Kallikrein-8 Proteolytically Processes Human Papillomaviruses in the Extracellular Space To Facilitate Entry into Host Cells

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...

Section: Discussionmentioning

confidence: 99%

Furin Cleavage of L2 during Papillomavirus Infection: Minimal Dependence on Cyclophilins

Bronnimann

Calton

Chiquette

et al. 2016

“…It is known that the DNA binding activity of L1 protein was mapped to the carboxy-terminal overlapping with the nuclear localization signal. Recently, it was also shown that L1 protein is partially proteolytically cleaved by kallekrein-8 on the cell surface after conformational changes have occurred (15). This cleavage was suggested to occur at a conserved consensus cleavage site in the carboxy-terminal arm of the L1 protein.…”

Section: Fig 3 L1 Protein Associated With Condensed Chromosomes Retaimentioning

confidence: 99%

“…The cleavage and loss of these 12 amino acids from the very N terminus are essential downstream in the entry process for endosomal escape. In addition, a fraction of L1 protein is cleaved by kallekrein-8 on the cell surface (15). Following these conformational changes, the virion is reported to associate with a number of non-HSPG secondary receptors, including integrins, tetraspanins, growth factor receptors, and annexin A2, that serve as a platform for internalization (16)(17)(18)(19)(20)(21)(22)(23)(24).…”

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confidence: 99%

Human Papillomavirus Major Capsid Protein L1 Remains Associated with the Incoming Viral Genome throughout the Entry Process

DiGiuseppe

Bienkowska-Haba

Guion

et al. 2017

During infectious entry, acidification within the endosome triggers uncoating of the human papillomavirus (HPV) capsid, whereupon host cyclophilins facilitate the release of most of the major capsid protein, L1, from the minor capsid protein L2 and the viral genome. The L2/DNA complex traffics to the trans-Golgi network (TGN). After the onset of mitosis, HPV-harboring transport vesicles bud from the TGN, followed by association with mitotic chromosomes. During this time, the HPV genome remains in a vesicular compartment until the nucleus has completely reformed. Recent data suggest that while most of L1 protein dissociates and is degraded in the endosome, some L1 protein remains associated with the viral genome. The L1 protein has DNA binding activity, and the L2 protein has multiple domains capable of interacting with L1 capsomeres. In this study, we report that some L1 protein traffics with L2 and viral genome to the nucleus. The accompanying L1 protein is mostly full length and retains conformation-dependent epitopes, which are recognized by neutralizing antibodies. Since more than one L1 molecule contributes to these epitopes and requires assembly into capsomeres, we propose that L1 protein is present in the form of pentamers. Furthermore, we provide evidence that the L1 protein interacts directly with viral DNA within the capsid. Based on our findings, we propose that the L1 protein, likely arranged as capsomeres, stabilizes the viral genome within the subviral complex during intracellular trafficking.IMPORTANCE After internalization, the nonenveloped human papillomavirus virion uncoats in the endosome, whereupon conformational changes result in a dissociation of a subset of the major capsid protein L1 from the minor capsid protein L2, which remains in complex with the viral DNA. Recent data suggest that some L1 protein may accompany the viral genome beyond the endosomal compartment. We demonstrate that conformationally intact L1 protein, likely still arranged as capsomeres, remains associated with the incoming viral genome throughout mitosis and transiently resides in the nucleus until after the viral DNA is released from the transport vesicle.KEYWORDS HPV entry, HPV16, L1 protein, L2 protein, mitosis, nuclear transport, virus trafficking P apillomaviruses are a family of nonenveloped DNA viruses that infect a wide range of hosts with a preference for epithelial cells. The papillomaviruses that infect humans (HPVs) are a particular health burden. While most infections with HPVs are cleared by the immune system, a persistent infection with the high-risk types is associated with increased risk of carcinomas. The high-risk type 16 accounts for

“…After primary binding, both capsid proteins undergo conformational changes initiated by interactions with HSPGs, chaperones, and cellular proteases (18)(19)(20)(21). The chaperone cyclophilin B facilitates exposure of the L2 N terminus (22), while furin cleaves the first 12 amino acids of L2 (23)(24)(25)(26).…”

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confidence: 99%

The Cytoskeletal Adaptor Obscurin-Like 1 Interacts with the Human Papillomavirus 16 (HPV16) Capsid Protein L2 and Is Required for HPV16 Endocytosis

Wüstenhagen

Hampe

Boukhallouk

et al. 2016

The human papillomavirus (HPV) capsid protein L2 is essential for viral entry. To gain a deeper understanding of the role of L2, we searched for novel cellular L2-interacting proteins. A yeast two-hybrid analysis uncovered the actin-depolymerizing factor gelsolin, the membrane glycoprotein dysadherin, the centrosomal protein 68 (Cep68), and the cytoskeletal adaptor protein obscurin-like 1 protein (OBSL1) as putative L2 binding molecules. Pseudovirus (PsV) infection assays identified OBSL1 as a host factor required for gene transduction by three oncogenic human papillomavirus types, HPV16, HPV18, and HPV31. In addition, we detected OBSL1 expression in cervical tissue sections and noted the involvement of OBSL1 during gene transduction of primary keratinocytes by HPV16 PsV. Complex formation of HPV16 L2 with OBSL1 was demonstrated in coimmunofluorescence and coimmunoprecipitation studies after overexpression of L2 or after PsV exposure. We observed a strong colocalization of OBSL1 with HPV16 PsV and tetraspanin CD151 at the plasma membrane, suggesting a role for OBSL1 in viral endocytosis. Indeed, viral entry assays exhibited a reduction of viral endocytosis in OBSL1-depleted cells. Our results suggest OBSL1 as a novel L2-interacting protein and endocytosis factor in HPV infection. IMPORTANCEHuman papillomaviruses infect mucosal and cutaneous epithelia, and the high-risk HPV types account for 5% of cancer cases worldwide. As recently discovered, HPV entry occurs by a clathrin-, caveolin-, and dynamin-independent endocytosis via tetraspanin-enriched microdomains. At present, the cellular proteins involved in the underlying mechanism of this type of endocytosis are under investigation. In this study, the cytoskeletal adaptor OBSL1 was discovered as a previously unrecognized interaction partner of the minor capsid protein L2 and was identified as a proviral host factor required for HPV16 endocytosis into target cells. The findings of this study advance the understanding of a so far less well-characterized endocytic pathway that is used by oncogenic HPV subtypes. Human papillomaviruses (HPVs) are small, nonenveloped DNA viruses that infect dividing basal keratinocytes of skin and mucosa via microlesions of the tissue. HPV is capable of inducing benign epithelial warts on the skin and mucosa, and infection with a high-risk HPV type may cause cervical and other anogenital and oropharyngeal cancers (1, 2). Cervical cancer is the third most common cancer in women worldwide and is associated with HPV infection, more precisely with high-risk HPV types such as HPV16, HPV18, and HPV31 (3). HPV is composed of a viral capsid with the major capsid protein L1, the minor capsid protein L2, and the viral genome. One icosahedral capsid contains 360 copies of L1, which can self-assemble to 72 pentamers, and up to 72 copies of the minor capsid protein L2, located inside the L1 shell (4-6). The capsid proteins L1 and L2 are key players in early events of infection, such as virus binding at the plasma membrane, cell entry, and t...