Large T antigen, polyomaviridae <p>The group of polyomaviruses is formed by the homonymous murine virus (Py) as well as other representative members such as the simian virus 40 (SV40) and the human BK and JC viruses [<cite idref="PUB00043699"/>]. Their large T antigen (T-ag) protein binds to and activates DNA replication from the origin of DNA replication (ori). Insofar as is known, the T-ag binds to the origin first as a monomer to its pentanucleotide recognition element. The monomers are then thought to assemble into hexamers and double hexamers, which constitute the form that is active in initiation of DNA replication. When bound to the ori, T-ag double hexamers encircle DNA [<cite idref="PUB00041759"/>]. T-ag is a multidomain protein that contains an N-terminal J domain, which mediates protein interactions (see <db_xref db="PROSITEDOC" dbkey="PDOC00553"/>, <db_xref db="INTERPRO" dbkey="IPR001623"/>), a central origin-binding domain (OBD), and a C-terminal superfamily 3 helicase domain (see <db_xref db="PROSITEDOC" dbkey="PDOC51206"/>, <db_xref db="INTERPRO" dbkey="IPR010932"/>) [<cite idref="PUB00040814"/>].</p><p>This group represents a large T antigen, Polyomaviridae type.</p><p>The oncogenic large tumour antigen (LTag) is a protein found in polyomaviruses which is essential for viral DNA replication [<cite idref="PUB00027648"/>]. LTag contains three domains, an N-terminal DnaJ domain which mediates protein interactions (<db_xref db="INTERPRO" dbkey="IPR001623"/>), a domain which binds the origin of DNA replication (<db_xref db="INTERPRO" dbkey="IPR003133"/>), and a C-terminal helicase domain. Replication is intitated by LTag assembling at the origin as a double hexamer that distorts and melts the origin DNA [<cite idref="PUB00027648"/>, <cite idref="PUB00027649"/>]. During elongation LTag acts as a helicase that unwinds duplex DNA at the replication forks [<cite idref="PUB00027650"/>].</p>