JC Virus Small t Antigen Binds Phosphatase PP2A and Rb Family Proteins and Is Required for Efficient Viral DNA Replication Activity

Abstract

BackgroundThe human polyomavirus, JC virus (JCV) produces five tumor proteins encoded by transcripts alternatively spliced from one precursor messenger RNA. Significant attention has been given to replication and transforming activities of JCV's large tumor antigen (TAg) and three T′ proteins, but little is known about small tumor antigen (tAg) functions. Amino-terminal sequences of tAg overlap with those of the other tumor proteins, but the carboxy half of tAg is unique. These latter sequences are the least conserved among the early coding regions of primate polyomaviruses.Methodology and FindingsWe investigated the ability of wild type and mutant forms of JCV tAg to interact with cellular proteins involved in regulating cell proliferation and survival. The JCV P99A tAg is mutated at a conserved proline, which in the SV40 tAg is required for efficient interaction with protein phosphatase 2A (PP2A), and the C157A mutant tAg is altered at one of two newly recognized LxCxE motifs. Relative to wild type and C157A tAgs, P99A tAg interacts inefficiently with PP2A in vivo. Unlike SV40 tAg, JCV tAg binds to the Rb family of tumor suppressor proteins. Viral DNAs expressing mutant t proteins replicated less efficiently than did the intact JCV genome. A JCV construct incapable of expressing tAg was replication-incompetent, a defect not complemented in trans using a tAg-expressing vector.ConclusionsJCV tAg possesses unique properties among the polyomavirus small t proteins. It contributes significantly to viral DNA replication in vivo; a tAg null mutant failed to display detectable DNA replication activity, and a tAg substitution mutant, reduced in PP2A binding, was replication-defective. Our observation that JCV tAg binds Rb proteins, indicates all five JCV tumor proteins have the potential to influence cell cycle progression in infected and transformed cells. It remains unclear how these proteins coordinate their unique and overlapping functions.

Molecular Biology, Epidemiology, and Pathogenesis of Progressive Multifocal Leukoencephalopathy, the JC Virus-Induced Demyelinating Disease of the Human Brain

“…Small t antigen has been less well studied, but has been shown to interact with the RB family of proteins, as well as protein phosphatase 2A (PP2A) (45). The interaction of small t antigen with PP2A appears to prevent the dephosphorylation of the late protein agnoprotein, and this allows for greater viral replication (440).…”

“…The interaction of small t antigen with PP2A appears to prevent the dephosphorylation of the late protein agnoprotein, and this allows for greater viral replication (440). Reduction in levels of small t antigen or PP2A results in reduction of DNA replication (45,440). Inhibition of the phosphatase activity of PP2A also drives the cell toward S phase, thus promoting viral replication.…”

Human polyomavirus JC small regulatory agnoprotein forms highly stable dimers and oligomers: Implications for their roles in agnoprotein function

“…The viral genome encodes several regulatory proteins, including large T antigen (LT-Ag), small t antigen (Sm t-Ag) T’ 165 , T’ 136 , T’ 135 and agnoprotein (Bollag et al, 2010; Bollag et al, 2006; Khalili, Sariyer, and Safak, 2008; Khalili et al, 2005; Major et al, 1992; Safak et al, 2001). Agnoprotein is a small basic protein, expressed by all three JCV, BK virus (BKV) and simian virus 40 (SV40) polyomaviruses.…”

La famille des protéine phosphatases PP2A

“…Les diverses protéines virales qui sont répertoriées dans le Tableau I (voir aussi [4][5][6][7][8][9][10][11][12][13][14][15][16]) peuvent s'associer à des protéines PP2A et former des complexes fonctionnels plurimoléculaires capables de détourner la signalisation cellulaire en provoquant soit l'inhibition de PP2A, ce qui en général active la survie de l'hôte infecté, soit son activation, ce qui induit l'apoptose de l'hôte. Par exemple, les virus transformants à ADN de la famille des papovae codent pour des protéines comme la protéine transformante moyen T du virus polyome et les antigènes petit t des virus polyome ou SV40.…”