Cutaneous and mucosal human papillomaviruses differ in net surface charge, potential impact on tropism

Abstract

Papillomaviruses can roughly be divided into two tropism groups, those infecting the skin, including the genus beta PVs, and those infecting the mucosa, predominantly genus alpha PVs. The L1 capsid protein determines the phylogenetic separation between beta types and alpha types and the L1 protein is most probably responsible for the first interaction with the cell surface. Virus entry is a known determinant for tissue tropism and to study if interactions of the viral capsid with the cell surface could affect HPV tropism, the net surface charge of the HPV L1 capsid proteins was analyzed and HPV-16 (alpha) and HPV-5 (beta) with a mucosal and cutaneous tropism respectively were used to study heparin inhibition of uptake. The negatively charged L1 proteins were all found among HPVs with cutaneous tropism from the beta- and gamma-PV genus, while all alpha HPVs were positively charged at pH 7.4. The linear sequence of the HPV-5 L1 capsid protein had a predicted isoelectric point (pI) of 6.59 and a charge of -2.74 at pH 7.4, while HPV-16 had a pI of 7.95 with a charge of +2.98, suggesting no interaction between HPV-5 and the highly negative charged heparin. Furthermore, 3D-modelling indicated that HPV-5 L1 exposed more negatively charged amino acids than HPV-16. Uptake of HPV-5 (beta) and HPV-16 (alpha) was studied in vitro by using a pseudovirus (PsV) assay. Uptake of HPV-5 PsV was not inhibited by heparin in C33A cells and only minor inhibition was detected in HaCaT cells. HPV-16 PsV uptake was significantly more inhibited by heparin in both cells and completely blocked in C33A cells.

Figure 1

Comparison of surface exposed charge differences of HPV-5 and HPV-16 L1 capsid proteins. The model of the HPV-5 L1 monomer is overlaid on the crystal structure of HPV-16 L1. Colour in this illustration display positive (blue) and negative (red) amino acids loops of L1. To model L1 capsid protein the structure file for the HPV-16 L1 monomer, 1 DZL.pdb [13], was downloaded and used as a template to model L1-monomers for HPV-5 using SWISS MODEL . Protein models were also ray-traced using POV-Ray . The 3D-models of L1 protein of an alpha-papillomavirus (HPV-16) and a beta-papillomavirus (HPV-5) visualize the number of charged amino acids on one L1 capsid protein that are more than 30% exposed in the surface when grouped into a pentamer.