Intracellular hepadnavirus nucleocapsids are selected for secretion by envelope protein-independent membrane binding

Hepadnaviruses are DNA viruses but, as pararetroviruses, their morphogenesi s initiates with the encapsidation of an RNA pregenome, and these viruses h ave therefore evolved mechanisms to exclude nucleocapsids that contain inco mpletely matured genomes from participating in budding and secretion. We pr ovide here evidence that binding of hepadnavirus core particles from the cy tosol to their target membranes is a distinct step in morphogenesis, discri minating among different populations of intracellular capsids. Using the du ck hepatitis B virus (DHBV) and a flotation assay, we found about half of t he intracellular capsids to be membrane associated due to an intrinsic memb rane-binding affinity. In contrast to free cytosolic capsids, this subpopul ation contained largely mature, double-stranded DNA genomes and lacked core protein hyperphosphorylation, both features characteristic for secreted vi rions. Against expectation, however, the selective membrane attachment obse rved did not require the presence of the large DHBV envelope protein, which has been considered to be crucial for nucleocapsid-membrane interaction. F urthermore, removal of surface-exposed phosphate residues from nonfloating capsids by itself did not suffice to confer membrane affinity and, finally, hyperphosphorylation was absent from nonenveloped nucleocapsids that were released from DHBV-transfected cells. Collectively, these observations argu e for a model in which nucleocapsid maturation, involving the viral genome, capsid structure, and capsid dephosphorylation, leads to the exposure of a membrane-binding signal as a step crucial for selecting the matured nucleo capsid to be incorporated into the capsid-independent budding of virus part icles.