Abstract
Human papillomavirus (HPV) entry is accompanied by multiple receptor-induced conformational changes (CCs) affecting both the major and minor capsid proteins, L1 and L2. Interaction of heparan sulfate (HS) with L1 is essential for successful HPV16 entry. Recently, cocrystallization of HPV16 with heparin revealed four distinct binding sites. Here we characterize mutant HPV16 to delineate the role of engagement with HS binding sites during infectious internalization. Site 1 (Lys278, Lys361), which mediates primary binding, is sufficient to trigger an L2 CC, exposing the amino terminus. Site 2 (Lys54, Lys356) and site 3 (Asn57, Lys59, Lys442, Lys443) are engaged following primary attachment and are required for infectious entry. Site 2 mutant particles are efficiently internalized but fail to undergo an L1 CC on the cell surface and subsequent uncoating in the endocytic compartment. After initial attachment to the cell, site 3 mutants undergo L1 and L2 CCs and then accumulate on the extracellular matrix (ECM). We conclude that the induction of CCs following site 1 and site 2 interactions results in reduced affinity for the primary HS binding site(s) on the cell surface, which allows engagement with site 3. Taken together, our findings suggest that HS binding site engagement induces CCs that prepare the virus for downstream events, such as the exposure of secondary binding sites, CCs, transfer to the uptake receptor, and uncoating.