LOCALIZATION OF THE C-TERMINUS OF THE ASSEMBLY DOMAIN OF HEPATITIS-B VIRUS CAPSID PROTEIN - IMPLICATIONS FOR MORPHOGENESIS AND ORGANIZATIONOF ENCAPSIDATED RNA

The capsid protein of hepatitis B virus, consisting of an ''assembly'' domain (residues 1-149) and an RNA-binding ''protamine'' domain (resi dues 150-183), assembles from dimers into icosahedral capsids of two d ifferent sizes, The C terminus of the assembly domain (residues 140-14 9) functions as a morphogenetic switch, longer C termini favoring a hi gher proportion of the larger capsids, it also connects the protamine domain to the capsid shell, We now have defined the location of this p eptide in capsids assembled in vitro by engineering a mutant assembly domain with a single cysteine at its C terminus (residue 150), labelin g it with a gold cluster and visualizing the cluster by cryo-electron microscopy, The labeled protein is unimpaired in its ability to form c apsids, Our density map reveals a single undecagold cluster under each fivefold and quasi-sixfold vertex, connected to sites at either end o f the undersides of the dimers, Considering the geometry of the vertic es, the C termini must be more crowded at the fivefolds. Thus, a bulky C terminus would be expected to favor formation of the larger (T = 4) capsids, which have a greater proportion of quasi-sixfolds. Capsids a ssembled by expressing the full-length protein in Escherichia coil pac kage bacterial RNAs in amounts equivalent to the viral pregenome, Our density map of these capsids reveals a distinct inner shell of density -the RNA, The RNA is connected to the protein shell via the C-terminal linkers and also makes contact around the dimer axes.