Simian virus 40 (SV40) was discovered in 1960 as a contaminant in early polio vaccines.
Its discovery coincided with an explosion of knowledge in the new field of molecular
biology, and SV40 was quickly adopted as a model to study eukaryotic genome structure,
expression, replication, and cell growth regulation in cultured cells [1]. With a
genome of only 5.2 kbp, SV40 relies heavily on host cell machinery to propagate, affording
investigators a powerful tool to discover key host proteins that the virus manipulates.
Indeed, a single multifunctional viral protein, the large tumor (T) antigen (Tag)
(Figure 1A), is sufficient to orchestrate the replication of the viral mini-chromosome
in infected monkey cells [2], [3]. The origin DNA binding domain of Tag binds specifically
to the viral origin of DNA replication, and the C-terminal helicase domain of Tag
unwinds parental DNA at SV40 replication forks. The development of a cell-free reaction
containing purified Tag and primate cell extract enabled the identification of ten
evolutionarily conserved host proteins that are necessary and sufficient, together
with Tag, to replicate SV40 DNA in vitro [3], [4].
Initiation: How Does Tag Recognize Origin DNA?
Assembly of Tag on the viral core origin of DNA replication (64 bp) is the first step
in replication [3], [5]. The core origin DNA is composed of three elements: a central
palindrome composed of four GAGGC sequences, flanked by a so-called EP element and
an asymmetric AT-rich element (Figure 1B). Binding of a Tag monomer to each GAGGC
in the central palindrome nucleates cooperative assembly of additional Tag to form
a double hexamer of ∼1 MDa (Figure 1B). The central lobe of the dodecamer consists
of the N-terminal 250 residues of both Tag hexamers ([6] and citations therein). The
C-terminal helicase lobe of each hexamer (residues ∼260–708) interacts with the EP
or AT element of the origin DNA. This pre-replication complex, in the presence of
Mg-ADP or -ATP, is sufficient to locally melt (EP element) or untwist (AT element)
duplex origin DNA. These local distortions are necessary, but not sufficient, to activate
bidirectional helicase activity of the Tag complex in vitro or in vivo.
10.1371/journal.ppat.1002994.g001
Figure 1
Assembly and activation of the SV40 pre-replication complex in vitro.
(A) Domain architecture of SV40 Tag. Three structured domains (yellow) (DnaJ chaperone
domain, origin DNA binding domain [OBD], and helicase domain), composed of the zinc
(Zn) and AAA+ ATPase sub-domains, are connected by flexible regions (white) (P, cluster
of phosphorylated residues that regulates origin activation; HR, host range function).
(B) Diagram of ADP-associated SV40 Tag double hexamer bound to the duplex SV40 core
origin of DNA replication (EP, central palindrome, AT), with non-origin DNA protruding
from the complex (adapted from [6]). (C) 3D cryo-electron microscopy reveals two conformations
(parallel, displaced) of ADP-associated hypo-phosphorylated SV40 Tag double hexamer
on SV40 origin DNA as in (B) (adapted from [6]). A hypothetical conformation for the
activated double hexamer is shown at the right. Dashed lines suggest potential paths
of the DNA strands through each protein conformation. (D) Stages of SV40 replication.
I, Tag dodecamer assembled on duplex SV40 DNA as in (B); II, hypo-phosphorylated Tag
dodecamer activated as in (C) unwinds DNA bidirectionally [13] and may assemble host
proteins (not shown here) into two sister replisomes that interact physically through
the central lobe of the Tag dodecamer; III, hyper-phosphorylation of Tag disrupts
interactions between the hexamers [7], [8], releasing the replisomes to progress independently
along the template chromatin; IV, replication forks converge slowly, accompanied by
DNA decatenation, to complete replication, which may involve additional host proteins
[3], [14], [18]–[21].
Activation of Replication: How Does the Tag Double Hexamer Unwind DNA?
Activation of the double hexamer on origin DNA requires a unique phosphorylation state
of Tag: phospho-Thr124, and unmodified Ser120 and 123 [7], [8]. Cooperative interactions
between the N-terminal regions of the two hexamers during assembly on the origin require
this same hypo-phosphorylated form of Tag, which, fortuitously, is expressed by recombinant
baculovirus. When hypo-phosphorylated Tag double hexamers assemble in the presence
of Mg-ADP, which prevents helicase activity, they adopt two distinct conformations
[6] (Figure 1C). In one conformation (parallel), the duplex core origin DNA is buried
in the central channel of the double hexamer. In each hexamer, the six origin DNA
binding domain (OBDs) form a left-handed spiral structure surrounding the central
palindrome [6], [9]. In the displaced conformation, the central lobe of the dodecamer
is more open, yielding a bent structure. Intriguingly, bacterially expressed Tag double
hexamer displays only the parallel conformation, consistent with its inability to
activate bidirectional origin unwinding [3], [6], [7]. Thus, we suggest that conformational
changes in the central lobe, in concert with local distortions in the EP and AT elements
bound to the helicase lobes, may allow single-stranded DNA (ssDNA) release from the
central channel of the double hexamer (Figure 1C, dashed lines). Hypothetically, the
displaced protein conformation could shift back to the parallel conformation without
fully recapturing both strands of ssDNA. Indeed, the observation that Tag double hexamer-ADP-origin
DNA complexes dissociate into single hexamers after exposure to a single-strand-specific
nuclease argues that ssDNA must become accessible outside of the protein complex [10].
The observed conformational flexibility [6] could thus generate an activated dodecamer
poised for bidirectional unwinding by steric exclusion, as proposed for the cellular
Mcm2-7 replicative helicase [11], [12]. Future studies to define the path of the DNA
through an active Tag helicase complex will be required to test this model.
