The assembly pathway for human papillomavirus L1 capsid protein is encoded in electrostatic interactions

Human papillomavirus L1 protein spontaneously folds and assembles into 60 nm virus-like particles (VLP), with T = 7d icosahedral symmetry at high salt concentrations but not in low salt. Rapid increase from low to high salt during the assembly process gives poor-quality VLPs, while slowly increasing salt, over as many as 48 h, gives rise to high-quality VLPs. Insight into this behavior is drawn from a computational model based on the cryogenic electron microscopy (cryo-EM) structure of the human papillomavirus Type 16 VLP. Electrostatic potential energy barriers were calculated using the linearized Poisson-Boltzmann equation. After initial, rapid oligomerization of monomers into symmetrical pentamers (capsomeres), these units form pairwise interactions in energetic order, lowest barrier first. But at high salt concentrations, the energetic barriers are more or less equal and irregular particles form, both in simulation and in experiments. The interaction interfaces were modified by mutation to favor the smaller, 30 nm T = 1 icosahedral symmetry and disfavor T = 7d symmetry. The results were smaller particles. By carefully controlling ionic strength, and/or by mutating interface residues by design, it is possible to make high quality, highly immunogenic VLPs directly from unpurified bacterial lysate, without requiring the use of a disassembly/reassembly step.