Hepatitis B is a small enveloped DNA virus that poses a major hazard to human health. The crystal structure of the T = 4 capsid has been solved at 3.3 A resolution, revealing a largely helical protein fold that is unusual for icosahedral viruses. The monomer fold is stabilized by a hydrophobic core that is highly conserved among human viral variants. Association of two amphipathic alpha-helical hairpins results in formation of a dimer with a four-helix bundle as the major central feature. The capsid is assembled from dimers via interactions involving a highly conserved region near the C terminus of the truncated protein used for crystallization. The major immunodominant region lies at the tips of the alpha-helical hairpins that form spikes on the capsid surface.

Hepatitis B virus, a major human pathogen with an estimated 300 million carriers worldwide, can lead to cirrhosis and liver cancer in cases of chronic infection. The virus consists of an inner nucleocapsid or core, surrounded by a lipid envelope containing virally encoded surface proteins. The core protein, when expressed in bacteria, assembles into core shell particles, closely resembling the native core of the virus. Here we use electron cryomicroscopy to solve the structure of the core protein to 7.4 A resolution. Images of about 6,400 individual particles from 34 micrographs at different levels of defocus were combined, imposing icosahedral symmetry. The three-dimensional map reveals the complete fold of the polypeptide chain, which is quite unlike previously solved viral capsid proteins and is largely alpha-helical. The dimer clustering of subunits produces spikes on the surface of the shell, which consist of radial bundles of four long alpha-helices. Our model implies that the sequence corresponding to the immunodominant region of the core protein lies at the tip of the spike and also explains other properties of the core protein.

Hepatitis B virus, a widespread and serious human pathogen, replicates by reverse transcription of an RNA intermediate. The virus consists of an inner nucleocapsid or core, surrounded by a lipid envelope containing virally encoded surface proteins. Using electron cryomicroscopy, we compare the structures of the bacterially expressed RNA-containing core particle and the mature DNA-containing core particle extracted from virions. We show that the mature core contains 240 subunits in a T ‫؍‬ 4 arrangement similar to that in expressed core (T is the triangulation number and the icosahedral shell contains 60 T subunits). During the infective cycle, the core assembles in an immature state around a complex of viral pregenomic RNA and polymerase. After reverse transcription with concomitant degradation of the RNA, the now mature core buds through a cellular membrane containing the surface proteins to become enveloped. Envelopment must not happen before reverse transcription is completed, so it has been hypothesized that a change in capsid structure may signal maturation. Our results show significant differences in structure between the RNAand DNA-containing cores. One such difference is in a hydrophobic pocket, formed largely from residues that, on mutation, lead to abnormal secretion. We suggest that the changes we see are related to maturation and control of envelopment, and we propose a mechanism based on DNA synthesis for their triggering.cryomicroscopy ͉ envelopment ͉ nucleocapsid ͉ viral maturation H epatitis B virus (HBV) is a major human pathogen and member of the Hepadnaviridae, a family of viruses that replicate by reverse transcription in the hepatoctyes of their hosts (1). During virus assembly, the core protein polymerizes around a complex consisting of pregenomic mRNA and viral polymerase to form an immature core. The RNA is reverse transcribed into single-stranded DNA and concomitantly degraded, then the partial second DNA strand is synthesized. Cores are then mature and can interact with the cytoplasmic parts of the surface proteins, already inserted into an inner cellular membrane, through which the core buds to form a virion (see Bruss, ref. 2).