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.
A transmission electron micrograph is essentially a projection of the specimen in the direction of view. In order to reconstruct a three-dimensional image of the specimen, it is necessary to be able to combine data from a number of different views. A formal solution of this problem is given in terms of Fourier transforms. Its realization requires data reduction and interpolation. The final solution is given by a least squares approach, which also indicates how many views must be included to give a valid reconstruction of a given particle to a given degree of resolution. Interpolation procedures of varying power are given, to be employed according to the economy with which the available data must be used. An alternative procedure is described for direct reconstruction without the use of Fourier transforms, but it is shown to be in general less practicable than the Fourier approach.
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