The virally encoded NS5B RNA-dependent RNA polymerase has emerged as a prime target in the search for specific HCV antivirals. A series of benzimidazole 5-carboxamide compounds inhibit the cellular RNA replication of a HCV subgenomic replicon and we have advanced our understanding of this class of inhibitors through a combination of complementary approaches that include biochemical cross-linking experiments with a photoreactive analogue followed by mass spectrometry analysis of the enzyme. A novel binding site has been localized for these inhibitors at the junction of the thumb domain and the N-terminal finger loop. Furthermore, the isolation and characterization of resistant replicon mutants that co-localize to this region distinguished this class of compounds from other non-nucleoside NS5B inhibitors that bind to distinct allosteric sites. Resistant mutations that emerged with the benzim- More than 2% of the world population are chronically infected with hepatitis C virus (HCV), 2 a flavivirus that is the etiological agent of non-A non-B hepatitis (1, 2). A large proportion of patients fail to achieve a sustained response to current therapies consisting of a combination of pegylated interferon and ribavirin. The discovery and development of specific anti-HCV chemotherapies aims to address this unmet clinical need and has focused on inhibitors of virally encoded functions. HCV encodes a linear polyprotein of ϳ3010 amino acids that is cleaved at multiple sites by cellular and viral proteases to produce structural and non-structural (NS) proteins (for review, see Ref.3). One of the non-structural proteins, NS5B, catalyzes the RNA-dependent RNA polymerization of a negative strand intermediate and the subsequent generation of multiple copies of the plus strand viral genome; this enzyme has emerged as a principal target for chemotherapeutic inhibition of HCV replication (4).The three-dimensional structure of the NS5B polymerase reveals an organization comparable with other nucleic acid polymerases with the familiar features of fingers, palm, and thumb domains that are organized in a "right-hand" motif (5-7). A distinct feature of the HCV polymerase (and closely related RNA-dependent RNA polymerase) active site cavity is the protrusion of a unique ␤-hairpin from the thumb subdomain that apparently plays a role in the initiation of de novo RNA synthesis as demonstrated by both structural and biochemical studies (8 -11). Another additional feature of the HCV polymerase is two loops that bridge the fingers and thumb subdomain and result in an encircled active site. This feature is now known to be shared by other RNA-dependent RNA polymerase from rhinovirus, bacteriophage 6, rabbit hemorrhagic disease virus, bovine viral diarrhea virus, Norwalk virus,. Interestingly, the interface between the HCV polymerase N-terminal 1 loop and the thumb subdomain is the location of a GTP binding site (8), although its precise biological role is unsolved.A number of different HCV polymerase inhibitors have emerged that can be broadly di...

Interaction between the E2 protein and E1 helicase of human papillomaviruses (HPVs) is essential for the initiation of viral DNA replication. We recently described a series of small molecules that bind to the N-terminal transactivation domain (TAD) of HPV type 11 E2 and inhibits its interaction with E1 in vitro and in cellular assays. Here we report the crystal structures of both the HPV11 TAD and of a complex between this domain and an inhibitor, at 2.5-and 2.4-Å resolution, respectively. The HPV11 TAD structure is very similar to that of the analogous domain of HPV16. Inhibitor binding caused no significant alteration of the protein backbone, but movements of several amino acid side chains at the binding site, in particular those of Tyr-19, His-32, Leu-94, and Glu-100, resulted in the formation of a deep hydrophobic pocket that accommodates the indandione moiety of the inhibitor. Mutational analysis provides functional evidence for specific interactions between Tyr-19 and E1 and between His-32 and the inhibitor. A second inhibitor molecule is also present at the binding pocket. Although evidence is presented that this second molecule makes only weak interactions with the protein and is likely an artifact of crystallization, its presence defines additional regions of the binding pocket that could be exploited to design more potent inhibitors.Human papillomaviruses (HPVs) 1 are the etiological agents of malignant and benign lesions of the differentiating squamous or mucosal epithelium, notably of cervical cancer. Approximately 25 HPV types replicate in mucosal tissues of the anogenital tract. HPV16, -18, and -31 are the most prevalent "high-risk" types found in pre-cancerous or malignant lesions of the cervix. HPV6 and -11 are the most common "low-risk" types, which cause benign genital warts (condyloma acuminata), a less serious condition but one of the most common sexually transmitted diseases (1). Currently, no specific antivirals are available for the treatment of HPV infections.The small circular double-stranded DNA genome of papillomavirus is actively maintained as a multicopy episome in the nucleus of infected epithelial cells. This process is dependent on replication of the viral genome by the viral E1 and E2 proteins, in conjunction with the host DNA replication machinery. E2 is a sequence-specific DNA-binding protein that has a number of functions in the viral lifecycle. In addition to its role in the initiation of viral DNA replication, E2 is involved in regulating the transcription of viral genes (2-7), and in the segregation of the viral genome during cell division (8, 9). As a replication initiation factor, E2 binds with high affinity to specific sites located within the viral origin (ori) to help recruit it to the E1 helicase (10 -13). Formation of a ternary complex between E1, E2, and the origin relies not only on the interaction of E1 and E2 with specific DNA sequences at the origin but is also critically dependent on a direct interaction between these two proteins (14 -18).The 40-kDa E2 prote...

