Purified JC virus T and T' proteins differentially interact with the retinoblastoma family of tumor suppressor proteins.

The amino termini of polyomavirus T antigens contain LXCXE and J domains, which are necessary for binding and inactivating the retinoblastoma family of tumor suppressors. Both of these motifs are found in the JC virus (JCV) early proteins T'(135), T'(136), and T'(165), leading to the suggestion that these recently discovered proteins complement the cell-cycle-deregulating function of the JCV large T antigen (TAg). To investigate this hypothesis, the three JCV T' proteins were produced in a baculovirus expression system and purified by immunoaffinity chromatography. To facilitate purification, hybridomas that secrete antibodies recognizing amino-terminal epitopes of JCV early proteins were produced. Potential interactions between the early viral proteins and the cellular proteins pRB, p107, and p130 were investigated by incubating purified JCV TAg and T' proteins with extracts of MOLT-4 cells, a human T cell line. The four viral proteins preferentially bound hypophosphorylated species of the cellular proteins and exhibited the highest binding affinity to p107 and the lowest affinity to pRB. TAg and T'(165) bound more pRB and less p107 than did T'(135) and T'(136); T'(165) also bound less p130 than the other three early proteins. Results of these in vitro interactions were compared to those obtained in vivo using POJ cells, a transformed human glial cell line that expresses JCV early proteins, relatively high levels of pRB and p107, and low levels of p130. Most of the pRB in POJ cells is hyperphosphorylated, and only a fraction of the hypophosphorylated form(s) of pRB is bound by the viral proteins. In contrast, only hypophosphorylated p130 is detected in the transformed cells, and most of this protein was found in complex with the viral proteins. Finally, nearly all of the p107 in POJ cells is bound by the JCV proteins.