Hepadnaviridae. Hepadnaviridae

Description of Hepadnaviridae adapted from ICTVdb Virion Properties Morphology Virions consist of an envelope and a nucleocapsid. Virus capsid is enveloped by a detergent sensitive lipoprotein (7 nm). Virions are spherical to pleomorphic (at times, filamentous forms occur and spherical structures that lack cores are common (HBsAg particles). Virions measure 40-48 nm in diameter, or 22 nm in diameter (for the HBsAg particles). The envelope has no surface projections. Capsid/nucleocapsid is round and exhibits icosahedral symmetry. The nucleocapsid is isometric and has a diameter of 30-35 nm. The capsid surface structure frequently reveals a regular pattern with distinctive features (concentric rings). The capsid consists of 180 capsomers. Physicochemical and Physical Properties Virions have a buoyant density in CsCl of 1.14-1.26 g cm-3. Nucleic Acid The genome is not segmented and contains a single molecule of circular. The genome is -RT. The genome is partially double-stranded DNA that forms a covalently closed circle (with 5' end of the full length minus strand which is linked to the viral DNA polymerase). The complete genome is 3020-3320 nucleotides long, or 1700-2800 nucleotides long (for the full and short length strand, respectively). The genome has a guanine + cytosine content of 48 %. The genome sequence has termini with cohesive ends (that match the uniquely located 5'-ends of the two strands which overlap by approximately 240 nucleotides and maintain the circular configuration of the DNA). The sequence has ENH an enhancer region and a direct repeat sequence (DR1 andDR2), or a U5-like sequence, a polyadenylated signal, and a putative glucocorticoid-responsive element; negative-sense or non-coding strand (complementary to the viral mRNA) is full-length (3.0-3.3 kb, positive sense strand (the viral mRNA) is variable in size and shorter than full-length. The double stranded genome has a nick at a unique site on full length negative strand opposite at a position 50 nucleotides, or 242 nucleotides downstream from the 5' end of the positive sense strand. The 5'-end of the negative-sense strand has a covalently attached terminal protein; positive-sense strand has a 5' capped oligoribonucleotide primer. Proteins The viral genome encodes structural proteins and non-structural proteins located in the envelope. Non-Structural Proteins: The virus codes for an RNA-dependent DNA polymerase. Lipids Lipids are present in significant amounts and are located in the envelope and empty particles (HBsAG components). The composition of viral lipids is known. The composition of viral lipids and host cell membranes are similar. The lipids are derived from a host membrane compartment intermediate between endoplasmatic reticulum membranes and Golgi membranes. Viral membranes include phospholipids, cholesterol, cholesterol esters, and triglycerides. Genome Organization and Replication Translation: Replication involves a reverse transcription step.