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.