Bovine Papillomavirus Type 1 Infection Is Mediated by SNARE Syntaxin 18

Abstract

Events that lead to viral infections include the binding of the virus to the target cells, internalization of the virus into the cells, and the ability of the viral genome to be expressed. These steps are mediated by cellular and viral proteins and are temporally regulated. The papillomavirus capsid consists of two virally encoded capsid proteins, L1 and L2. Much is known about the role of the major capsid protein L1 compared to what is known of the role of the L2 protein. We identified the interaction of the L2 protein with SNARE protein syntaxin 18, which mediates the trafficking of vesicles and their cargo between the endoplasmic reticulum, the cis-Golgi compartment, and possibly the plasma membrane. Mutations of L2 residues 41 to 44 prevented the interaction of L2 protein with syntaxin 18 in cotransfection experiments and resulted in noninfectious pseudovirions. In this paper, we describe that syntaxin 18 colocalizes with infectious bovine papillomavirus type 1 (BPV1) pseudovirions during infection but does not colocalize with the noninfectious BPV1 pseudovirions made with an L2 mutant at residues 41 to 44. We show that an antibody against BPV1 L2 residues 36 to 49 (␣L2 36-49) binds to in vitro-generated BPV1 pseudoviral capsids and does not coimmunoprecipitate syntaxin 18-and BPV1 L2-transfected proteins. ␣L2 36-49 was able to partially or completely neutralize infection of BPV1 pseudovirions and genuine virions. These results support the dependence of syntaxin 18 during BPV1 infection and the ability to interfere with infection by targeting the L2-syntaxin 18 interaction and further define the infectious route of BPV1 mediated by the L2 protein.Papillomaviruses (PVs) induce a variety of lesions such as cutaneous or genital warts in humans and exophytic papillomas of cutaneous or mucosal epithelia in animals. Human PV (HPV) infection is the most common sexually transmitted disease in the United States (approximately 5.5 million cases per annum) (31), and specific HPV genotypes (high risk) are the etiologic agents of cervical carcinoma, a major killer of women worldwide (23,(37)(38)(39). Bovine PV (BPV) infection causes benign tumors, which can result in squamous cell carcinoma in the presence of environmental factors such as bracken (7).The PV capsids consist primarily of two virally encoded proteins, L1 and L2, at an estimated ratio of 30:1 (17, 49). The L1/L2 ratio suggests that there is one L2 at each of 12 capsid vertices (49). Each viral particle contains a single doublestranded circular DNA genome of about 8 kb bound by histone proteins (15, 24), and the virus is assembled in the nucleus of squamous epithelial cells into particles that are 52 to 55 nm in diameter (9).The expression of the viral L1 protein in the absence of other viral proteins results in the packaging of viral DNA at inefficient levels (5, 50). The addition of L2 to viral particle production increases DNA packaging into virions (28, 53) and contributes to the infectious process of the virus (2, 18). Antibodies to L2 have proven to be ...

Bovine Papillomavirus Type 1: from Clathrin to Caveolin

“…Using polyomavirus-like particles encapsidating reporter DNA, investigators previously found two pathways for viral trafficking, one leading to the efficient delivery of the pseudogenome and the other along a noninfectious route (21). Our laboratory has described the internalization and trafficking of noninfectious mutant PsVs, which appear to traffic identically to wild-type (WT) PsVs until about 4 h postinternalization, when the WT virions arrive at the syntaxin 18-positive region of the cell and the mutant virions appear to be retained in the lysosomes (22). Indeed, it has been shown that for roughly every 2,000 wartderived BPV1 virions, only 1 viral particle is infectious (33).…”

“…Our laboratory identified BPV1 pseudovirions (PsVs) in the ER using transmission electron microscopy, and immunofluorescence studies showed colocalization between BPV1 and the ER marker calnexin (4,22). We characterized the interaction between the ER-resident protein syntaxin 18 and the BPV1 minor capsid protein L2, an interaction necessary for infection (22).…”

“…We characterized the interaction between the ER-resident protein syntaxin 18 and the BPV1 minor capsid protein L2, an interaction necessary for infection (22). Cells transfected with FLAG-tagged dominant negative syntaxin 18 prior to BPV1 infection were incapable of supporting a productive viral infection (4).…”

Biology and natural history of human papillomavirus infection

“…33 On the other hand, it has been proposed that HPV 16 initially enters via clatrin-coated pits but the traffic occurs through caveosomes to eventually reach the endoplasmic reticulum. 51,52 Moreover, it has been suggested that the capsids might be internalized via a novel pathway involving tetraspanin-enriched microdomains. 53 The uncoating is not observed until 8-12 hours after cellsurface binding, and it seems that L2 plays a critical role in the endosome escape.…”

“…Spliced viral mRNA was first detected at 12 hours postinfection, and in most assay systems infection is not robustly detected until at least 24 hours after capsid binding. 46,51 The pathway involved in internalization and intracellular trafficking is still unclear, but it seems to occur slowly and asynchronously over a span of several hours. 50 Clathrin-mediated endocytosis has been pointed out to be like the endocytotic pathway for the majority of HPV types.…”