Hepadnaviruses are DNA infections but as pararetroviruses their morphogenesis initiates with

Hepadnaviruses are DNA infections but as pararetroviruses their morphogenesis initiates with the encapsidation of an RNA pregenome and these viruses have therefore evolved mechanisms to exclude nucleocapsids that contain incompletely matured genomes from participating in budding and secretion. capsids. Using the duck hepatitis B computer virus (DHBV) and a flotation assay we found about half of the intracellular capsids to be membrane associated BMS-794833 due to an intrinsic membrane-binding affinity. In contrast to free cytosolic capsids this subpopulation contained largely mature double-stranded DNA genomes and lacked core protein hyperphosphorylation both features characteristic for secreted virions. Against expectation however the selective membrane attachment observed did not require the presence of the large DHBV envelope protein which BMS-794833 has been considered to be crucial for nucleocapsid-membrane conversation. Furthermore 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 argue for any model in which nucleocapsid maturation involving the viral genome capsid structure and capsid dephosphorylation prospects to the exposure of a membrane-binding transmission as BMS-794833 a step crucial for selecting the matured nucleocapsid to be incorporated into the capsid-independent budding of computer virus particles. Enveloped viruses acquire their outer coat by budding at cellular membranes a step generally thought to depend around the interaction between the viral envelope proteins and internal viral matrix and nucleocapsid components (7). However some viruses such as retroviruses and rhabdoviruses are able to release membrane-coated particles also in the absence of viral envelope KRT20 proteins (5 9 23 Moreover other viruses including coronavirus herpes simplex virus type 1 and in particular the hepadnaviruses release empty envelope particles devoid of nucleocapsids in addition to infectious computer virus (28 30 Hepatitis B viruses (HBVs; hepadnaviruses) are small enveloped viruses and a causative agent of acute and chronic viral hepatitis (6). Their nucleocapsid or core particle which is composed of a single core protein species contains a largely double-stranded DNA genome and the covalently attached viral polymerase and is surrounded by a membrane shell with two or three viral envelope proteins embedded. In addition to these infectious computer virus particles hepadnavirus-infected cells secrete in abundant extra nucleocapsid-free enveloped particles suggesting that hepadnavirus budding may be an envelope protein-driven process. On the other hand it has been shown that budding and secretion of total computer virus particles require the presence of the large viral envelope protein (L-protein) (2 27 This has led to the assumption that nucleocapsids enter the export pathway by attaching to cytosolically uncovered preS ectodomains of membrane-anchored L chains at the ERGIC (endoplasmic reticulum-to-Golgi intermediate compartment) into which they bud (11 19 Hepadnaviruses replicate their genome via reverse transcription of an RNA intermediate a process occurring already in the producer cell and thereby differing from your related retroviral life cycle (17 25 Intracellular core particles thus contain the viral genome at numerous stages of maturation while secreted computer virus has been found to contain only the mature replication end product a largely double-stranded DNA molecule. These observations have been taken to show that completion of genome replication is usually a prerequisite for capsid envelopment and they predict that core particles containing a mature viral DNA genome display signals for selective budding and export (25). Support for this prediction comes from more recent experiments demonstrating a block to computer virus production for capsids unable to total DNA synthesis due to mutational inactivation of the viral polymerase (8 31 While this model has been generally accepted the nature of the predicted maturation transmission and its cellular or viral conversation partner(s) have remained unknown as has the mechanism resulting in selective export of mature capsids. However it has been extrapolated that genome maturation could lead to the exposure of L-protein binding sites around the particle surface involving changes in the overall nucleocapsid structure (25 31 Alternatively or additionally more-subtle changes have been considered to transmission capsid maturation such as a switch in core protein hyperphosphorylation (characterized by the.