Structures of Hepatitis B Virus Subviral Particles and Identification of Potential Druggable Pockets for the Discovery of Antiviral Agents.

Abstract

The small envelope protein of hepatitis B virus (S-HBs) is the principal component of the spherical and filamentous noninfectious subviral particles (SVPs) produced by infected hepatocytes, likely functioning as immune tolerizing agents critical to the establishment and maintenance of chronic infection. The structural features of S-HBs in the context of SVPs have been recently elucidated by several groups using high resolution cryo-EM techniques. In this review, we resolve the commonalities and differences between these reports. The basic unit of SVPs is an S-HBs dimer which is stabilized by hydrophobic interactions between adjacent protomer helices (TH2/TH2 and TH1/EH4, nomenclature by Wang et al., 2024), forming a helical transmembrane core. The adjacent protomer cytosolic and antigenic loops (CYL and AGL, respectively) contribute to dimer stability through salt bridges and intermolecular disulfide bonds; the CYL likely features a zinc finger motif coordinated by C48, H60, C65 and C69, which were shown to be essential. Oligomerization of dimers into trimeric and multiple tetrameric arrangements of dimers during SVP morphogenesis is aided by conformational plasticity within the helical core. Taking previous cell biological studies into account, it is envisioned that in the ER membrane topology and dimerization of S-HBs occur co-translationally, initiating an SVP budding process that completes in the downstream ERGIC and/or Golgi compartments. We report on the identification of two possible non-symmetrical small-molecule binding pockets at the dimer interface, and one in the CYL zing finger region. The druggability of these pockets, as well as unresolved issues in SVP biology, are discussed.