Functional mapping of the DNA binding domain of bovine papillomavirus E1 protein

The E1 proteins

E1, an ATP-dependent DNA helicase, is the only enzyme encoded by papillomaviruses (PVs). It is essential for replication and amplification of the viral episome in the nucleus of infected cells. To do so, E1 assembles into a double-hexamer at the viral origin, unwinds DNA at the origin and ahead of the replication fork and interacts with cellular DNA replication factors. Biochemical and structural studies have revealed the assembly pathway of E1 at the origin and how the enzyme unwinds DNA using a spiral escalator mechanism. E1 is tightly regulated in vivo, in particular by post-translational modifications that restrict its accumulation in the nucleus. Here we review how different functional domains of E1 orchestrate viral DNA replication, with an emphasis on their interactions with substrate DNA, host DNA replication factors and modifying enzymes. These studies have made E1 one of the best characterized helicases and provided unique insights on how PVs usurp different host-cell machineries to replicate and amplify their genome in a tightly controlled manner.

Cell culture assay for transient replication of human and animal papillomaviruses

This unit contains protocols for evaluation of replication functionality of papillomavirus genomes or subgenomic fragments. Replication is measured after transient cotransfection of the genome (or subgenomic fragment) with expression vectors encoding the viral E1 and E2 proteins. Input DNA is methylated at the adenine of GATC sequences by propagation in E. coli. DNA that replicates in mammalian cells will lose the adenine methylation and become DpnI-resistant, while residual methylated input DNA will remain DpnI-sensitive. After transfection, DNA extraction, and DpnI digestion, replicated DNA can be detected by Southern blotting as a full-length plasmid, since it is resistant to digestion. This assay can be used to map the genomic location of a functional origin or to evaluate replication activity of mutations in either the origin DNA sequences or the E1 or E2 proteins.

The DNA-binding domain of human papillomavirus type 18 E1. Crystal structure, dimerization, and DNA binding

High risk types of human papillomavirus, such as type 18 (HPV-18), cause cervical carcinoma, one of the most frequent causes of cancer death in women worldwide. DNA replication is one of the central processes in viral maintenance, and the machinery involved is an excellent target for the design of antiviral therapy. The papillomaviral DNA replication initiation protein E1 has origin recognition and ATP-dependent DNA melting and helicase activities, and it consists of a DNA-binding domain and an ATPase/helicase domain. While monomeric in solution, E1 binds DNA as a dimer. Dimerization occurs via an interaction of hydrophobic residues on a single alpha-helix of each monomer. Here we present the crystal structure of the monomeric HPV-18 E1 DNA-binding domain refined to 1.8-A resolution. The structure reveals that the dimerization helix is significantly different from that of bovine papillomavirus type 1 (BPV-1). However, we demonstrate that the analogous residues required for E1 dimerization in BPV-1 and the low risk HPV-11 are also required for HPV-18 E1. We also present evidence that the HPV-18 E1 DNA-binding domain does not share the same nucleotide and amino acid requirements for specific DNA recognition as BPV-1 and HPV-11 E1.

Characterization of the minimal DNA binding domain of the human papillomavirus e1 helicase: fluorescence anisotropy studies and characterization of a dimerization-defective mutant protein

The E1 helicase of papillomaviruses is required for replication of the viral double-stranded DNA genome, in conjunction with cellular factors. DNA replication is initiated at the viral origin by the assembly of E1 monomers into oligomeric complexes that have unwinding activity. In vivo, this process is catalyzed by the viral E2 protein, which recruits E1 specifically at the origin. For bovine papillomavirus (BPV) E1 a minimal DNA-binding domain (DBD) has been identified N-terminal to the enzymatic domain. In this study, we characterized the DBD of human papillomavirus 11 (HPV11), HPV18, and BPV E1 using a quantitative DNA binding assay based on fluorescence anisotropy. We found that the HPV11 DBD binds DNA with an affinity and sequence requirement comparable to those of the analogous domain of BPV but that the HPV18 DBD has a higher affinity for nonspecific DNA. By comparing the DNA-binding properties of a dimerization-defective protein to those of the wild type, we provide evidence that dimerization of the HPV11 DBD occurs only on two appropriately positioned E1 binding-sites and contributes approximately a 10-fold increase in binding affinity. In contrast, the HPV11 E1 helicase purified as preformed hexamers binds DNA with little sequence specificity, similarly to a dimerization-defective DBD. Finally, we show that the amino acid substitution that prevents dimerization reduces the ability of a longer E1 protein to bind to the origin in vitro and to support transient HPV DNA replication in vivo, but has little effect on its ATPase activity or ability to oligomerize into hexamers. These results are discussed in light of a model of the assembly of replication-competent double hexameric E1 complexes at the origin.

Papillomavirus E1 proteins: form, function, and features

The E1 proteins are the essential origin recognition proteins for papillomavirus (PV) replication. E1 proteins bind to specific DNA elements in the viral origin of replication and assemble into hexameric helicases with the aid of a second viral protein, E2. The resultant helicase complex initiates origin DNA unwinding to provide the template for subsequent syntheses of progeny DNA. In addition to ATP-dependent helicase activity, E1 proteins interact with and recruit several host cell replication proteins to viral origin, including DNA polymerase alpha and RPA. This review will compare the basic structures and features of the human (HPV) and bovine (BPV1) papillomaviruses with an emphasis on mechanisms of replication function.

Hydrophobic residue contributions to sequence-specific DNA binding by the bovine papillomavirus helicase E1

Previously, mutational analyses of the DNA binding domain of the bovine papillomavirus E1 protein (E1DBD) identified several hydrophobic residues that are critical for DNA binding activity (M. West, D. Flanery, K. Woytek, D. Rangasamy, and V. G. Wilson, 2001, J. Virol. 75, 11948-11960). Hydrophobic interactions of nonpolar amino acid side chains can contribute to the function of DNA binding proteins through both conformational effects and direct interaction with nucleotides. To further investigate the role of hydrophobic residues in E1DBD function, a more extensive site-directed mutational analysis of hydrophobic amino acids was conducted. Alanine substitutions were made at residues V196, F197, F217, F, 237, V246, L249, and F276, and the mutants were tested for DNA binding activity in vitro and in vivo. The E1 F237A and F276A mutants were completely defective for site-specific DNA binding, while the other mutants retained partial to full wild-type binding activity. Consistent with their DNA binding defect, the F237A and F276A mutants were severely impaired for the ability to support transient in vivo replication of an origin plasmid. Combined with our previous study, five critical hydrophobic residues have been identified: F175, V193, F237, V246, and F276. These five residues localize to two internal clusters in the E1DBD structure designated hydrophobic clusters A (HCA; includes F175, V193, and F276) and B (HCB; includes F237 and V246). Amino acid side chains from residues in HCA and HCB have little surface accessibility and it is unlikely that they are involved in direct contact with DNA. HCA is distal to the DNA binding surface and presumably contributes to global conformational organization of the E1DBD. HCB is positioned beneath the DNA contact surface and we propose that it serves as an anchor or platform device to stabilize the DNA-binding element. A comparable hydrophobic cluster is present in the corresponding position in the T antigen DBD and likely serves a similar function.