Activation of ATPase activity of simian virus 40 large T antigen by the covalent affinity analog of ATP, fluorosulfonylbenzoyl 5'-adenosine

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.

Characterization of the nucleotide binding properties of SV40 T antigen using fluorescent 3'(2')-O-(2,4,6-trinitrophenyl)adenine nucleotide analogues

ATP binding to the large tumor (T) antigen encoded by the simian virus 40 (SV40) genome plays an essential role in the replication of viral DNA [Fanning, E., and Knippers, R. (1992) Annu. Rev. Biochem. 61, 55-85]. To better explore the functions of T antigen during the replication process, we have studied the interactions of T antigen with fluorescent 3'(2')-O-(2,4,6-trinitrophenyl) (TNP) adenine nucleotide analogues. Binding of TNP-ATP and TNP-ADP was accompanied by an 8-fold fluorescence enhancement and a concomitant blue shift (11 nm) of the maximal emission wavelength; the intrinsic protein tryptophan fluorescence was quenched maximally by 50%. Both signals were utilized to characterize the nucleotide binding activity of T antigen. TNP-ATP and TNP-ADP bound to the ATP binding site with dissociation constants of 0.35 microM and 2.6 microM. TNP substitution enhanced the affinity of ADP for T antigen by approximately 11-fold. The binding stoichiometry was 1 mol of TNP nucleotide per mole of monomer T antigen. The binding of TNP-ATP was more temperature dependent than that of TNP-ADP. The enthalpy change contributed nearly half of the energy for TNP-ATP binding, whereas binding of TNP-ADP was primarily entropy driven. Both TNP-ATP and TNP-ADP were strong inhibitors of the T antigen ATPase activity, confirming the high affinities of the TNP nucleotides for the ATP binding site. Like the parent nucleotides, they also induced T antigen hexamer formation. Using the TNP nucleotides as fluorescent probes, we have measured the affinity of various nucleotides and analogues for T antigen. The results indicate that the nucleotide binding specificity of T antigen was similar to that of the prokaryotic helicases Dna B and Rep, suggesting closely related ATP binding sites in the three DNA helicases.

DnaJ/hsp40 chaperone domain of SV40 large T antigen promotes efficient viral DNA replication

The amino-terminal domain of SV40 large tumor antigen (TAg) is required for efficient viral DNA replication. However, the biochemical activity associated with this domain has remained obscure. We show here that the amino-terminal domain of TAg shares functional homology with the J-domain of DnaJ/hsp40 molecular chaperones. DnaJ proteins function as cofactors by regulating the activity of a member of the 70-kD heat shock protein family. Genetic analyses demonstrated that amino-terminal sequences of TAg comprise a novel J-domain that mediates a specific interaction with the constitutively expressed hsc70 and show that the J-domain is also required for efficient viral DNA replication in vivo. Furthermore, we demonstrated that the J-domain of two human DnaJ homologs, HSJ1 or DNAJ2, could substitute functionally for the amino-terminus of TAg in promoting viral DNA replication. Together, our findings suggest that TAg uses its J-domain to support SV40 DNA replication in a manner that is strikingly similar to the use of Escherichia coli DnaJ by bacteriophage lambda in DNA replication. However, TAg has evolved a more efficient strategy of DNA replication through an intrinsic J-domain to associate directly with a partner chaperone protein. Our observations provide evidence of a role for chaperone proteins in the process of eukaryotic DNA replication.

An N-terminal deletion mutant of simian virus 40 (SV40) large T antigen oligomerizes incorrectly on SV40 DNA but retains the ability to bind to DNA polymerase alpha and replicate SV40 DNA in vitro

A peptide encompassing the N-terminal 82 amino acids of simian virus 40 (SV40) large T antigen was previously shown to bind to the large subunit of DNA polymerase alpha-primase (I. Dornreiter, A. Höss, A. K. Arthur, and E. Fanning, EMBO J. 9:3329-3336, 1990). We report here that a mutant T antigen, T83-708, lacking residues 2 to 82 retained the ability to bind to DNA polymerase alpha-primase, implying that it carries a second binding site for DNA polymerase alpha-primase. The mutant protein also retained ATPase, helicase, and SV40 origin DNA-binding activity. However, its SV40 DNA replication activity in vitro was reduced compared with that of wild-type protein. The reduction in replication activity was accompanied by a lower DNA-binding affinity to SV40 origin sequences and aberrant oligomerization on viral origin DNA. Thus, the first 82 residues of SV40 T antigen are not strictly required for its interaction with DNA polymerase alpha-primase or for DNA replication function but may play a role in correct hexamer assembly and efficient DNA binding at the origin.

