The Genetics and Antigenic Structure of Viral Capsid Proteins: Hepatitis B Virus Core Antigen and Coat Protein of RNA Phage fr

The cloning and expression in E. coli of genes for capsid proteins such as the core antigen (HBcAg) of hepatitis B virus and the coat protein (CP) of the RNA phage fr allowed us to approach the problem of self-organization of these multifunctional proteins from the standpoint of protein engineering by using mutations introduced into specific gene regions. The two cloned genes were sequenced, the functional sense of certain gene regions was predicted by theoretical and comparative evolutionary methods, and the hypothetical functional maps of proteins encoded by the genes were constructed. Their correctness of these maps was proved by insertion of oligonucleotides of various lengths into different gene sites. Both these genes present an ideal model for our purposes, since their products are synthesized in E. coli in large amounts and self-assembled correctly without any other viral components being present. HBcAg and fr CP form particles similar in size and electron microscopic parameters despite the dramatic differences in their primary and secondary structure. HBcAg retained its ability for self-assembly even after large insertions into the C-terminal part of the molecule. The fr capsids were less prone to such insertions, irrespective of the insertion site: in the N-terminus, in the internal gene regions corresponding to antigenic determinants, or in the C-terminus. The processing of recombinant proteins was found to lead to fragments with altered immunological activity (eAg in the case of HBcAg, frmono in the case of fr CP) and incapable of self-assembly. The prospects of protein engineering from viral capsid proteins aimed at constructing new antigens will be considered.