Sequence Requirements for the Assembly of Simian Virus 40 T Antigen and the T-Antigen Origin Binding Domain on the Viral Core Origin of Replication

The regions of the simian virus 40 (SV40) core origin that are required for stable assembly of virally encoded T antigen (T-ag) and the T-ag origin binding domain (T-ag-obd(131-260)) have been determined. Binding of the purified T-ag-obd(131-260) is mediated by interactions with the central region of the core origin, site II. In contrast, T-ag binding and hexamer assembly requires a larger region of the core origin that includes both site II and an additional fragment of DNA that may be positioned on either side of site II. These studies indicate that in the context of T-ag, the origin binding domain can engage the pentanucleotides in site II only if a second region of T-ag interacts with one of the flanking sequences. The requirements for T-ag double-hexamer assembly are complex; the nucleotide cofactor present in the reaction modulates the sequence requirements for oligomerization. Nevertheless, these experiments provide additional evidence that only a subset of the SV40 core origin is required for assembly of T-ag double hexamers.     

Assembly of T-Antigen Double Hexamers on the Simian Virus 40 Core Origin Requires Only a Subset of the Available Binding Sites

Initiation of simian virus 40 (SV40) DNA replication is dependent upon the assembly of two T-antigen (T-ag) hexamers on the SV40 core origin. To further define the oligomerization mechanism, the pentanucleotide requirements for T-ag assembly were investigated. Here, we demonstrate that individual pentanucleotides support hexamer formation, while particular pairs of pentanucleotides suffice for the assembly of T-ag double hexamers. Related studies demonstrate that T-ag double hexamers formed on “active pairs” of pentanucleotides catalyze a set of previously described structural distortions within the core origin. For the four-pentanucleotide-containing wild-type SV40 core origin, footprinting experiments indicate that T-ag double hexamers prefer to bind to pentanucleotides 1 and 3. Collectively, these experiments demonstrate that only two of the four pentanucleotides in the core origin are necessary for T-ag assembly and the induction of structural changes in the core origin. Since all four pentanucleotides in the wild-type origin are necessary for extensive DNA unwinding, we concluded that the second pair of pentanucleotides is required at a step subsequent to the initial assembly process.     

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.     

Mechanisms of simian virus 40 T-antigen activation by phosphorylation of threonine 124.

Previous studies have shown that phosphorylation of simian virus 40 (SV40) T antigen at threonine 124 enhances the binding of T antigen to the SV40 core origin of replication and the unwinding of the core origin DNA via hexamer-hexamer interactions. Here, we report that threonine 124 phosphorylation enhances the interaction of T-antigen amino acids 1 to 259 and 89 to 259 with the core origin of replication. Phosphorylation, therefore, activates the minimal DNA binding domain of T antigen even in the absence of domains required for hexamer formation. Activation is mediated by only one of three DNA binding elements in the minimal DNA binding domain of T antigen. This element, including amino acids 167, 215, and 219, enhances binding to the unique arrangement of four pentanucleotides in the core origin but not to other pentanucleotide arrangements found in ancillary regions of the SV40 origin of replication. Interestingly, the same four pentanucleotides in the core origin are necessary and sufficient for phosphorylation-enhanced DNA binding. Further, we show that phosphorylation of threonine 124 promotes the assembly of high-order complexes of the minimal DNA binding domain of T antigen with core origin DNA. We propose that phosphorylation induces conformational shifts in the minimal DNA binding domain of T antigen and thereby enhances interactions among T-antigen subunits oriented by core origin pentanucleotides. Similar subunit interactions would enhance both assembly of full-length T antigen into binary hexamer complexes and origin unwinding.     

Localized melting and structural changes in the SV40 origin of replication induced by T-antigen.

