page_title: Small tumor antigen
hierarchical_section_title: Small tumor antigen
caption_attribution_description: English: The SV40 polyomavirus small tumor antigen (STag) J domain (yellow) and unique region (blue). Key residues in the interface between the two domains are shown as sticks. Two bound zinc ions are shown as pink spheres, with their coordinating cysteine residues shown as sticks. The interface with the PP2A B56 subunit present in the same crystal structure is located to the bottom of the figure (compare File:2pf4 pp2a stag.png). Rendered from PDB ID 2PF4. PLoS Biol. 2007 Aug;5(8):e202. Structural basis of PP2A inhibition by small t antigen. Cho US, Morrone S, Sablina AA, Arroyo JD, Hahn WC, Xu W. PMID 17608567.
context_page_description: The small tumor antigen is a protein encoded in the genomes of polyomaviruses, which are small double-stranded DNA viruses. STag is expressed early in the infectious cycle and is usually not essential for viral proliferation, though in most polyomaviruses it does improve replication efficiency. The STag protein is expressed from a gene that overlaps the large tumor antigen such that the two proteins share an N-terminal DnaJ-like domain but have distinct C-terminal regions. STag is known to interact with host cell proteins, most notably protein phosphatase 2A, and may activate the expression of cellular proteins associated with the cell cycle transition to S phase. In some polyomaviruses - such as the well-studied SV40, which natively infects monkeys - STag is unable to induce neoplastic transformation in the host cell on its own, but its presence may increase the transforming efficiency of LTag. In other polyomaviruses, such as Merkel cell polyomavirus, which causes Merkel cell carcinoma in humans, STag appears to be important for replication and to be an oncoprotein in its own right.
context_section_description: The small tumor antigen (also called the small T-antigen and abbreviated STag or ST) is a protein encoded in the genomes of polyomaviruses, which are small double-stranded DNA viruses. STag is expressed early in the infectious cycle and is usually not essential for viral proliferation, though in most polyomaviruses it does improve replication efficiency. The STag protein is expressed from a gene that overlaps the large tumor antigen (LTag) such that the two proteins share an N-terminal DnaJ-like domain but have distinct C-terminal regions. STag is known to interact with host cell proteins, most notably protein phosphatase 2A (PP2A), and may activate the expression of cellular proteins associated with the cell cycle transition to S phase. In some polyomaviruses - such as the well-studied SV40, which natively infects monkeys - STag is unable to induce neoplastic transformation in the host cell on its own, but its presence may increase the transforming efficiency of LTag. In other polyomaviruses, such as Merkel cell polyomavirus, which causes Merkel cell carcinoma in humans, STag appears to be important for replication and to be an oncoprotein in its own right.
caption: The structure of part of the small tumor antigen from the SV40 polyomavirus, showing the J domain in yellow and the STag unique region in blue. Bound zinc ions are shown as pink spheres. Coordinating cysteine residues and residues in the domain interface are shown as sticks.[1]
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