Tantigen

T antigen, commonly written as T antigen or T-antigen, is a family of viral regulatory oncoproteins encoded by polyomaviruses, including SV40 and related human polyomaviruses. These nonstructural proteins are produced from the virus’s early region and exist mainly as Large T antigen (LT) and Small T antigen (ST). They modulate host cell processes to facilitate viral replication and, in some contexts, cellular transformation.

Large T antigen is essential for viral DNA replication. It binds the origin of replication and, with host factors, unwinds DNA and recruits replication machinery. LT contains an N‑terminal J domain that interacts with Hsp70 and a C‑terminal region that binds tumor suppressors such as pRb and p53. Through these interactions LT drives cell cycle progression toward S phase and can contribute to oncogenic transformation by disabling pRb and p53 pathways.

Small T antigen normally modulates signaling by binding protein phosphatase 2A (PP2A), altering phosphorylation cascades and promoting cellular proliferation. ST often collaborates with LT to enhance transformation and can influence growth independently of viral replication in some polyomaviruses.

Clinical and research relevance: LT and ST are central to polyomavirus biology and oncogenic potential. In Merkel cell polyomavirus, truncated LT lacking the helicase domain is expressed in tumors, supporting transformation while preventing productive viral replication. In laboratories, T antigens are used to study cell cycle control and to immortalize cells. The proteins were first identified as tumor antigens in early virology and immunology research.