The mechanism used by polyomavirus and other viral SF3 helicases to unwind DNA at replication forks remains unknown. Using AlphaFold2, we have determined the structure of a representative SF3 helicase, the SV40 T-antigen (T-ag). This model has been analyzed in terms of the features of T-ag required for helicase activity, particularly the proximity of the T-ag origin binding domain (OBD) to the replication fork and the distribution of basic residues on the surface of the OBD that are known to play roles in DNA unwinding. These and related studies provide additional evidence that the T-ag OBDs have a role in the unwinding of DNA at the replication fork. Nuclear magnetic resonance and modeling experiments also indicate that protonated histidines on the surface of the T-ag OBD play an important role in the unwinding process, and additional modeling studies indicate that protonated histidines are essential in other SF3 and SF6 helicases. Finally, a model for T-ag's helicase activity is presented, which is a variant of the "rope climber." According to this model, the hands are the N-terminal OBD domains that interact with the replication fork, while the C-terminal helicase domains contain the feet that bind to single-stranded DNA.

Importance: Enzymes termed helicases are essential for the replication of DNA tumor viruses. Unfortunately, much remains to be determined about this class of enzymes, including their structures and the mechanism(s) they employ to unwind DNA. Herein, we present the full-length structure of a model helicase encoded by a DNA tumor virus. Moreover, this AI-based structure has been analyzed in terms of its basic functional properties, such as the orientation of the helicase at replication forks and the relative locations of the amino acid residues that are critical for helicase activity. Obtaining this information is important because it permits proposals regarding how DNA is routed through these model helicases. Also presented is structural evidence that the conclusions drawn from our detailed analyses of one model helicase, encoded by one class of tumor viruses, are likely to apply to other viral and eukaryotic helicases.