Replication of simian virus 40 (SV40) DNA is dependent upon the binding of the viral T-antigen to the SV40 origin of replication. Structural changes in the origin of replication induced by binding of T-antigen were probed by chemical modifications of the DNA. In the presence of ATP, T-antigen rendered two of three domains in the SV40 core origin hypersensitive to attack by either dimethyl sulfate or potassium permanganate (KMnO4). One of these domains, the early palindrome, was shown to contain an 8-bp region of melted DNA as determined from methylation of cytosine residues and by nuclease S1 cleavage of methylated DNA. DNA melting was not dependent upon either the hydrolysis of ATP or the binding of T-antigen to an adjacent site (site I). A second domain, the A/T element, was extensively modified by KMnO4 but no significant melting was detected. Rather, the pattern of modification indicates that T-antigen caused a conformational change of the double-stranded DNA in this region. These results suggest that T-antigen, in the presence of ATP, destabilizes the SV40 origin by melting and structurally deforming two flanking regions within the core origin sequence. These DNA structural changes may provide access to other replication factors, allowing complete denaturation of the SV40 origin and the initiation of SV40 DNA replication.     

Cooperative assembly of simian virus 40 T-antigen hexamers on functional halves of the replication origin.

The cofactor ATP stimulates the formation of T-antigen double hexamers on the simian virus 40 core origin of replication (I. A. Mastrangelo, P. V. C. Hough, J. S. Wall, M. Dodson, F. B. Dean, and J. Horwitz, Nature [London] 338:658-662, 1989). We report here the pathway for the assembly of hexamers and double hexamers on the core origin. ATP triggers the cooperative assembly of hexamers on the early and late halves of the origin even when they are completely isolated. Hexamer assembly nucleates at T-antigen recognition pentanucleotides in the early half of the origin. In intact origins, assembly of the first hexamer on the early half of the origin cooperatively stimulates the assembly of a second hexamer on the adjacent late half of the origin. Thus, monomer-monomer and hexamer-hexamer interactions of T antigen, allosterically activated by ATP, constitute two distinct types of cooperative interaction with the origin. Finally, we show that the assembly of T-antigen hexamers on isolated half origins leads to the same array of structural changes that T antigen induces in intact origins. We conclude that the origin is divided into complementary halves that each promote the assembly of functional T-antigen hexamers.     

The DNA-binding domain of simian virus 40 tumor antigen has multiple functions.

The DNA-binding domain of simian virus 40 tumor antigen has been previously shown to participate in a number of different activities. Besides being involved in binding to sequences at the viral replication origin, this domain appears to be required for nonspecific DNA binding, for structurally distorting origin DNA (melting and untwisting), and possibly for oligomerization of the protein into hexamers and double hexamers. We now provide evidence that it also takes part in unwinding origin DNA sequences, contributes a function specifically related to in vivo DNA replication, and perhaps supports the assembly of the virus or release of the virus from the cell. This 100-amino-acid domain appears to be an excellent model system for studying how a small region of a protein could have a number of distinct activities.     

Structural basis for the cooperative assembly of large T antigen on the origin of replication

Large T antigen (LTag) from simian virus 40 (SV40) is an ATP-driven DNA helicase that specifically recognizes the core of the viral origin of replication (ori), where it oligomerizes as a double hexamer. During this process, binding of the first hexamer stimulates the assembly of a second one. Using electron microscopy, we show that the N-terminal part of LTag that includes the origin-binding domain does not present a stable quaternary structure in single hexamers. This disordered region, however, is well arranged within the LTag double hexamer after specific ori recognition, where it mediates the interactions between hexamers and constructs a separated structural module at their junction. We conclude that full assembly of LTag hexamers occurs only within the dodecamer, and requires the specific hexamer-hexamer interactions established upon binding to the origin of replication. This mechanism provides the structural basis for the cooperative assembly of LTag double hexamer on the cognate viral ori.     

Spacing is crucial for coordination of domain functions within the simian virus 40 core origin of replication.

The simian virus 40 core origin of replication is composed of distinct domains that are bracketed by DNA spacers. We created a matched set of insertion mutations in spacer sites to study the spatial relationships among origin domains. Insertions larger than a single base pair severely inhibit replication regardless of the helical phasing between domains. Replication-defective mutations reduce T-antigen binding and T-antigen-induced KMnO4 modifications of DNA to various extents. Mutations in the early half of the origin reduce T-antigen functions in the entire origin, whereas mutations in the late half reduce functions only in that half. Surprisingly, some mutations that severely inhibit DNA replication reduce T-antigen-induced melting and other structural changes within origin DNA to only a limited extent. In contrast, all replication-defective origin mutations prevent T antigen from extending the primary replication bubble beyond the limits of the core origin of replication. We conclude, therefore, that T-antigen-induced events within the core origin must be spatially coordinated for conversion of T-antigen hexamers bound to the core origin into mobile helicase units.     

