The human papillomavirus type 16 (HPV16) L2 protein acts as a chaperone to ensure that the viral genome (vDNA) traffics from endosomes to the trans-Golgi network (TGN) and eventually the nucleus, where HPV replication occurs. En route to the nucleus, the L2/vDNA complex must translocate across limiting intracellular membranes. The details of this critical process remain poorly characterized. We have developed a system based on subcellular compartmentalization of the enzyme BirA and its cognate substrate to detect membrane translocation of L2-BirA from incoming virions. We find that L2 translocation requires transport to the TGN and is strictly dependent on entry into mitosis, coinciding with mitotic entry in synchronized cells. Cell cycle arrest causes retention of L2/vDNA at the TGN; only release and progression past G2/M enables translocation across the limiting membrane and subsequent infection. Microscopy of EdU-labeled vDNA reveals a rapid and dramatic shift in vDNA localization during early mitosis. At late G2/early prophase vDNA egresses from the TGN to a pericentriolar location, accumulating there through prometaphase where it begins to associate with condensed chromosomes. By metaphase and throughout anaphase the vDNA is seen bound to the mitotic chromosomes, ensuring distribution into both daughter nuclei. Mutations in a newly defined chromatin binding region of L2 potently blocked translocation, suggesting that translocation is dependent on chromatin binding during prometaphase. This represents the first time a virus has been shown to functionally couple the penetration of limiting membranes to cellular mitosis, explaining in part the tropism of HPV for mitotic basal keratinocytes.

During cellular invasion, human papillomavirus type 16 (HPV16) must transfer its viral genome (vDNA) across the endosomal membrane prior to its accumulation at nuclear PML bodies for the establishment of infection. After cellular uptake, the capsid likely undergoes pH-dependent disassembly within the endo-/lysosomal compartment, thereby exposing hidden domains in L2 that facilitate membrane penetration of L2/vDNA complexes. In an effort to identify regions of L2 that might physically interact with membranes, we have subjected the L2 sequence to multiple transmembrane (TM) domain prediction algorithms. Here, we describe a conserved TM domain within L2 (residues 45 to 67) and investigate its role in HPV16 infection. In vitro, the predicted TM domain adopts an alpha-helical structure in lipid environments and can function as a real TM domain, although not as efficiently as the bona fide TM domain of PDGFR. An L2 double point mutant renders the TM domain nonfunctional and blocks HPV16 infection by preventing endosomal translocation of vDNA. The TM domain contains three highly conserved GxxxG motifs. These motifs can facilitate homotypic and heterotypic interactions between TM helices, activities that may be important for vDNA translocation. Disruption of some of these GxxxG motifs resulted in noninfectious viruses, indicating a critical role in infection. Using a ToxR-based homo-oligomerization assay, we show a propensity for this TM domain to self-associate in a GxxxG-dependent manner. These data suggest an important role for the self-associating L2 TM domain and the conserved GxxxG motifs in the transfer of vDNA across the endo-/lysosomal membrane.A ll nonenveloped viruses are faced with a cellular membrane barrier through which they must transfer their genetic material to establish a successful infection. These viruses behave as metastable particles, rigid enough to survive transmission through the extracellular milieu but structurally poised to undergo conformational rearrangements or partial disassembly upon encountering specific factors during cellular entry, including the engagement of host cell receptors, proteolytic and chaperone activity, and low pH of the intracellular endosomal compartment. In response to these factors, capsids rearrange to expose hidden and often hydrophobic membrane translocation domains (MTDs), thereby ensuring that the coordinated process of membrane penetration occurs only when the viral capsid has reached the appropriate intracellular locale. Through convergent evolution, viruses have become equipped with a variety of different MTDs, designed to accomplish the feat of membrane penetration in several ways (recently reviewed in reference 1). Amphipathic helices, myristoylated and/or hydrophobic peptides, and phospholipase activity have all been documented to facilitate membrane penetration through pore formation, local membrane disruption, or gross fragmentation of membranes although many molecular details of these MTD-driven activities remain obscure and are ongoing subjects of inve...