Nonspecific DNA binding activity of simian virus 40 large T antigen: evidence for the cooperation of two regions for full activity

We generated a series of COOH-terminal truncated simian virus 40 large tumor (T) antigens by using oligonucleotide-directed site-specific mutagenesis. The mutant proteins [T(1-650) to T(1-516)] were expressed in insect cells infected with recombinant baculoviruses. T(1-623) and shorter proteins [T(1-621) to T(1-516)] appeared to be structurally changed in a region between residues 269 and 522, as determined by increased sensitivities to trypsin digestion and by altered reactivities to several monoclonal antibodies. These same mutant proteins bound significantly less nonorigin plasmid DNA (15%) and calf thymus DNA (25%) than longer proteins [T(1-625) to T(1-708)]. However, all mutant T antigens exhibited a nearly wild-type level of viral origin-specific DNA binding and binding to a helicase substrate DNA. This indicated that binding to origin and helicase substrate DNAs is separable from about 85% of nonspecific binding to double-stranded DNA. As an independent confirmation that a region distinct from the origin-binding domain (amino acids 147 to 247) is involved in nonspecific DNA binding, we found that up to 96% of this latter activity was specifically inhibited in wild-type T antigen by several monoclonal antibodies which collectively bind to the region between residues 269 and 522. In order to investigate the relationship between the origin-binding domain and the second region, we performed origin-specific DNA binding assays with increasing amounts of calf thymus DNA as competitor. The results suggest that this second region is not an independent nonspecific DNA binding domain. Rather, it most likely cooperates with the origin-binding domain to give rise to wild-type levels of nonspecific DNA binding. Our results further suggest that most of the nonspecific binding to double-stranded DNA is involved in a function other than direct recognition and binding to the pentanucleotides at the replication origin on simian virus 40 DNA.

Mechanisms of interference with simian virus 40 (SV40) DNA replication by trans-dominant mutants of SV40 large T antigen

Mutations at multiple sites within the simian virus 40 (SV40) early region yield large T antigens which interfere trans dominantly with the replicative activities of wild-type T antigen. A series of experiments were conducted to study possible mechanisms of interference with SV40 DNA replication caused by these mutant T antigens. First, the levels of wild-type T antigen expression in cells cotransfected with wild-type and mutant SV40 DNAs were examined; approximately equal levels of wild-type T antigen were seen, regardless of whether the cotransfected mutant was trans dominant or not. Second, double mutants that contained the mutation of inA2827, a strong trans-dominant mutation with a 12-bp linker inserted at the position encoding amino acid 520, and various mutations in other parts of the large-T-antigen coding region were constructed. The trans-dominant interference of inA2827 was not affected by second mutations within the p105Rb binding site or the amino or carboxy terminus of large T antigen. Mutation of the nuclear localization signal partially reduced the trans dominance of inA2827. The large T antigen of mutant inA2815 contains an insertion of 4 amino acids at position 168 of large T; this T antigen fails to bind SV40 DNA but is not trans dominant for DNA replication. The double mutant containing the mutations of both inA2815 and in A2827 was not trans dominant. The large T antigen of dlA2433 lacks amino acids 587 to 589, was unstable, and failed to bind p53. Combining the dlA2433 mutation with the inA2827 mutation also reversed the trans dominance completely, but the effect of the dlA2433 mutation on trans dominance can be explained by the instability of this double mutant protein. In addition, we examined several mutants with conservative point mutations in the DNA binding domain and found that most of them were not trans dominant. The implications of the results of these experiments on possible mechanisms of trans dominance are discussed.

