HBcAg has been expressed in E.coli were it assembles in the bacterial cytoplasm into icosahedral capsids, which contain bound host nucleic acid. Deletion of the polybasic C-terminal 34 residues (protamine domain) also produces assembly competent protein. The capsids from C-terminal truncated protein (Cp149) do not contain nucleic acid and their structure has been previously determined by cryo-electron microscopy and image analysis and by X-ray crystallography. Native HBeAg is also C-terminally truncated at position 149 and in addition contains a 10 residue N-terminal extension derived from partial processing of precursor protein (pre-C). Although detailed knowledge of the function and structure of HBeAg are unknown it has clinical importance as a serological marker. Using surface plasmon resonance (Biacore) to measure antibody antigen interactions, a kinetic-affinity map of a panel of monoclonal antibodies (mAbs) against HBV nucleocapsid proteins was determined. Monoclonal antibodies (murine origin) binding to the assembled (HBcAg) and non-assembled forms of the capsids (HBeAg) were identified and new combinations useful for clinical diagnosis described. Previous structural determinations of nucleocapsid-antibody immune complexes by cryo-electron microscopy helped to more clearly explain the immunological distinction between the assembled HBcAg and unassembled HBeAg antigen. This work has been extended to included human antibodies and the preliminary results indicate binding to distinct regions on the capsid surface (epitopes) which we had previously identified using the model murine antibodies. This work, together our previous description of an immune complex of capsids and an antibody representative of the surface immunoglobulin from naive B-cells, provide one of the most complete structural pictures of the interaction of antibodies with a viral protein system related to an important human disease. Using a rabbit antibody library, monoclonal antibodies (mAbs) were selected then humanized to produce chimeric mAb fragment antigen binding portions (Fab). Fabs against the HBV capsid proteins were selected for high affinity binding to the capsid subunits. The binding of the antibodies prevent the assembly of the capsids (a central functional and structural component of the HBV virus) and may with development provide useful anti-HBV reagents. Direct determination of the biophysical properties of the HBV capsid protein is limited due to the size of the multiprotein megadalton complex. In collaboration with Albert Heck (Utrecht University) macromolecular tandem and ion mobility mass spectrometry was used to study the stability and conformational diversity of HBV capsids. Very precise mass measurements were made and the exact molecular stoichiometries of HBV nucleocapsid complexes determined. In an extension of this work, the stability of the capsids was probed by the direct determination of the slow exchange of constituent subunits using isotopically labeled protein. The methods and approaches used have general applicability to the study of large molecular assemblies used in nano- and biotechnology.
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