Papillomaviruses are internalized via clathrin-dependent endocytosis. However, the mechanism by which viral genomes pass endosomal membranes has not been elucidated. In this report we show that the minor capsid protein L2 is required for egress of viral genomes from endosomes but not for initial uptake and uncoating and that a 23-amino-acid peptide at the C terminus of L2 is necessary for this function. Pseudogenomes encapsidated by L1 and L2 lacking this peptide accumulated in vesicular compartments similar to that observed with L1-only viral particles, and these mutant pseudoviruses were noninfectious. This L2 peptide displayed strong membrane-disrupting activity, induced cytolysis of bacteria and eukaryotic cells in a pHdependent manner, and permeabilized cells after exogenous addition. Fusions between green fluorescent protein and the L2 peptide integrated into cellular membranes like the wild type but not like C-terminal mutants of L2. Our data indicate that the L2 C terminus facilitates escape of viral genomes from the endocytic compartment and that this feature is conserved among papillomaviruses. Furthermore, the characteristic of this peptide differs from the classical virus-encoded membrane-penetrating peptides.

Cell surface heparan sulfate proteoglycans (HSPGs) serve as primary attachment receptors for human papillomaviruses (HPVs). To demonstrate that a biologically functional HPV-receptor interaction is restricted to a specific subset of HSPGs, we first explored the role of HSPG glucosaminoglycan side chain modifications. We demonstrate that HSPG O sulfation is essential for HPV binding and infection, whereas de-N-sulfated heparin interfered with VLP binding but not with HPV pseudoinfection. This points to differences in VLP-HSPG and pseudovirion-HSPG interactions. Interestingly, internalization kinetics of VLPs and pseudovirions, as measured by fluorescence-activated cell sorting analysis, also differ significantly with approximate half times of 3.5 and 7.5 h, respectively. These data suggest that differences in HSPG binding significantly influence postbinding events. We also present evidence that pseudovirions undergo a conformational change after cell attachment. A monoclonal antibody (H33.J3), which displays negligible effectiveness in preattachment neutralization assays, efficiently neutralizes cell-bound virions. However, no difference in H33.J3 binding to pseudovirions and VLPs was observed in enzyme-linked immunosorbent assay and virus capture assays. In contrast to antibody H33.B6, which displays equal efficiencies in pre-and postattachment neutralization assays, H33.J3 does not block VLP binding to heparin, demonstrating that it interferes with steps subsequent to virus binding. Our data strongly suggest that H33.J3 recognizes a conformation-dependent epitope in capsid protein L1, which undergoes a structural change after cell attachment.Human papillomaviruses (HPVs) are highly species-specific epitheliotropic DNA viruses. Of the more than 100 different genotypes, HPV type 16 (HPV16), HPV18, HPV31, HPV33, HPV35, HPV45, and HPV58 are most closely associated with cervical epithelial neoplasias and members of the group of HPV imposing a "high risk" for malignant progression to invasive genital carcinomas (30). The nonenveloped papillomavirus is composed of 360 copies of the major capsid protein L1, organized in 72 capsomeres, and probably 12 copies of the minor capsid protein L2 (1, 43). The encapsidated genome is an 8,000-bp circularized double-stranded DNA associated with cellular histones.Despite their considerable clinical significance, the initial steps leading to infection with these viruses, as well as the mechanisms involved in virus entry into host cells, have not yet been completely elucidated due to the limited growth properties of HPV in cell cultures and the ubiquitous expression of HPV-binding proteins. The use of virus-like particles (VLPs) (20,25,36,46,49) has helped in the study of the initial interaction of papillomavirus particles with cell surfaces. It was established that VLPs of many HPV types compete for binding to the same highly conserved proteinaceous attachment receptor. In contrast to L1, L2 protein was not essential for binding, since L1 VLPs bound as efficiently as L1L2 VLPs (32,3...