e Cell invasion by human papillomavirus type 16 (HPV16) is a complex process relying on multiple host cell factors. Here we describe an investigation into the role of cellular protein disulfide isomerases (PDIs) by studying the effects of the commonly used PDI inhibitor bacitracin on HPV16 infection. Bacitracin caused an unusual time-dependent opposing effect on viral infection. Enhanced cellular binding and entry were observed at early times of infection, while inhibition was observed at later times postentry. Bacitracin was rapidly taken up by host cells and colocalized with HPV16 at late times of infection. Bacitracin had no deleterious effect on HPV16 entry, capsid disassembly, exposure of L1/L2 epitopes, or lysosomal trafficking but caused a stark inhibition of L2/viral DNA (vDNA) endosomal penetration and accumulation at nuclear PML bodies. ␥-Secretase has recently been implicated in the endosomal penetration of L2/vDNA, but bacitracin had no effect on ␥-secretase activity, indicating that blockage of this step occurs through a ␥-secretase-independent mechanism. Transient treatment with the reductant ␤-mercaptoethanol (␤-ME) was able to partially rescue the virus from bacitracin, suggesting the involvement of a cellular reductase activity in HPV16 infection. Small interfering RNA (siRNA) knockdown of cellular PDI and the related PDI family members ERp57 and ERp72 reveals a potential role for PDI and ERp72 in HPV infection. Human papillomaviruses (HPVs) are one of the most common sexually transmitted infections in the world. HPVs are small 55-nm icosahedral nonenveloped double-stranded DNA (dsDNA) viruses that replicate in differentiating cutaneous and mucosal epithelium. Infection of mucosal epithelium by oncogenic HPV genotypes can lead to cervical, anogenital, and other head and neck cancers. HPV type 16 (HPV16) is the most common of the high-risk types and is alone responsible for over 50% of cervical cancers worldwide (77). Although HPVs have been known to be the etiological agent of cervical cancer for nearly 30 years, and despite intensive research in recent years, the infectious entry pathway of HPV16 is still not well defined. Our current understanding of HPV cellular invasion reveals a complex and prolonged process, complicated by differences between in vitro cell culture systems and the recently described in vivo mouse cervicovaginal challenge model (33,37,50,62).The HPV capsid is assembled from 360 molecules of the L1 protein, arranged as 72 pentamers. L1 monomers from neighboring pentamers are disulfide bonded to each other as dimers and trimers, providing stability to the capsid (45). The minor capsid protein L2 is localized within a central cavity beneath the L1 pentamers. L2 can be present at a maximum stoichiometry of one L2 molecule per L1 pentamer or 72 molecules per virion; however, most preparations of virus contain submaximal levels of L2, typically 20 to 25 copies per virion (6). Packaged within the capsid is the ϳ8-kb viral genome (viral DNA [vDNA]), condensed as chromatin with cellu...

Despite an abundance of evidence supporting an important role for the cleavage of minor capsid protein L2 by cellular furin, direct cleavage of capsid-associated L2 during human papillomavirus 16 (HPV16) infection remains poorly characterized. The conserved cleavage site, close to the L2 N terminus, confounds observation and quantification of the small cleavage product by SDS-PAGE. To overcome this difficulty, we increased the size shift by fusing a compact protein domain, the Propionibacterium shermanii transcarboxylase domain (PSTCD), to the N terminus of L2. The infectious PSTCD-L2 virus displayed an appreciable L2 size shift during infection of HaCaT keratinocytes. Cleavage under standard cell culture conditions rarely exceeded 35% of total L2. Cleavage levels were enhanced by the addition of exogenous furin, and the absolute levels of infection correlated to the level of L2 cleavage. Cleavage occurred on both the HaCaT cell surface and extracellular matrix (ECM). Contrary to current models, experiments on the involvement of cyclophilins revealed little, if any, role for these cellular enzymes in the modulation of furin cleavage. HPV16 L2 contains two consensus cleavage sites, Arg5 ( 2 RHKR 5 ) and Arg12 ( 9 RTKR 12 ). Mutant PSTCD-L2 viruses demonstrated that although furin can cleave either site, cleavage must occur at Arg12, as cleavage at Arg5 alone is insufficient for successful infection. Mutation of the conserved cysteine residues revealed that the Cys22-Cys28 disulfide bridge is not required for cleavage. The PSTCD-L2 virus or similar N-terminal fusions will be valuable tools to study additional cellular and viral determinants of furin cleavage. IMPORTANCEFurin cleavage of minor capsid protein L2 during papillomavirus infection has been difficult to directly visualize and quantify, confounding efforts to study this important step of HPV infection. Fusion of a small protein domain to the N terminus greatly facilitates direct visualization of the cleavage product, revealing important characteristics of this critical process. Contrary to the current model, we found that cleavage is largely independent of cyclophilins, suggesting that cyclophilins act either in parallel to or downstream of furin to trigger exposure of a conserved N-terminal L2 epitope (RG-1) during infection. Based on this finding, we strongly caution against using L2 RG-1 epitope exposure as a convenient but indirect proxy of furin cleavage. H uman papillomaviruses (HPVs) are currently the most common sexually transmitted infection in the United States (1). These viruses infect and replicate in differentiating mucosal and cutaneous epithelia, and a subset of the mucosa-tropic viruses, the high-risk HPVs, cause Ͼ99% of cervical cancers in women and are associated with other anogenital and nasopharyngeal cancers in both women and men (2). In all, the high-risk HPVs account for an astounding 5% of total cancers worldwide (3).HPVs are nonenveloped viruses with a 55-nm icosahedral capsid composed of 72 pentamers of the major capsid p...

Cathepsin L (CatL) and cathepsin B (CatB) are lysosomal proteases that many viruses utilize for capsid disassembly. We tested whether CatL and CatB are required for infection by human papillomavirus type 16 (HPV16). CatL-and CatB-deficient mouse embryonic fibroblasts had higher levels of infection when compared with wild-type cells. Similar results were obtained in HaCaT keratinocytes treated with CatL-or CatB-specific small interfering RNA. Thus, CatL and CatB are not required for HPV16 infection but instead appear to restrict infection.