Due to the association of human papillomaviruses (HPV) with development of multiple carcinomas, especially cervical carcinomas, early diagnosis and prevention of infection with HPV are of great medical and economic importance. Knowledge of the early steps of papillomavirus infection, which results in infectious entry, will help develop means to prevent HPV-induced lesions. Since HPV are difficult to propagate in cell culture, surrogate infection systems with marker-encoding viral capsids, called HPV pseudovirions, have been developed and successfully used in investigating the HPV entry pathway as well as in testing of substances interfering with HPV infection (2, 33). These studies have led to the identification of specifically modified heparan sulfate proteoglycans (HSPGs) as primary attachment receptors for papillomaviruses (13,15) and to heparin and other sulfated polysaccharides as inhibitors of HPV infection (1, 7, 13). Recently, carrageenan, an unbranched sulfated polysaccharide from algae with saccharide linkages reminiscent of galactosaminoglycans, has been reported to inhibit HPV infection primarily by preventing the binding of virions to the cell (4). Dispirotripiperazine (DSTP) derivatives represent another substance class with proven antiviral potential. DSTP27 (an N,NЈ-bisheteryl derivative of DSTP), one of the most active derivatives of this new class of low-molecular-weight antiherpetic compounds, interacts with specific forms of cell surface HSPGs (26). In addition to the inhibition of herpes virus attachment and infection, DSTP27 efficiently blocks the attachment and uptake of members from other virus families that depend on HSPGs as primary attachment molecules (25). In contrast to the HS analogs such as heparin and pentosan polysulfate that have short-lived effects, pretreatment of cells with DSTP27 induces a longlasting antiviral effect. Based on computer modeling, DSTP27 possibly interacts with two O-sulfate groups located on neighboring saccharides of the HS chain (27). Using the octosaccharide essential for HS-mediated entry of herpes simplex virus type 1 (HSV-1) into host cells (20), these computational studies further show that DSTP27 may additionally interact with a carbonyl group, thus increasing the strength of compound binding.Since HPV bind specifically to sulfated polysaccharide residues of cell surface HSPGs, particularly 2-O-and 6-O-sulfated HS chains in addition to N-sulfated residues (27), DSTP27 was predicted to work as a potent inhibitor of HPV infection. In this report we demonstrate that DSTP27 efficiently prevents HPV infection when applied several hours pre-or postinfection of cells. This is achieved by two putatively different mech-* Corresponding author. Mailing address:

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.

Human papillomavirus type 33 (HPV-33) pseudovirus infection is a slow process dependent on the initial interaction with cell-surface heparan sulfate (T. Giroglou, L. Florin, F. Schafer, R. E. Streeck, and M. Sapp, 2001a, J. Virol. 75, 1565-1570). We have now further dissected the initial steps of pseudovirus uptake using removal of cell-surface proteoglycans and selective inhibition of entry pathways. Treatment of cells with heparinase I, but not with phosphoinositol-specific phospholipase C (PIPLC), prevented binding of papillomavirus-like particles and infection with HPV-33 pseudovirions, indicating that GPI-linked proteoglycans (glypicans) are not required for productive infection. The slow entry of pseudovirions was inhibited by cytochalasin D and nocodazole in a concentration-dependent manner, suggesting actin polymerization and intact microtubuli be required. Inhibitors of the caveolae-mediated uptake did not significantly affect pseudoinfection. Interestingly, pseudoinfection was blocked by selective inhibitors of endosomal acidification up to 12 h postinfection. Together, our results suggest that binding of HPV pseudovirions to heparan sulfate proteoglycans, most likely syndecans, is followed by delayed internalization via the endosomal pathway.

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...

Efficient infection of cells by human papillomaviruses (HPVs)and pseudovirions requires primary interaction with cell surface proteoglycans with apparent preference for species carrying heparan sulfate (HS) side chains. To identify residues contributing to virus/cell interaction, we performed point mutational analysis of the HPV16 major capsid protein, L1, targeting surface-exposed amino acid residues. Replacement of lysine residues 278, 356, or 361 for alanine reduced cell binding and infectivity of pseudovirions. Various combinations of these amino acid exchanges further decreased cell attachment and infectivity with residual infectivity of less than 5% for the triple mutant, suggesting that these lysine residues cooperate in HS binding. Single, double, or triple exchanges for arginine did not impair infectivity, demonstrating that interaction is dependent on charge distribution rather than sequence-specific. The lysine residues are located within a pocket on the capsomere surface, which was previously proposed as the putative receptor binding site. Fab fragments of binding-neutralizing antibody H16.56E that recognize an epitope directly adjacent to lysine residues strongly reduced HS-mediated cell binding, further corroborating our findings. In contrast, mutation of basic surface residues located in the cleft between capsomeres outside this pocket did not significantly reduce interaction with HS or resulted in assembly-deficient proteins. Computer-simulated heparin docking suggested that all three lysine residues can form hydrogen bonds with 2-O-, 6-O-, and N-sulfate groups of a single HS molecule with a minimal saccharide domain length of eight monomer units. This prediction was experimentally confirmed in binding experiments using capsid protein, heparin molecules of defined length, and sulfate group modifications. Human papillomaviruses (HPVs)2 are non-enveloped epitheliotropic viruses mainly causing benign tumors of the skin and mucosa. Some of them, including HPV type 16 (HPV16), are the primary etiologic agent for anogenital tumors, especially cervical carcinoma (1). To initiate a successful life cycle, viruses need to attach to host cells. This is achieved by binding to cell surface receptor molecules, which can be proteins or sugar or lipid components. Specific attachment will then initiate internalization, which is followed by amplification of the viral genome and finally production of progeny virus. Certain HPVs of the genus of ␣-Papillomavirus have been shown to use cell surface proteoglycans as initial attachment receptors; the target glycosaminoglycan (GAG) chains have been tentatively identified as heparan sulfate (HS) (2-4).Cell surface heparan sulfate proteoglycans (HSPGs), mainly syndecans and glypicans, are composed of a protein core with covalently attached HS chains. These unbranched polysaccharides are generated through a process initiated by polymerization of glucuronic acid and N-acetylglucosamine units in alternating sequence (5). This backbone undergoes a series of modifications, incl...