cell and modulates the transcriptional activity of AR.The nuclear receptor (NR) superfamily of ligand-activated transcription factors directs the expression of genes that control a wide range of processes, including development, metabolism, and reproductive biology (24). NRs such as the androgen receptor (AR) reside initially in the cytoplasm, where they bind their cognate ligand, undergo chaperone-assisted conformational changes, engage with the nuclear import machinery, and translocate to the nucleus (8). NRs undergo sequencespecific binding to response element DNAs and recruit regulatory proteins to enhancers and promoters of target genes en route to assembling preinitiation complexes, attracting RNA polymerase II, and initiating transcription (14). There is recent evidence that NR recruitment of regulatory proteins needed for transcription occurs in an ordered and cyclical fashion (25).Members of the NR superfamily have a modular structure consisting of an N-terminal domain, a central DNA binding domain (DBD), a flexible hinge region, and a C-terminal ligand-binding domain (LBD) (10). The AR makes contact with regulatory proteins through subdomains with activation functions in the N-terminal domain activation function region 1 (AF-1) and the C-terminal LBD (AF-2). The AR DBD mediates binding to androgen response element DNA, which contains two palindromic hexanucleotide half-sites separated by a three-nucleotide spacer. The AR LBD forms the ligand-binding pocket, provides binding sites for heat shock proteins, and mediates intramolecular interactions with the N-terminal domain (17, 31).Ligand binding is the primary mechanism for controlling the transactivation of NRs, and for some steroid hormone receptors, including AR, ligand binding controls subcellular localization as well (2). There is, however, a growing body of evidence that NR activity can be regulated by posttranslational modification. The phosphorylation of NRs can regulate ligand sensiti...

RETRACTED: Cancer-Associated PP2A Aα Subunits Induce Functional Haploinsufficiency and Tumorigenicity

“…A recent study defined a PP2A-dependent role for polyoma small t in disrupting ARF-mediated activation of p53 (48). In addition, SV40 small t induces a conformational change in the PP2A A subunit that mediates PP2A binding to the androgen receptor (49). Although the interaction of small t with PP2A complexes is involved in regulating many signaling pathways, only some of those pathways are responsible for the transforming function of small t. Here we showed that the introduction of small t or suppression of PP2A B56g or Aa all lead to loss of AC-B56g complexes, constitutive phosphorylation of AKT, and cell transformation, suggesting a common mechanism for PP2A alterations and human cell transformation.…”

c-Jun Supports Ribosomal RNA Processing and Nucleolar Localization of RNA Helicase DDX21

“…Moreover, it seems evident that direct binding of c-Jun to the C-terminal tail of DDX21 enhances the intrinsic RNA binding affinity of DDX21. Importantly, a similar "hit-and-run" mechanism resulting in conformational changes at the binding surface has been suggested recently to regulate protein-protein interactions (25,26).…”

Lessons in Signaling and Tumorigenesis from Polyomavirus Middle T Antigen

“…Lipin, a target of insulin signaling, is dephosphorylated as a result of its interaction with MT (Schaffhausen et al, unpublished). The idea that MT might direct PP2A toward substrates is supported by the observations that SV40 ST increases the activity of PP2A for substrates such as histone H1 (339) and the androgen receptor (337). In a second scenario, MT might perturb the activity of PP2A complexes in cells, either in terms of inhibiting activity or by changing the range of B-subunit associations.…”