Two domains of the epstein-barr virus origin DNA-binding protein, EBNA1, orchestrate sequence-specific DNA binding

The EBNA1 (for Epstein-Barr nuclear antigen 1) protein of Epstein-Barr virus governs the replication and partitioning of the viral genomes during latent infection by binding to specific recognition sites in the viral origin of DNA replication. The crystal structure of the DNA binding portion of the EBNA1 protein revealed that this region comprises two structural motifs; a core domain, which mediates protein dimerization and is structurally homologous to the DNA binding domain of the papillomavirus E2 protein, and a flanking domain, which mediated all the observed sequence-specific contacts. To test the possibility that the EBNA1 core domain plays a role in sequence-specific DNA binding not revealed in the crystal structure, we examined the effects of point mutations in potential hydrogen bond donors located in an alpha-helix of the EBNA1 core domain whose structural homologue in E2 mediates sequence-specific DNA binding. We show that these mutations severely reduce the affinity of EBNA1 for its recognition site, and that the core domain, when expressed in the absence of the flanking domain, has sequence-specific DNA binding activity. Flanking domain residues were also found to contribute to the DNA binding activity of EBNA1. Thus, both the core and flanking domains of EBNA1 play direct roles in DNA recognition.     

Mechanism of activation of simian virus 40 DNA replication by protein phosphatase 2A.

The catalytic subunit of protein phosphatase 2A (PP2Ac) stimulates the initiation of replication of simian virus 40 DNA in vitro by dephosphorylating T antigen at specific phosphoserine residues (K. H. Scheidtmann, D. M. Virshup, and T. J. Kelly, J. Virol. 65:2098-2101, 1991). To better define the biochemical mechanism responsible for this stimulation, we investigated the effect of PP2Ac on the interaction of T antigen with wild-type and mutant origins of replication. Analysis of the binding of T antigen to the wild-type origin as a function of protein concentration revealed that binding occurs in two relatively discrete steps: the assembly of a T-antigen hexamer on one half-site of the origin, followed by the assembly of the second hexamer on the other half-site. The major effect of PP2Ac was to stimulate binding of the second hexamer, so that the binding reaction became much more cooperative. This observation suggests that dephosphorylation of T antigen by PP2Ac primarily affects interactions between the two hexamers bound to the origin. Pretreatment with PP2Ac increased the ability of the bound T antigen to unwind the origin of replication but had no effect on the intrinsic helicase activity of the protein. Thus, dephosphorylation of PP2Ac appears to increase the efficiency of the initial opening of the origin by T antigen. An insertion mutation at the dyad axis in the simian virus 40 origin, which altered the structural relationship of the two halves of the origin, abolished the effect of the phosphatase on the cooperativity of binding and completely prevented origin unwinding. These findings suggest that the ability of T antigen to open the viral origin of DNA replication is critically dependent on the appropriate functional interactions between T-antigen hexamers and that these interactions are regulated by the phosphorylation state of the viral initiator protein.     

Large T-antigen double hexamers imaged at the simian virus 40 origin of replication

The initial step of simian virus 40 (SV40) DNA replication is the binding of the large tumor antigen (T-Ag) to the SV40 core origin. In the presence of Mg(2+) and ATP, T-Ag forms a double-hexamer complex covering the complete core origin. By using electron microscopy and negative staining, we visualized for the first time T-Ag double hexamers bound to the SV40 origin. Image processing of side views of these nucleoprotein complexes revealed bilobed particles 24 nm long and 8 to 12 nm wide, which indicates that the two T-Ag hexamers are oriented head to head. Taking into account all of the biochemical data known on the T-Ag-DNA interactions at the replication origin, we present a model in which the DNA passes through the inner channel of both hexamers. In addition, we describe a previously undetected structural domain of the T-Ag hexamer and thereby amend the previously published dimensions of the T-Ag hexamer. This domain we have determined to be the DNA-binding domain of T-Ag.     

