During cell invasion, human papillomaviruses use large CD151 patches on the cell surface. Here, we studied whether these patches are defined architectures with features for virus binding and/ or internalization. Super-resolution microscopy reveals that the patches are assemblies of closely associated nanoclusters of CD151, integrin α3 and integrin α6. Integrin α6 is required for virus attachment and integrin α3 for endocytosis. We propose that CD151 organizes viral entry platforms with different types of integrin clusters for different functionalities. Since numerous viruses use tetraspanin patches, we speculate that this building principle is a blueprint for cell-surface architectures utilized by viral particles. www.nature.com/scientificreports www.nature.com/scientificreports/ cluster increases at viral particle attachment sites. Our data suggests that viral particles use an entry platform containing integrin α6 for virus attachment and integrin α3 for internalisation. integrin α3 is required for HPV16 infection of keratinocytes. To clarify the role of integrin α3 in the infection of keratinocytes, we infected integrin α3 depleted HaCaT cells by incubation with HPV16 pseudovirions (PsVs). For positive control, cells were depleted from integrin α6, which is required for infection of HeLa and KH-SV cells 5,21 . In Western blot analysis, knockdown reduced integrin α6 and integrin α3 protein levels by 87% and 97%, respectively (Fig. S1 ). Intracellular processing of the L1 virus capsid protein was monitored by Western blot analysis and by microscopy. In the Western blots, we quantified the ~25 kDa cleavage product of L1, which is generated only after internalization and lysosomal degradation of the virus 22 . As seen in Fig. 1A , knockdown of integrin α3 and integrin α6 reduced the cleavage product by 73% and 44%, respectively. For microscopy, we employed antibody-detection of the L1-7 epitope, which only becomes detectable after intracellular capsid disassembly 4,23 . Knockdown of either one of the integrins reduced the number of L1-7 positive organelles to about 65% (Fig. 1B,C) . Moreover, organelles were slightly dimmer (Fig. S2) , suggesting that apart from less forming endocytic organelles the viral load per organelle diminishes. Next, we employed a luciferase-based infection assay to test whether less uptake and processing of the L1 protein would be associated with a lower infection rate. Infection is inhibited by 88% and 67% after integrin α3 and integrin α6 knockdown, respectively (Fig. 1D) . It should be noted that integrin knockdown has secondary effects (Fig. S3) . Still, after correcting for these secondary effects, knockdown of each integrin more than halves the infection rate (Fig. S3) . These experiments demonstrate that apart from integrin α6, integrin α3 plays a role in HPV infection of HaCaT cells as well. However, in Western blot analysis testing PsVs cell-surface primary binding only the knockdown of integrin α6 strongly inhibits binding by 40% (Fig. 2) , suggesting each of these integrins is differently involved during infection. (2020) 10:5356 | https://doi.supplemented without or with PsVs (2 * 10 7 vge per well) and incubated for 1 h at 4 °C to allow L1 attachment to the cells. Cells were washed extensively three times with ice-cold PBS. Afterwards, cells were scraped from the wells and transferred to a reaction tube, washed three times with ice-cold PBS to remove unbound viral particles and lysed in Lämmli buffer (63 mM Tris-HCl, 2% w/v SDS, 10% w/v Glycerol, pH 6.8 in ddH 2 O) supplemented with 5% β-mercaptoethanol. Cell lysates were vortexed, boiled for 10 min at 95 °C and stored at −20 °C until SDS-PAGE and Western blot analysis. Samples were analyzed using a 4% stacking gel and a 12% (L1-cleavage assay) or 8% (for integrin detection and PsV-binding assay) polyacrylamide running gel. SDS-PAGE was performed in a MiniPROTEAN Tetra Cell (Bio-Rad) in SDS running buffer (25 mM Tris, 0.1% w/v SDS, 192 mM glycine, pH 8.3 in ddH 2 O). Gel electrophoresis started at 70 V. Voltage was raised to 150 V after samples had left the stacking gel. After the run, gels were incubated for at least 10 min with ice-cold Towbin buffer (25 mM Tris, 192 mM glycine, 20% v/v MeOH, pH 8.3 in ddH 2 O). Nitrocellulose membranes (0.2 µm pore-size, Carl Roth, Karlsruhe, Germany, cat# HP40.1) were also equilibrated in Towbin buffer for 30 min at 4 °C. Then, proteins were blotted in a MiniPROTEAN Tetra Cell equipped with Mini Trans-Blot Module in cooled Towbin buffer under constant agitation. Transfer was performed for 45 min (L1-cleavage assay and PsV-binding assay) or for 2 h (for integrin detection) at 100 V. Afterwards, membranes were washed in PBS and blocked for 1 h in 50% Odyssey Blocking Buffer (Li-Cor, cat# 927-40000) in PBS. Then, membranes were incubated with primary antibodies diluted in 50% Odyssey Blocking Buffer in PBS supplemented with 0.1% Tween-20 for 1 h at RT or overnight at 4 °C under constant agitation. Membranes were washed four times with PBS-T (0.1% Tween-20 in PBS) and incubated with secondary antibodies diluted 1:10,000 in 50% Odyssey Blocking Buffer in PBS supplemented with 0.1% Tween-20 for 1 h at RT. Afterwards, membranes were washed three times in PBS-T and one time in PBS. Blots were imaged with a Li-Cor Odyssey Classic Imaging System using the 700 nm and 800 nm channels. Detected bands were quantified using ImageJ software and corrected for local background.
Output: Anatomy of a viral entry platform differentially functionalized by integrins α3 and α6