Human papillomavirus (HPV) DNA replication is initiated by recruitment of the E1 helicase by the E2 protein to the viral origin. Screening of our corporate compound collection with an assay measuring the cooperative binding of E1 and E2 to the origin identified a class of small molecule inhibitors of the protein interaction between E1 and E2. Isothermal titration calorimetry and changes in protein fluorescence showed that the inhibitors bind to the transactivation domain of E2, the region that interacts with E1. These compounds inhibit E2 of the low risk HPV types 6 and 11 but not those of high risk HPV types or of cottontail rabbit papillomavirus. Functional evidence that the transactivation domain is the target of inhibition was obtained by swapping this domain between a sensitive (HPV11) and a resistant (cottontail rabbit papillomavirus) E2 type and by identifying an amino acid substitution, E100A, that increases inhibition by ϳ10-fold. This class of inhibitors was found to antagonize specifically the E1-E2 interaction in vivo and to inhibit HPV DNA replication in transiently transfected cells. These results highlight the potential of the E1-E2 interaction as a small molecule antiviral target.Papillomaviruses are a family of small double-stranded DNA viruses that induce benign and malignant hyperproliferative lesions of the differentiating epithelium (reviewed in Refs. 1-4). Approximately 25 types of human papillomavirus (HPV) 1 infect the anogenital region. These HPV types have been classified as low risk or high risk types depending on whether they cause benign warts or lesions that can progress to invasive cancer, respectively. Current therapies to remove HPV-induced lesions include a variety of ablative or cytodestructive procedures and the use of immunomodulatory molecules such as imiquimod (5, 6) to stimulate a host immune response. Small molecule antivirals for the treatment of HPV infections do not currently exist.The life cycle of HPV is coupled to the cellular differentiation program that occurs in the epithelium (7). Maintenance of the viral genome in infected cells of the basal layer is essential for the viral life cycle and the ensuing pathology. Maintenance of the HPV episome in primary keratinocyte cultures depends on the function of E1 and E2, a 3Ј-5Ј helicase (8) and a sequencespecific DNA-binding protein (9), respectively, which are required for replication of the genome (10). HPV DNA replication is initiated by the co-operative binding of E1 and E2 to specific DNA sequences within the viral origin (11-14). Formation of this E1-E2-ori ternary complex is dependent on the interaction of both proteins with DNA but also on a critical interaction between the N-terminal transactivation domain (TAD) of E2 and the C-terminal enzymatic domain of E1 (15-22). Assembly of this initial E1-E2-ori complex serves as a starting point for the recruitment of additional E1 molecules (23, 24) and their assembly into hexamers and double hexamers that have ATPase and helicase activity (25,26).Any of the protein-pr...

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We have reinvestigated the use of ionizing radiations to measure the molecular mass of water-soluble or membrane proteins. The test was performed by using the most straightforward aspect of the technique, which consists of SDS/PAGE analysis of the protein-fragmentation process. We found that exposure of purified standard proteins to increasing doses of ionizing radiation causes progressive fragmentation of the native protein into defined peptide patterns. The coloured band corresponding to the intact protein was measured on the SDS gel as a function of dose to determine the dose (D37 t) corresponding to 37 % of the initial amount of unfragmented protein deposited on the gel. This led to a calibration curve between I/D37t and the known molecular mass of the standard proteins whose best fit gave Mr = 1.77 x 106/D37. tat -78°C, i.e. 3500 higher than the generally accepted value at that temperature obtained from inactivation studies.However, we have to conclude that this method is useless to determine the state of aggregation of a protein, since, for all the oligomers tested, the best fit was obtained by using the protomeric molecular mass, suggesting that there is no energy transfer between protomers. Furthermore, SDS greatly increases the fragmentation rate of proteins, which suggests additional calibration problems for membrane proteins in detergent or in the lipid bilayer. But the main drawback of the technique arises from our observation that some proteins behaved anomalously, leading to very large errors in the apparent target size as compared with true molecular mass (up to 100 0w). It is thus unreliable to apply the radiation method for absolute molecular-mass determination. We then focused on the novel finding that discrete fragmentation of proteins occurs at preferential sites, and this was studied in more detail with aspartate transcarbamylase. N-Terminal sequencing of several radiolysis fragments of the catalytic chain of the enzyme revealed that breaks along the polypeptide chains are localized close to the C-terminal end. Examination of the three-dimensional structure of aspartate transcarbamylase suggests that radiolysis sites (fragile bonds) might be localized in connecting loops.