Simian virus 40 T-antigen DNA helicase is a hexamer which forms a binary complex during bidirectional unwinding from the viral origin of DNA replication

The role of simian virus 40 (SV40) large tumor antigen (T antigen) as a DNA helicase at the replication fork was studied. We found that a T-antigen hexamer complex acts during the unidirectional unwinding of appropriate DNA substrates and is localized directly in the center of the fork, contacting the adjacent double strand as well as the emerging single strands. When bidirectional DNA unwinding, initiated at the viral origin of DNA replication, was analyzed, a larger T-antigen complex that is simultaneously active at both branch points of an unwinding bubble was observed. The size and shape of this helicase complex imply that the T-antigen dodecamer complex, assembled at the origin and active in the localized melting of duplex DNA, is subsequently also used to continue DNA unwinding bidirectionally. Then, however, the dodecamer complex does not split into two hexamer subunits that track along the DNA; rather, the DNA is threaded through the intact complex, with the concomitant extrusion of single-stranded loops.

Specific mutation of a regulatory site within the ATP-binding region of simian virus 40 large T antigen

In an attempt to distinguish simian virus 40 (SV40) large T antigen (T) binding to ATP from hydrolysis, specific mutations were made in the ATP-binding site of T according to our model for the site (M. K. Bradley, T. F. Smith, R. H. Lathrop, D. M. Livingston, and T. A. Webster, Proc. Natl. Acad. Sci. USA 84:4026-4030, 1987). Two acidic residues predicted to make contact with the magnesium phosphate were changed to alanines. The mutated T gene was completely defective for viral DNA synthesis and for virion production, and it was dominant defective for viral DNA replication. The defective T gene encoded a stable product (2905T) that oncogenically transformed mouse cell lines. 2905T, immunoprecipitated from transformed-cell extracts, bound SV40 origin DNA specifically and, surprisingly, it was active as an ATPase. A recombinant baculovirus was constructed for the production and purification of the mutant protein for detailed biochemical analyses. 2905T had only 10% of the ATPase and helicase of wild-type T. The Km of 2905T for ATP in ATPase assays was the same as the Km of wild-type T. ATP activated the ATPase activity of wild-type T, but not of 2905T. As tested by gel bandshift assay, 2905T bound to SV40 origin DNA and to individual sites I and II with affinities similar to that of the wild type. However, ATP did not modulate the DNA-binding activity of mutant T to site II. Therefore, this mutation in the ATP-binding site in T resulted in defects in the interaction between the protein and ATP that appeared to be responsible for the determination of the active state of T for DNA binding versus ATPase.

The zinc finger region of simian virus 40 large T antigen is needed for hexamer assembly and origin melting

Simian virus 40 large T antigen contains a single sequence element with an arrangement of cysteines and histidines that is characteristic of a zinc finger motif. The finger region maps from amino acids 302 through 320 and has the sequence C-302 L K C-305 I K K E Q P S H Y K Y H-317 E K H-320. Previous genetic analysis has shown that the cysteine and histidine sequences and the contiguous S H Y K Y region in the finger are important for DNA replication in vivo. We show here that representative mutations in either of these elements of the finger prevent the assembly of large T antigen into stable hexamers in vitro. These same mutations have a characteristic effect on the interaction of T antigen with the simian virus 40 core origin of replication. The mutant T antigens bind to the central pentanucleotide domain of the core origin but fail to melt the adjacent inverted repeat domain and to untwist the adenine-thymine domain. These defects would prevent the formation of a replication bubble and the initiation of DNA replication. Finger mutations have lesser effects on the helicase function of T antigen and no observable effect on binding of T antigen to the mouse p53 protein. We propose that the zinc finger region contributes to protein-protein interactions essential for the assembly of stable T-antigen hexamers at the origin of replication and that hexamers are needed for subsequent alterations in the structure of origin DNA. We cannot exclude the possibility that the zinc finger region also makes specific contacts with components of origin DNA.