Identification of simian virus 40 T-antigen residues important for specific and nonspecific binding to DNA and for helicase activity.

We have previously identified three regions (called elements) in the DNA-binding domain of simian virus 40 large tumor (T) antigen which are critical for binding of the protein to the recognition pentanucleotides GAGGC at the viral replication origin. These are elements A (residues 147 to 159), B1 (185 to 187), and B2 (203 to 207). In this study, we generated mutants of simian virus 40 in order to make single-point substitution mutations at nearly every site in these three elements. Each mutation was tested for its effect on virus replication, and T antigen was produced from all replication-negative mutants. The mutant proteins were assayed for binding to several different DNA substrates and for helicase activity. We found that within each element, mutations at some sites had major effects on DNA binding while mutations at other sites had moderate, mild, or minimal effects, suggesting that some residues are more important than others in mediating DNA binding. Furthermore, we provide evidence that certain residues in elements A and B2 (Ala-149, Phe-159, and His-203) participate in nonspecific double-stranded and helicase substrate (single-stranded) DNA binding while others (Ser-147, Ser-152, Asn-153, Thr-155, Arg-204, Val-205, and Ala-207) are involved in sequence-specific binding at the origin. The residues in element B1 (primarily Ser-185 and His-187) take part only in nonspecific DNA binding. The amino acids important for nonspecific DNA binding are also required for helicase activity, and we hypothesize that they make contact with the sugar-phosphate backbone of DNA. On the other hand, those involved in sequence-specific binding are not needed for helicase activity. Finally, our analysis showed that three residues (Asn-153 and Thr-155 in element A and Arg-204 in element B2) may be the most important for sequence-specific binding. They are likely to make direct or indirect contacts with the pentanucleotide sequences at the origin.