E is encoded by the largest and most
E1 is encoded by the largest and most conserved open-reading frame (ORF) of the PV genome. The protein ranges in size from 600 to 650 amino acids, depending on the PV type. Overall, the protein can be divided into three functional segments: an N-terminal regulatory region that is essential for optimal replication in vivo but dispensable in vitro (Amin et al., 2000, Ferran and McBride, 1998, Morin et al., 2011, Sun et al., 1998), a central origin-binding domain (known as the DNA-binding domain, DBD) that recognizes specific sites in the ori (Auster and Joshua-Tor, 2004, Chen and Stenlund, 1998, Leng et al., 1997, Sarafi and McBride, 1995, Sun et al., 1998, Thorner et al., 1993, Titolo et al., 2003), and a C-terminal enzymatic domain sufficient for self-assembly into hexamers that display ATPase activity and are capable of unwinding short DNA duplexes (Fig. 1) (Castella et al., 2006b, Enemark and Joshua-Tor, 2006, Titolo et al., 2000, White et al., 2001). The DBD and C-terminal helicase domain (HD) are sufficient for ori-dependent DNA replication in vitro and form the core of the molecular motor that drives viral DNA replication (Amin et al., 2000).
Crystal structure of the E1 DNA helicase and the mechanism of DNA unwinding
Initiation of papillomavirus DNA replication Although it is generally accepted that replication of the viral genome in undifferentiated cells, in transient assays or in vitro involves a bi-directional replication fork, it has been suggested that amplification of the PV genome during the productive-phase of the life 5-Carboxymethylester-UTP occurs through a rolling-circle mechanism (Flores and Lambert, 1997). Because the vast majority of published studies have investigated how the viral genome is replicated during the establishment and maintenance stages of the viral life cycle, much more is known about the bi-directional mode of DNA replication that is characteristic of these stages. In this section, we will review how bi-directional replication of the PV genome is initiated by E1 and E2 and the mechanism by which E1 assembles at the ori as a functional double-hexamer. Most in vitro studies on the assembly of E1 and E2 at the origin have been performed with the BPV1 proteins and a fragment of the BPV1 origin, designated as the minimal ori, which contains a single E2-binding site (E2BS), an E1-binding region and an AT-rich sequence (Fig. 5A). This minimal ori supports BPV1 DNA replication in transient assays, albeit at slightly lower levels that the complete ori which contains an additional E2BS (Fig. 5A) (Ustav et al., 1993). The current model of initiation of BPV1 DNA replication is schematized in Fig. 5B and described below together with relevant data obtained with the E1 and E2 proteins of other PV types.
Interaction of E1 with the cellular DNA replication machinery Small DNA tumor viruses, including PVs, encode only a few proteins and must therefore rely extensively on their host for the other functions required to complete their life cycles. Consequently, most papillomavirus proteins engage in multiple protein interactions with cellular factors to carry out their activities. E1 is no exception and has been reported to interact with several members of the cellular DNA replication machinery (Table 1). These include the DNA polymerase α-primase (Pol α-prim) complex, replication protein A (RPA), and topoisomerase I (Topo I). In vitro, papillomavirus DNA replication also requires replication factor C (RFC), proliferating-cell nuclear antigen (PCNA) and DNA polymerase δ, even though interaction of these factors with E1 has not been reported (Kuo et al., 1994, Melendy et al., 1995, Müller et al., 1994). The process of viral DNA synthesis is schematized in Fig. 7 and the various interactions that E1 makes with key replication factors are reviewed below.
Interaction of E1 with other cellular proteins In addition to the known DNA replication factors mentioned above, E1 has also been reported to associate with other cellular proteins to assist and/or regulate viral DNA replication (Table 1). Among these are p80, hSNF5, histone H1, E1BP and p56, described below.