Observed Structural Heterogeneity of Human Hepatitis B Virus Surface Antigen Particles by Cryo-electron Microscopy

Authors

• JR Gallagher
• AK Harris

Abstract

Design of vaccine nanoparticles is an area of research that is gaining interest, due to the fact that conserved epitopes can be engineered to be displayed on their surface. Hepatitis B virus (HBV) surface antigen particles (sAg) isolated from sera were used as the first HBV vaccine, and may be developed into a platform to display designed antigens. Particles of sAg had been previously reported also to have octahedral symmetry by cryo-EM analysis, yet the molecular detail of the complex remained to be solved. We used single-particle cryo-EM to characterize sAg particles of the Ad subtype isolated from human serum. Model-free Analysis of 125,405 particles using RELION did not produce classes with apparent 2-fold, 3-fold, and 4-fold views that would be expected for particles with octahedral symmetry. However, the particles did appear to have two density layers, with the outer layer having protruding surface spikes, and an apparent low-density or empty center. Particle diameters were in the range of 20 to 25 nm. We selected the subset of particles that were 22 nm in diameter for asymmetric reconstruction and found that the resolution was limited to about 20 Å, even as micrograph Thon rings were visible to 8 Å. Our results suggest that human sAg particles might not have strict symmetry to which the constituent sAg molecules are organized, yet this may be developed into a strength for vaccine design in that particles of varied diameters and antigen spacings are generated from a single nanoparticle platform.

Scientific Focus Area: Structural Biology

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