The C-Terminal Arm of the Human Papillomavirus Major Capsid Protein Is Immunogenic and Involved in Virus-Host Interaction

Abstract

SUMMARY Cervical cancer is the second most prevalent malignant tumor among women worldwide. High-risk human papillomaviruses (HPVs) are believed to be the major causative pathogens of mucosal epithelial cancers including cervical cancer. The HPV capsid is made up of 360 copies of major (L1) and 72 copies of minor (L2) capsid proteins. To date, limited high-resolution structural information about the HPV capsid has hindered attempts to understand details concerning the mechanisms by which HPV assembles and infects cells. In this study, we have constructed a pseudo-atomic model of the HPV59 L1-only capsid and demonstrate that the C-terminal arm of L1 participates in virus-host interactions. Moreover, when conjugated to a scaffold protein, keyhole limpet hemocyanin (KLH), this arm is immunogenic in vivo. These results provide new insights that will help elucidate HPV biology, and hence pave a way for the design of next-generation HPV vaccines.

Crystal Structures of Two Immune Complexes Identify Determinants for Viral Infectivity and Type-Specific Neutralization of Human Papillomavirus

“…Cryo-electron microscopy (cryo-EM) structures of the whole virus capsid have provided critical insight into the mechanism of HPV assembly ( 9 – 12 ), and crystal structures of the T = 1 L1-only VLP (HPV16) and L1 pentamers (HPV11, -16, -18, and -35) have illustrated how the HPV L1 monomer forms a canonical, eight-stranded β-barrel (BIDG-CHEF) joined by six highly variable loops (BC, CD, DE, EF, FG, and HI), five of which (all but CD) are located on the surface of the L1 pentamer ( 13 , 14 ). Biochemical and serological assays have further revealed that the neutralizing epitopes for HPV capsids are type restricted and mainly clustered on these six hypervariable loops of L1 ( 15 – 20 ).…”

N-terminal truncations on L1 proteins of human papillomaviruses promote their soluble expression in Escherichia coli and self-assembly in vitro

“…4a–d , lower); this has been observed elsewhere for VLPs generated from different expression systems 35 , 39 , 40 . A comparison of the available density maps of all ten HPV types (plus HPV 59) 41 revealed structural differences in the diameters of the E. coli -generated HPV VLPs (HPV 6: 53.6 nm, HPV 11: 57.5 nm, HPV 16: 50.6 nm, HPV 18: 57.1 nm, HPV 31: 58.0 nm, HPV 33: 58.8 nm, HPV 45: 56.5 nm, HPV 52: 56.6 nm, HPV 58: 58.1 nm and HPV 59: 58.3 nm) (Fig. 4e ); this may be a consequence of the varied N-terminal truncations that would perhaps normally contribute to the interactions between capsomers during particle assembly 41 , 42 .…”

“…4f ), which may be possibly caused by the differences in particle homogeneity among the different types. The variance in the dataset sizes used in the reconstructions (~3,000 from 21,000 particles were selected to build a 6 Å HPV 59 VLPs cryoEM structures 41 , whereas only ~1000 particles were used in the final reconstruction of HPV 58 at 12.5 Å) might be another possibility to explain the resolution variance.

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“…4f ), and this may reflect differences in particle homogeneity among the different types. The varied sizes of the datasets used in the reconstructions (~3,000 particles for the 6-Å HPV 59 VLPs 41 , vs. ~1,000 particles for the 12.5-Å HPV 58) might have also affected the resolution.…”

“…As described by Li et al 41 , purified HPV 33, 45, 52, and 58 VLPs, at a concentration of ~3 mg/mL, were vitrified on Quantifoil holey carbon grids in an FEI Vitrobot. Images were recorded on an FEI Falcon II direct detector camera at a nominal ×93,000 magnification in an FEI TF30 FEG microscope at 300 kV, with underfocus settings estimated to be between 1 and 3 μm, and with an electron dose of 25 e/Å 2 .…”