Chaperone Proteins Abrogate Inhibition of the Human Papillomavirus (HPV) E1 Replicative Helicase by the HPV E2 Protein

Abstract

Human papillomavirus (HPV) DNA replication requires the viral origin recognition protein E2 and the presumptive viral replicative helicase E1. We now report for the first time efficient DNA unwinding by a purified HPV E1 protein. Unwinding depends on a supercoiled DNA substrate, topoisomerase I, singlestranded-DNA-binding protein, and ATP, but not an origin. Electron microscopy revealed completely unwound molecules. Intermediates contained two single-stranded loops emanating from a single protein complex, suggesting a bidirectional E1 helicase which translocated the flanking DNA in an inward direction. We showed that E2 protein partially inhibited DNA unwinding and that Hsp70 or Hsp40, which we reported previously to stimulate HPV-11 E1 binding to the origin and promote dihexameric E1 formation, apparently displaced E2 and abolished inhibition. Neither E2 nor chaperone proteins were detected in unwinding complexes. These results suggest that chaperones play important roles in the assembly and activation of a replicative helicase in higher eukaryotes. An E1 mutation in the ATP binding site caused deficient binding and unwinding of origin DNA, indicating the importance of ATP binding in efficient helicase assembly on the origin.Human and animal papillomaviruses are prevalent pathogens. Efficient origin (ori)-dependent replication of viral DNA requires the virus-encoded E1 and E2 proteins as well as cellular replication proteins (11,27,46,59,68). As such, these DNA viruses may serve as a model for higher eukaryotic DNA replication, as do simian virus 40 (SV40) and polyomavirus. The papillomavirus ori consists of several E2 binding sites (BS) flanking one E1 BS. E1 recruits the DNA polymerase ␣/primase (6, 12, 41) and the single-stranded-DNA-binding protein RPA (25). The human papillomavirus (HPV) E1 protein is required during initiation and elongation and is thought to be the replicative helicase (33). However, HPV E1 proteins are poor helicases in strand displacement assays (26, 65), and there has been no report of DNA-unwinding activity. In contrast, the bovine papillomavirus 1 (BPV-1) E1 exhibits helicase activity in both assays (51, 69).We previously reported that purified HPV-11 E1 protein expressed in insect Sf9 cells binds to ori with low affinity and specificity and also binds to DNA nonspecifically (33). Electron microscopy (EM) shows that E1 binds ori primarily as a hexamer and, at a low frequency, as a dihexamer (34). The human heat shock proteins Hsp70, Hdj2, and Hdj1 greatly stimulate E1 binding to ori. Hdj1 and Hdj2 encode members of the Hsp40 family of proteins that normally function as cochaperones of the Hsp70 proteins and greatly stimulate the ATPase activity of Hsp70 (for a review, see reference 21). However, in the case of the HPV-11 E1-ori association, their effects are independent and additive. Most strikingly, EM has revealed that Hsp40 but not Hsp70 promotes E1 dihexamer formation on ori (34). The BPV-1 E1 has been reported to be a hexameric helicase (48). However, EM revealed a bilobed comp...

The X-ray structure of the papillomavirus helicase in complex with its molecular matchmaker E2

“…Consequently, disruption of the intimate E1:E2 complex on the DNA must occur before monomer E1:E1 interactions commence. Therefore, our results suggest that previous reports of HPV-11 E2 remaining bound to origin DNA during the assembly of E1 (Lin et al 2002) is a consequence of E2 being tethered at a distance from the E1 sites following disruption of the E1 · E2 association. In contrast, the close proximity of the E2-binding sites to the location of E1 assembly in BPV likely necessitates physical displacement of E2 from the origin.…”

“…For BPV1, following the binding of additional E1 molecules, E2 is displaced from the viral origin (Lusky et al 1994;Sanders and Stenlund 1998). For HPV-11, it has been reported that E2 remains associated with the DNA during the assembly of E1 molecules, even though E2 might serve as a potential roadblock for extensive unwinding (Lin et al 2002). These differences raise several important questions.…”

“…Following formation of the E1 · E2 complex at the viral origin, a series of poorly understood reactions ensue by which E1 molecules in the prereplication complex reorganize and convert into an active, doubly hexameric helicase (Fouts et al 1999;Lin et al 2002). Conversion requires the binding of additional E1 molecules to the origin and is accelerated by ATP (Sanders and Stenlund 1998).…”

ATP-Dependent Minor Groove Recognition of TA Base Pairs Is Required for Template Melting by the E1 Initiator Protein

“…Viral initiator proteins such as the simian virus 40 (SV40) large T antigen (T-Ag) and the papillomavirus E1 protein have long been known to melt their respective origins of DNA replication, as detected by oxidation with KMnO 4 (4,5,14,22,24,25). However, little information exists about which forms of these proteins execute melting, which parts of the polypeptide are responsible for the melting activity, and whether this process is DNA sequence dependent (5, 27).The E1 proteins from papillomaviruses are ϳ70-kDa polypeptides which, in addition to DNA melting activity, have DNA helicase activity (19,20,30,33,35,37,38,40) and also bind DNA. DNA binding by E1 is the result of two different DNA binding activities.…”

“…The E1 proteins from papillomaviruses are ϳ70-kDa polypeptides which, in addition to DNA melting activity, have DNA helicase activity (19,20,30,33,35,37,38,40) and also bind DNA. DNA binding by E1 is the result of two different DNA binding activities.…”