Polyanionic Sugars Drastically Affect Assembly of Human Papilloma Virus Virus-Like Particles

The L1 capsid protein of Human papilloma virus 16 (HPV16) forms pentamers that assemble into virus-like particles (VLPs). With many viruses, interaction of the virus with polysaccharides modulates association of the virus with a host cell. For HPV16, soluble polyanionic saccharides heparin and carrageenan bind to L1 and inhibit cell entry. Based on the observed multivalent binding of heparin, we hypothesized that anionic polysaccharides could act as scaffolds to promote VLP assembly. We observed that heparin, a highly sulfated, flexible polymer, increased the initial rate of assembly and stabilized assembled L1 VLPs. Heparin also notably increased the susceptibility of L1 pentamers to proteolysis. Conversely, κ-carrageenan, a less sulfated, rigid polymer, blocked assembly and destabilized VLPs. We examined the effects of a small panel of anions on assembly to identify their important features. We observed that smaller anions required much higher concentration for an effect, and that their chemical nature also had an impact. These data demonstrate that artificial binding partners can control VLP assembly through their interaction with the exterior of capsid protein and that structural details beyond size and electrostatics are critical to accelerating or inhibiting self-assembly.