Preformed hexamers of SV40 T antigen are active in RNA and origin-DNA unwinding

Preformed hexamers of simian virus 40 (SV40) large tumor antigen (T antigen) constitute the bulk of T antigen in infected cells and are stable under physiological conditions. In spite of this they could not be assigned a function in virus replication or transformation. We report that preformed hexamers represent the active T antigen RNA helicase. Monomers and smaller oligomeric forms of T antigen were inactive due to the lack of hexamer formation under RNA unwinding conditions. In contrast to the immunologically related cellular DEAD-box protein p68, the T antigen RNA helicase is found to act in a much more processive way and it does not catalyze rearrangements of structured RNAs. Thereby, it rather seems to resemble other virus-encoded RNA helicases, like vaccinia virus NPH-II. Surprisingly, in our hands preformed hexamers also strikingly bound to and unwound the SV40 replication origin, pointing to a possible role of preformed hexamers in the initiation step of viral DNA replication. Furthermore, we have detected an extra hexamer-specific, high-affinity T antigen ATP binding site with a very slow exchange rate constant, the function of which is discussed.