Structural and biochemical insights into the regulation of protein phosphatase 2A by small t antigen of SV40

Chen, Yu; Xu, Yanhui; Bao, Qing; Xing, Yongna; Zhu, Li; Lin, Zheng; Stock, Jeffry B.; Jeffrey, Philip D.; Shi, Yigong

doi:10.1038/nsmb1254

Cited by 78 publications

(85 citation statements)

References 45 publications

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“…It is becoming evident that the function of PP2A relies on the B subunit because the various B subunit proteins determine substrate specificity and PP2A complex subcellular localization (2, 3, 7, 10 -13). The recent crystal structure of PP2A supports this premise (4,(13)(14)(15). Thus the PP2A family of protein phosphatase isoforms can best be defined by its regulatory B subunit.…”

mentioning

confidence: 57%

PKR Regulates B56α-mediated BCL2 Phosphatase Activity in Acute Lymphoblastic Leukemia-derived REH Cells

Ruvolo

Kurinna

Karanjeet

et al. 2008

Journal of Biological Chemistry

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Protein phosphatase 2A (PP2A) is a heterotrimer comprising catalytic, scaffold, and regulatory (B) subunits. There are at least 21 B subunit family members. Thus PP2A is actually a family of enzymes defined by which B subunit is used. The B56 family member B56␣ is a phosphoprotein that regulates dephosphorylation of BCL2. The stress kinase PKR has been shown to phosphorylate B56␣ at serine 28 in vitro, but it has been unclear how PKR might regulate the BCL2 phosphatase. In the present study, PKR regulation of B56␣ in REH cells was examined, because these cells exhibit robust BCL2 phosphatase activity. PKR was found to be basally active in REH cells as would be predicted if the kinase supports B56␣-mediated dephosphorylation of BCL2. Suppression of PKR promoted BCL2 phosphorylation with concomitant loss of B56␣ phosphorylation at serine 28 and inhibition of mitochondrial PP2A activity. PKR supports stress signaling in REH cells, as suppression of PKR promoted chemoresistance to etoposide. Suppression of PKR promoted B56␣ proteolysis, which could be blocked by a proteasome inhibitor. However, the mechanism by which PKR supports B56␣ protein does not involve PKR-mediated phosphorylation of the B subunit at serine 28 but may involve eIF2␣ activation of AKT. Phosphorylation of serine 28 by PKR promotes mitochondrial localization of B56␣, because wild-type but not mutant S28A B56␣ promoted mitochondrial PP2A activity. Cells expressing wildtype B56␣ but not S28A B56␣ were sensitized to etoposide. These results suggest that PKR regulates B56␣-mediated PP2A signaling in REH cells.Protein phosphatase 2A (PP2A) 2 is an important regulator of apoptosis and may be a tumor suppressor (1-4). How PP2A might function as a tumor suppressor is not clear, as the enzyme on the one hand is required for cell survival but on the other is active in processes responsible for cell death (2-7). An explanation for this paradox is that PP2A is not a single enzyme but rather a family of protein phosphatase isoforms (3,4,8,9). PP2A is a heterotrimer composed of a catalytic (C) subunit, a scaffold (A) subunit, and a regulatory (B) subunit. The C and A subunits form a catalytic complex that interacts with one of at least 21 diverse B subunit members from three major families (i.e. B55, B56, and PR72/130; see Refs. 2, 3, and 9). It is becoming evident that the function of PP2A relies on the B subunit because the various B subunit proteins determine substrate specificity and PP2A complex subcellular localization (2, 3, 7, 10 -13). The recent crystal structure of PP2A supports this premise (4, 13-15). Thus the PP2A family of protein phosphatase isoforms can best be defined by its regulatory B subunit.PP2A function is regulated both negatively and positively by post-translational modification (4, 16 -18). Phosphorylation of B56␥ by ERK at serine 337 inhibits PP2A function (13, 18). Interestingly, this regulatory ERK site is conserved in all of the known B56 family members except B56␣. Although ERK may not regulate B56␣ function negatively, phosphory...

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mentioning

confidence: 57%

PKR Regulates B56α-mediated BCL2 Phosphatase Activity in Acute Lymphoblastic Leukemia-derived REH Cells

Ruvolo

Kurinna

Karanjeet

et al. 2008

Journal of Biological Chemistry

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“…Recently, the structural basis for this inhibition was provided: small t consists of an N-terminal J-domain and a C-terminal unique domain that contains two zinc-binding motifs. Whereas the J-domain and the second zinc-binding motif interact with the PR61/B'g-binding site on Aa, the first zinc-binding motif is in a position to interact directly with and inhibit the activity of the C subunit [13,14]. Therefore, it seems that members of both the PR55/B and PR61/B 0 subunit families can be replaced by viral tumor antigens in living cells, thereby providing a proof-of-principle that PP2A-subunit assembly is not a static phenomenon in vivo.…”