The crystal structure of the SV40 T-antigen origin binding domain in complex with DNA

DNA replication is initiated upon binding of “initiators” to origins of replication. In simian virus 40 (SV40), the core origin contains four pentanucleotide binding sites organized as pairs of inverted repeats. Here we describe the crystal structures of the origin binding domain (obd) of the SV40 large T-antigen (T-ag) both with and without a subfragment of origin-containing DNA. In the co-structure, two T-ag obds are oriented in a head-to-head fashion on the same face of the DNA, and each T-ag obd engages the major groove. Although the obds are very close to each other when bound to this DNA target, they do not contact one another. These data provide a high-resolution structural model that explains site-specific binding to the origin and suggests how these interactions help direct the oligomerization events that culminate in assembly of the helicase-active dodecameric complex of T-ag.     

Characterization of the DNA-binding properties of the origin-binding domain of simian virus 40 large T antigen by fluorescence anisotropy

The affinity of the origin-binding domain (OBD) of simian virus 40 large T antigen for its cognate origin was measured at equilibrium using a DNA binding assay based on fluorescence anisotropy. At a near-physiological concentration of salt, the affinities of the OBD for site II and the core origin were 31 and 50 nM, respectively. Binding to any of the four 5'-GAGGC-3' binding sites in site II was only slightly weaker, between 57 and 150 nM. Although the OBD was shown previously to assemble as a dimer on two binding sites spaced by 7 bp, we found that increasing the distance between both binding sites by 1 to 3 bp had little effect on affinity. Similar results were obtained for full-length T antigen in absence of nucleotide. Addition of ADP-Mg, which promotes hexamerization of T antigen, greatly increased the affinity of full-length T antigen for the core origin and for nonspecific DNA. The implications of these findings for the assembly of T antigen at the origin and its transition to a non-specific DNA helicase are discussed.     

The simian virus 40 core origin contains two separate sequence modules that support T-antigen double-hexamer assembly

Using subfragments of the simian virus 40 (SV40) core origin, we demonstrate that two alternative modules exist for the assembly of T-antigen (T-ag) double hexamers. Pentanucleotides 1 and 3 and the early palindrome (EP) constitute one assembly unit, while pentanucleotides 2 and 4 and the AT-rich region constitute a second, relatively weak, assembly unit. Related studies indicate that on the unit made up of pentanucleotide 1 and 3 and the EP assembly unit, the first hexamer forms on pentanucleotide 1 and that owing to additional protein-DNA and protein-protein interactions, the second hexamer is able to form on pentanucleotide 3. Oligomerization on the unit made up of pentanucleotide 2 and 4 and the AT-rich region is initiated by assembly of a hexamer on pentanucleotide 4; subsequent formation of the second hexamer takes place on pentanucleotide 2. Given that oligomerization on the SV40 origin is limited to double-hexamer formation, it is likely that only a single module is used for the initial assembly of T-ag double hexamers. Finally, we discuss the evidence that nucleotide hydrolysis is required for the remodeling events that result in the utilization of the second assembly unit.     

T-antigen binding to site I facilitates initiation of SV40 DNA replication but does not affect bidirectionality.

SV40 origin auxiliary sequence 1 (aux-1) encompasses T-antigen (T-ag) binding site I and facilitates origin core (ori-core) activity in whole cells or cell extracts. Aux-1 activity depended completely upon its sequence, orientation and spacing relative to ori-core. Aux-1 activity was lost either by inserting 10 base pairs between aux-1 and ori-core or by placing either orientation of aux-1 on the opposite side of ori-core. Reversing the orientation of aux-1 in its normal position actually inhibited replication. Easily unwound DNA sequences that stimulate yeast or E. coli origins of replication could not replace aux-1. Aux-1 did not affect bidirectional replication. Replication remained bidirectional even when aux-1 was inactivated, and deletion of aux-1 did not affect selection of RNA-primed DNA synthesis initiation sites in the origin region: the transition from discontinuous to continuous DNA synthesis that marks the origin of bidirectional replication occurred at the same nucleotide locations in both wild-type and aux-1 deleted origins. These results support a model for initiation of SV40 DNA replication in which T-ag binding to aux-1 (T-ag binding site I) facilitates the efficiency with which T-ag initiates replication at ori-core (T-ag binding site II) without affecting the mechanism by which initiation of DNA replication occurs.