Asymmetrizing an icosahedral virus capsid by hierarchical assembly of subunits with designed asymmetry

Abstract

Symmetrical protein complexes are ubiquitous in natural biological systems. Many have been reengineered in vitro for chemical and medical applications. Symmetrical viral capsids and their assembly are frequent platforms for these investigations. Lacking a means to create asymmetric capsids may limit broader applications. Here, starting with the homodimeric Hepatitis B Virus capsid protein, we developed a heterodimer, designed a hierarchical assembly pathway, and produced asymmetric capsids. We showed that the heterodimers assemble into hexamers, and such preformed hexamers can nucleate co-assembly, leading to “Janus” capsids with two discrete patches. We removed the hexamer patches specifically and observed asymmetric holey capsids by cryo-EM reconstruction. The resulting holes can be refilled with new engineered dimers. This programmed assembly pathway provides windows for specific engineering and modification inside and outside of the capsid. This strategy can also be generalized to other capsid assembly systems.