Elongation and Termination: Is Movement of Sister Replication Forks Coupled?
Both unphosphorylated and phosphorylated forms of Tag assemble double hexamers on
duplex SV40 origin DNA (Figure 1D, I). However, only the hypo-phosphorylated form
of Tag displays cooperative interactions between the two hexamers and undergoes remodeling
to activate the helicase to unwind with 3′ to 5′ polarity (Figure 1C, right). In vitro,
purified hypo-phosphorylated Tag can unwind origin DNA bidirectionally without disrupting
the cooperative interactions between the two hexamers, resulting in “rabbit-ear” DNA
structures detectable by electron microscopy [13]. If this looped template were replicated,
DNA synthesis at the two sister replisomes might be coupled (Figure 1D, II). However,
in infected primate cells, most of the Tag is additionally phosphorylated on Ser120
and Ser123. Alanine substitution of either residue abolishes viral DNA replication
in vivo [7], [14], implying that modification of both sites is important for replication.
Since phosphorylation of Ser120 or Ser123 disrupts cooperative interactions between
hexamers, we suggest that hyper-phosphorylation of Tag uncouples the two replisomes
soon after initiation of replication (Figure 1D, III). Since hyper-phosphorylation
of Tag has no detectable effect on its unidirectional helicase activity [3], [7],
[8], the sister replication forks could migrate independently and converge to complete
replication in vivo (Figure 1D, IV).
SV40: A Simple Model for Host DNA Replication?
Investigation of SV40 replication has been motivated in part by anticipation that
it would provide insight into host replication proteins and mechanisms. The architecture,
dimensions, and assembly of Tag and yeast Mcm2-7 double hexamers on their cognate
origin DNAs are closely related [6], [15], [16]. Much of the protein machinery at
SV40 and host replication forks is also remarkably similar [2]–[4] (Figure 2A). Furthermore,
the SV40 genome replicates in vivo as a mini-chromosome packaged in host nucleosomes
and utilizes a variety of chromatin remodeling proteins and histone chaperones. Yet,
the SV40 replisome clearly excludes several key components of host replication forks,
e.g., the leading strand DNA polymerase ε, Mcm10, and Cdc45 ([17], [18]; G. Sowd,
unpublished data), and all of the host proteins essential for SV40 replication in
vitro (Figure 2A) function in host DNA repair, as well as replication, pathways. Lastly,
SV40 infection induces host DNA damage signaling that is required to replicate viral
chromatin in vivo [14], [18], [19]. These observations have prompted a re-evaluation
of the viral replication strategy as a model for host chromosomal replication, and
suggest the possibility that the virus may co-opt host repair pathways.
10.1371/journal.ppat.1002994.g002
Figure 2
Viral exploitation of host DNA genome maintenance proteins.
(A) Diagram of a minimal replication protein assembly (replisome) at a viral and a
host fork. Topoisomerases, nucleosomes, and chromatin modifiers known to act at both
forks are not shown (adapted from [24]). (B) DNA damage signaling in SV40 DNA replication
centers at 48 hours post-infection, but not in host DNA replication centers. Mock-infected
or SV40-infected BSC40 monkey cells were labeled with 10 µM EdU (a thymidine analog)
for 5 minutes to visualize newly replicated DNA. Soluble proteins were pre-extracted
and cells were fixed [18]. EdU (teal) was coupled to a fluorescent dye using click
chemistry (Invitrogen) and DNA was stained with DAPI. Chromatin-bound Tag (green)
and histone γH2AX (red) were stained for indirect immunofluorescence as described
[18]. Cells were visualized with a 63× objective at a 0.6 µm z-axis slice using an
Apotome (Zeiss). Scale bars represent 10 µm.
Host Genome Maintenance: A Niche for Viral Chromatin Replication?
Recently, fluorescence microscopy of SV40 chromatin replication in infected cells
has revealed that Tag and the host proteins required for SV40 replication in vitro
co-localize in prominent subnuclear foci that enlarge with time after infection in
permissive cells [18] (Figure 2B). Moreover, thymidine analogs, e.g., EdU, that are
incorporated into nascent viral chromatin co-localize with these proteins, suggesting
that these foci represent viral replication centers ([20], [21]; G. Sowd, unpublished
data) (Figure 2B). Intriguingly, a variety of host DNA damage signaling and repair
proteins, e.g., γH2AX, Mre11, Nbs1, Rad51, and FancD2, also reside in SV40 replication
centers [18], [20], [21] (Figure 2B). Although punctate foci of such genome maintenance
proteins are observed in chromatin of uninfected cells exposed to DNA damaging agents,
such foci are generally much smaller than SV40 replication centers [18]. Of note,
the association of host genome maintenance proteins with viral replication centers
is not unique to SV40 or polyomaviral infections, but also occurs in cells infected
by other DNA viruses, including adeno-, papilloma-, and herpesviruses [22], [23].
These findings suggest that host damage signaling and genome maintenance pathways
serve important, though still poorly understood, roles in viral propagation, and raise
questions about how viruses activate damage signaling. The localization of host genome
maintenance proteins at SV40 replication centers suggests the possibility that viral
chromatin may masquerade as “damage” to attract host proteins needed for replication
(Figure 2A). A second possibility is that replicating viral chromatin may suffer actual
DNA damage that host genome maintenance proteins could then repair. In either case,
the activation of DNA damage checkpoints controlled by ATR and ATM signaling may arrest
SV40-infected cells in a pseudo-S/G2 phase state that provides conditions favorable
for viral DNA amplification [19], [21], [23]. Thus, much remains to be learned about
how SV40 infection activates DNA damage signaling and uses it to facilitate viral
propagation.