MAPPING OF HOMOLOGOUS INTERACTION SITES IN THE HEPATITIS-B VIRUS COREPROTEIN

Hepatitis B virus consists of an outer envelope and an inner capsid, o r core, that wraps around the small genome plus the viral replication enzyme. The icosahedrally symmetric nucleocapsid is assembled from mul tiple dimeric subunits of a single 183-residue capsid protein, which m ust therefore contain interfaces for monomer dimerization and for dime r multimerization, The atomic structure of the protein is not known, b ut electron microscopy-based image reconstructions suggested a hammerh ead shape for the dimer and, very recently, led to a tentative model f or the main chain trace. Here we used a combination of interaction scr eening techniques and functional analyses of core protein variants to define, at the primary sequence level, the regions that mediate capsid assembly. Both the two-hybrid system and the pepscan technique identi fied a strongly interacting region I between amino acids (aa) 78 and 1 17 that probably forms part of the dimer interface. Surprisingly, muta tions in this region, in the contest of a C-terminally truncated but a ssembly-competent core protein variant, had no detectable effect on as sembly. By contrast, mutations in a second region, bordered by aa 113 and 143, markedly influenced capsid stability, strongly suggesting tha t this region II is the main contributor to dimer multimerization, Bas ed on the electron microscopic data, it must therefore be located at t he basal tips of the dimer, experimentally supporting the proposed mai n chain trace.