Section: Pp2a a Structural Centipede With Multiple Functionsmentioning

confidence: 99%

“…Therefore, it seems that members of both the PR55/B and PR61/B 0 subunit families can be replaced by viral tumor antigens in living cells, thereby providing a proof-of-principle that PP2A-subunit assembly is not a static phenomenon in vivo. However, competition with B-type subunits could be indirect, and cooperation with cellular factors might be required for viral proteins to disassemble PP2A holoenzymes in the cell [14].…”

Section: Pp2a a Structural Centipede With Multiple Functionsmentioning

confidence: 99%

PP2A holoenzyme assembly: in cauda venenum (the sting is in the tail)

Janssens

Longin

Goris

2008

Trends in Biochemical Sciences

365

424

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“…Interactions between either SV40 or Py sT and PP2A have been particularly well studied (26). SV40 sT binds to the scaffolding A and catalytic C subunits and displaces B=/B56/ PR61 and B56␥ regulatory B subunits, causing activation of the PI3K/Akt, Wnt, and c-Myc signaling pathways (27)(28)(29). Never- …”

Section: Merkel Cell Polyomavirus Is a Newly Discovered Human Cancer mentioning

confidence: 99%

Restricted Protein Phosphatase 2A Targeting by Merkel Cell Polyomavirus Small T Antigen

Kwun

Shuda

Camacho

et al. 2015

J Virol

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Merkel cell polyomavirus (MCV) is a newly discovered human cancer virus encoding a small T (sT) oncoprotein. We performed MCV sT FLAG-affinity purification followed by mass spectroscopy (MS) analysis, which identified several protein phosphatases (PP), including PP2A A and C subunits and PP4C, as potential cellular interacting proteins. PP2A targeting is critical for the transforming properties of nonhuman polyomaviruses, such as simian virus 40 (SV40), but is not required for MCV sT-induced rodent cell transformation. We compared similarities and differences in PP2A binding between MCV and SV40 sT. While SV40 sT coimmunopurified with subunits PP2A A␣ and PP2A C, MCV sT coimmunopurified with PP2A A␣, PP2A A␤, and PP2A C. Scanning alanine mutagenesis at 29 sites across the MCV sT protein revealed that PP2A-binding domains lie on the opposite molecular surface from a previously described large T stabilization domain (LSD) loop that binds E3 ligases, such as Fbw7. MCV sT-PP2A interactions can be functionally distinguished by mutagenesis from MCV sT LSD-dependent 4E-BP1 hyperphosphorylation and viral DNA replication enhancement. MCV sT has a restricted range for PP2A B subunit substitution, inhibiting only the assembly of B56␣ into the phosphatase holoenzyme. In contrast, SV40 sT inhibits the assembly of B55␣, B56␣ and B56 into PP2A. We conclude that MCV sT is required for Merkel cell carcinoma growth, but its in vitro transforming activity depends on LSD interactions rather than PP2A targeting. A pproximately 30 widely expressed serine/threonine (Ser/Thr) protein phosphatases (PPases) have been identified (1, 2). Protein phosphatase 2A (PP2A) is a combinatorial family of Ser/Thr phosphatases that are highly conserved in eukaryotes. The PP2A core enzyme structure is comprised of a 36-kDa catalytic C subunit (PP2A C) bound to a 65-kDa A regulatory subunit (PP2A A). This core heterodimer can be purified, but the PP2A holoenzyme generally contains a third, regulatory B subunit that provides substrate specificity and subcellular localization (3-7). Among the many combinatorial possibilities, there are two C catalytic subunits, at least two 〈 scaffolding subunits, and multiple B substrate recognition subunits (8), which allow for potentially hundreds of different phosphatase specificities and activities. Phosphatidylinositol 3-kinase (PI3K)/Akt, mitogen-activated protein kinase, c-Myc, and other cancer signaling pathways that impact cellular processes, including cell cycle progression and cellular tumorigenesis are regulated by specific PP2A isoforms (9). Aberrant expression and mutations in PP2A subunits have been implicated in human cancers (8,(10)(11)(12)(13)(14)(15)(16). IMPORTANCE Merkel cell polyomavirus is a newly discovered human cancer virus that promotes cancer, in part, through expression of itsPP2A activity is regulated in part by posttranslational modifications of subunits (17-23). Methylation of PP2A C at Leu309 activates the holoenzyme to allow binding of the B subunit (17), while phosphorylation o...

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Structural and biochemical insights into the regulation of protein phosphatase 2A by small t antigen of SV40

Cited by 78 publications

References 45 publications

PKR Regulates B56α-mediated BCL2 Phosphatase Activity in Acute Lymphoblastic Leukemia-derived REH Cells

PKR Regulates B56α-mediated BCL2 Phosphatase Activity in Acute Lymphoblastic Leukemia-derived REH Cells

PP2A holoenzyme assembly: in cauda venenum (the sting is in the tail)

Restricted Protein Phosphatase 2A Targeting by Merkel Cell Polyomavirus Small T Antigen

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