Association of protein phosphatase 2A with polyoma virus medium tumor antigen.

Abstract

The polyoma virus medium and small tumor antigens, as well as simian virus 40 small tumor antigen, form specific complexes with two cellular proteins designated 61-and 37-kDa proteins. In this report, we demonstrate that the 61-and 37-kDa proteins correspond to the A and C subunits, respectively, of the serine-and threonine-specific protein phosphatase 2A (PP2A). On the one hand, antibodies raised against the 61-kDa protein reacted specifically with the purified A subunit of PP2A. Furthermore, the amino acid sequences of seven tryptic peptides from the A subunit were almost identical to sequences of the 61-kDa protein as deduced from the corresponding cDNA sequence. On the other hand, antibodies against the purified C subunit (catalytic subunit) of PP2A reacted specifically with the medium tumor antigen-associated 37-kDa protein. These data suggest a role of PP2A in cell transformation by polyoma virus and sinmian virus 40.The importance of protein phosphorylation in signaling pathways that control cell proliferation and carcinogenesis is well established (for review, see ref. 1). Many oncogenes and growth factor receptors are tyrosine-specific protein kinases and two oncogenes, mos and raf, are serine/threoninespecific protein kinases (2,3). Little is known about the role of protein dephosphorylation in growth control, although it seems likely that reversible regulatory mechanisms involving specific phosphorylation events also require specific mechanisms for dephosphorylation. It has recently been shown that several tyrosine-specific protein phosphatases comprise a family ofreceptor-linked enzymes, which may be important in attenuating the effects of tyrosine phosphorylation on cell proliferation (for review, see ref. 4). Four major forms of serine/threonine-specific phosphatases have been characterized in mammalian tissues, which are differentiated by substrate specificity, sensitivity to inhibitors, regulatory properties, antibody reactivity, and primary sequence (for review, see ref. 5). Protein phosphatase 2A (PP2A) comprises a significant portion of total activity in many tissues, has a broad substrate specificity, and is active in the absence of divalent cations. It consists of a 36-to 38-kDa catalytic subunit (termed C) and two additional subunits of 60 and 55 kDa, termed A and B, respectively (5). cDNA clones encoding the catalytic subunit have been isolated from a number of tissues and species (6-8). Analysis of the primary amino acid sequence has shown that the type 2A catalytic subunit is highly conserved between species (nearly 100o identity) and that it is a member of a gene family that includes the catalytic subunit of types 1 and 2B protein phosphatases (5, 9). Currently, very little is known about the structure or function of the A and B subunits. The presence or absence of the A, or A and B, subunits alters the activity and substrate preference of the catalytic subunit (10-12).The transforming proteins of DNA tumor viruses frequently form complexes with cellular proteins. These interact...

Association between src-kinases and the polyoma virus oncogene middle T-antigen requires PP2A and a specific sequence motif

“…This area also binds PP2A (Campbell et al, 1995;Eckhart, 1993, 1995;Pallas et al, 1990;Walter et al, 1990) (see Figure 1). All mutations that disrupt PP2A binding to MT also a ect the MT/src association, indicating that these two interactions are closely related, though it is unclear how or why they are linked.…”

“…It is now clear that MT transforms cells by stimulating many of the pathways used by tyrosine kinase associated growth factor receptors (Dilworth, 1995), and so can be used to study receptor signalling. MT binds the regulatory, A, and catalytic, C, subunits of protein phosphatase 2A (PP2A) (Pallas et al, 1990;Walter et al, 1990), and three members of the srcfamily of tyrosine kinases, pp60 c-src (Courtneidge and Smith, 1983), pp62 c-yes (Kornbluth et al, 1987), and pp59 c-fyn (Cheng et al, 1988;Horak et al, 1989;Kypta et al, 1988). As a consequence, the kinase activity of pp60 c-src , and probably pp62 c-yes , is stimulated (Bolen et al, 1984;Courtneidge, 1985).…”

Role of Janus kinase-2 in insulin-mediated phosphorylation and inactivation of protein phosphatase-2A and its impact on upstream insulin signalling components

“…For example, PP-2A has been shown to bind the β2-adrenergic receptor [11], casein kinase 2α [12], homeobox gene 11 [13], tau [14], translation termination eukaryotic release factor 1 [15], adenovirus e4orf4 protein [16], α4 protein [17], neurofilament proteins [18,19] and small t and middle t antigens encoded by DNA tumour viruses [20,21]. Recent studies by Westphal et al [22] identified a calmodulin IV (CaMIV)-PP-2A complex in which PP-2A dephosphorylates…”

Protein kinase B/Akt is activated by polyomavirus middle-T antigen via a phosphatidylinositol 3-kinase-dependent mechanism

“…Oncogenic transformation by polyomavirus has been used as a paradigm for the study of mitogenic signaling in mammalian cells since this protein activates many of the cellular pathways also targeted by tyrosine kinase growth factor receptors. Signaling by middle-T requires interactions with cellular proteins such as the serine/ threonine protein phosphatase 2A (PP2A) (Pallas et al, 1990;Walter et al, 1990), Src family tyrosine kinases (Courtneidge and Smith, 1983 and reviewed in Kiefer et al, 1994), the adaptor protein SHC which binds to phosphotyrosine 250 of middle-T via its phosphotyrosine binding (PTB) domain (Dilworth et al, 1994;Campbell et al, 1994), PI 3-kinase (Whitman et al, 1985;Talmage et al, 1989;Courtneidge et al, 1989) and phospholipase Cg-1 (Su et al, 1995). When associated with middle-T, SHC gets phosphorylated at tyrosine residues.…”