JC virus small T antigen binds phosphatase PP2A and Rb family proteins and is required for efficient viral DNA replication activity

• Published in: PloS one Vol. 5; no. 5; p. e10606
• Main Authors: Bollag, Brigitte, Hofstetter, Catherine A, Reviriego-Mendoza, Marta M, Frisque, Richard J
• Format: Journal Article
• Language: English
• Published: United States Public Library of Science 12-05-2010; Public Library of Science (PLoS)
• Subjects: Alternative splicing; Amino Acid Sequence; Amino acids; Animals; Antigen (tumor-associated); Antigen T (large); Antigens; Antigens, Viral, Tumor - chemistry; Antigens, Viral, Tumor - metabolism; Biochemistry; Cell cycle; Cell Line; Cell proliferation; Cell survival; Cellular proteins; Cytomegalovirus - genetics; Deoxyribonucleic acid; DNA; DNA biosynthesis; DNA Replication; DNA, Viral - metabolism; Enzymes; Genome, Viral - genetics; Genomes; Genomics; Humans; JC virus; JC Virus - genetics; JC Virus - physiology; Kinases; Localization; Mice; Molecular biology; Molecular Sequence Data; mRNA; Mutation - genetics; Phosphatase; Phosphatases; Phosphoprotein phosphatase; Phosphorylation; Polyomavirus; Primates; Proline; Promoter Regions, Genetic - genetics; Protein Binding; Protein phosphatase; Protein Phosphatase 2 - metabolism; Proteins; Rats; Replication; Retinoblastoma protein; Retinoblastoma Protein - metabolism; Ribonucleic acid; RNA; Simian virus 40; Tags; Transformed cells; Tumor antigens; Tumor proteins; Tumor suppressor genes; Tumorigenesis; Tumors; Virology; Virology/Effects of Virus Infection on Host Gene Expression; Virology/Host Antiviral Responses; Virology/Mechanisms of Resistance and Susceptibility, including Host Genetics; Virology/Viral and Gene Regulation; Virology/Virulence Factors and Mechanisms; Virology/Viruses and Cancer; Virus Replication - physiology; Viruses; United States > US; Pennsylvania
• ISSN: 1932-6203; 1932-6203
• DOI: 10.1371/journal.pone.0010606

The 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.
We 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.
JCV 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.