J. 57kT forms of MCPyV large T antigen. MCPyV large T antigen could bind to Rb but was unable to bind to p53. Furthermore, MCPyV-truncated large T Calcium N5-methyltetrahydrofolate antigen was more effective than full-length and 57kT large T antigen in promoting the growth of human and mouse fibroblasts. In contrast, expression of the MCPyV large T antigen C-terminal 100 residues could inhibit the growth of several different cell types. These data imply that the deletion of the C terminus of MCPyV large T antigen found in MCC serves not only to disrupt viral replication but also results in the loss of a distinct growth-inhibitory function intrinsic to this region. INTRODUCTION Merkel cell carcinoma (MCC) is an aggressive skin malignancy with an annual incidence of 3 per million in the United States (1). Risk factors for developing MCC include advanced age, prolonged UV exposure, and immunosuppression due to HIV, hematologic malignancy, or solid-organ transplantation (2, 3). Recently, Merkel cell polyomavirus (MCPyV) was discovered to be clonally integrated in at least 80% of MCC, raising the possibility that this pathogen contributes to carcinogenesis (4, 5). MCPyV is usually a typical polyomavirus with a circular, double-stranded DNA genome made up of an early region that expresses large and small T antigens, a late region that encodes 3 viral coat proteins, VP1, VP2, and VP3, and a regulatory region that contains the origin of replication and a bidirectional Calcium N5-methyltetrahydrofolate promoter for the early and late genes. MCPyV was the fifth polyomavirus recognized in humans, preceded by BKPyV, JCPyV, KIPyV, and WUPyV (6C9). Since then, 6 additional human polyomaviruses have been discovered, including HPyV6, HPyV7, TSPyV, HPyV9, MWPyV, and STLPyV (10C15). Although JCPyV and BKPyV have been detected in a variety of human cancers (16, 17), Merkel cell polyomavirus is the only polyomavirus DNA clonally integrated in human malignancy. Expression of Merkel large and small T antigens can be detected in most MCC specimens (5, 18, 19). The T antigens from several polyomaviruses have oncogenic activity. Notably, the simian computer virus 40 (SV40) large and small T antigens can transform a variety of rodent and human cells. Expression of SV40 large and small T antigens, together with human telomerase reverse transcriptase and an oncogenic form of H-RAS, can fully transform normal human fibroblasts (20, 21). At a minimum, the SV40 large T antigen transforming activity is dependent on binding to cellular tumor suppressor proteins, including p53 (TP53) and users of the Rb family (RB1, RBL1, and RBL2) (22). SV40 small T antigen binding to the serine/threonine phosphatase PP2A results in the perturbation in phosphorylation state of several host cell factors, including c-Myc (23, 24). Much like other polyomaviruses, the MCPyV large T antigen contains an N-terminal J domain name, an LXCXE or Rb binding motif, a DNA binding domain name, and a helicase domain name (25, 26). MCPyV small T antigen contains the J domain name and the unique region not shared with large T antigen (19). In addition, the MCPyV early region undergoes option splicing, resulting in expression of 57kT, which deletes most of the centrally located DNA binding and helicase domains of large T antigen (Fig. 1), but it retains the J domain name and LXCXE motif in-frame with the C-terminal 100 residues expressed from Exon Rabbit polyclonal to GJA1 3 (25, 27). Open in a separate windows Fig 1 Mutations truncate MCPyV large T antigen in MCC. Full-length (LT) and 57kT forms of large T antigen and small T antigen (ST) are shown. LT contains DnaJ (J), Rb-binding (LXCXE), nuclear localization transmission (NLS), DNA binding (DBD), and helicase domains. MCV350 and MCV339 are from the original statement of MCPyV in MCC (4). Predicted total numbers of residues are indicated on the right. Only mutations causing quit codons or deletions in large T antigen are shown. Given the oncogenic properties of the canonical polyomavirus SV40 large and small T antigens, it is likely that this MCPyV T antigens also have transforming activities. MCPyV large T antigen has been shown to bind specifically to RB1 as well as Calcium N5-methyltetrahydrofolate VPS39 (vacuolar protein sorting 39 homolog, or Vam6) (25, 28). Furthermore, RNA interference (RNAi)-mediated knockdown of MCPyV large T and small T antigen results in decreased growth of MCC cell lines and as xenografts (19, 29). The LXCXE motif is required for MCPyV large T antigen for binding to Rb and for sustained growth of a Merkel cell carcinoma.