We now demonstrate that the L2 protein is able to interact with the microtubule network via the motor protein dynein. L2 protein was found attached to microtubules after uncoating of incoming human papillomavirus pseudovirions. Based on immunofluorescence and coimmunoprecipitation analyses, the L2 region interacting with dynein is mapped to the C-terminal 40 amino acids. Mutations within this region abrogating the L2/dynein interaction strongly reduce the infectivity of pseudoviruses, indicating that this interaction mediates the minus-end-directed transport of the viral genome along microtubules towards the nucleus.Infection by DNA viruses replicating in the nucleus requires the passage of the cytosol, a high diffusion barrier due to its viscosity and the presence of a dense network of microtubules, actin, and intermediate filaments. A common strategy for intracytoplasmic transport adopted by viruses to overcome this obstacle has been to utilize the cellular transport machinery to move along microtubules (MTs). MTs are nucleated at the microtubule organizing center (MTOC) via their minus ends, whereas the plus ends extend towards the cell periphery (18). Minus-end-directed transport is mostly catalyzed by the large motor protein complex dynein (30).Dynein-mediated transport along MTs has been reported for a variety of viruses (8,19,25,26). Papillomavirus infection also requires a functional MT network (6, 24). The nonenveloped papillomaviruses comprise a large virus family, and some of the human viruses (e.g., human papillomavirus type 16 [HPV16], HPV18, and HPV33) are associated with various forms of cancer (16). The papillomavirus capsid consists of the major capsid protein L1, the main structural component of the viral protein shell, and a minor capsid protein L2, which serves important nonstructural functions during virus assembly and infection (3,7,10,11,15,29,33). During assembly, L2 recruits L1 to the subnuclear promyelocytic leukemia protein (PML) bodies (PML oncogenic domains and nuclear domain 10) (7, 11), where replication of virus DNA and papillomavirus morphogenesis probably take place (28). During infection, L2 is proteolytically processed by furin convertase (20). L2 protein also mediates the egress of the viral genome from endosomes (14) and accompanies it to PML bodies (5), suggesting that L2 is involved in the intracytoplasmic transport. Here we show that L2 protein interacts with dynein via its C-terminal region. Viruses harboring deletions or point mutations in the dynein binding motif exhibit strongly reduced infectivity, suggesting a role for L2 dynein interaction in papillomavirus infection.The L2 protein contains at least two nuclear localization signals (NLSs) and an additional signal for association with PML bodies (2). Nuclear translocation of L2 synthesized in the cytoplasm is rapid and requires the host cell chaperone Hsc70 (9). In addition to nuclear L2, a small amount of L2 was found at the MTOC when costaining was performed using L2-specific mouse monoclonal antibody 33L2-1 (32) ...

Constitutively active mutant KRas displays a reduced rate of GTP hydrolysis via both intrinsic and GTPaseactivating protein-catalysed mechanisms, resulting in the perpetual activation of Ras pathways. We describe a fragment screening campaign using X-ray crystallography that led to the discovery of three fragment binding sites on the Ras:SOS complex. The identification of tool compounds binding at each of these sites allowed exploration of two new approaches to Ras pathway inhibition by stabilising or covalently modifying the Ras:SOS complex to prevent the reloading of Ras with GTP. Initially, we identified ligands that bound reversibly to the Ras:SOS complex in two distinct sites, but these compounds were not sufficiently potent inhibitors to validate our stabilisation hypothesis. We conclude by demonstrating that covalent modification of Cys118 on Ras leads to a novel mechanism of inhibition of the SOS-mediated interaction between Ras and Raf, and is effective at inhibiting the exchange of labelled GDP in both mutant (G12C and G12V) and wild type Ras.

Fragment-based lead generation (FBLG) has recently emerged as an alternative to traditional high throughput screening (HTS) to identify initial chemistry starting points for drug discovery programs. In comparison to HTS screening libraries, the screening sets for FBLG tend to contain orders of magnitude fewer compounds, and the compounds themselves are less structurally complex and have lower molecular weight. This report summarises the advent of FBLG within the industry and then describes the FBLG experience at AstraZeneca. We discuss (1) optimising the design of screening libraries, (2) hit detection methodologies, (3) evaluation of hit quality and use of ligand efficiency calculations, and (4) approaches to evolve fragment-based, low complexity hits towards drug-like leads. Furthermore, we exemplify our use of FBLG with case studies in the following drug discovery areas: antibacterial enzyme targets, GPCRs (melanocortin 4 receptor modulators), prostaglandin D2 synthase inhibitors, phosphatase inhibitors (protein tyrosine phosphotase 1B), and protease inhibitors (b-secretase).