B. Background on HDV Antigens:HDV encodes two
proteins of its own: a large and small hepatitis delta antigen (HdAg-S and
HdAg-L) on a single coding sequence. The small antigen is expressed
earlier in infection and mediates replication. The HdAg-L is expressed
later in infection, and is involved in mediating assembly by interacting
with Hepatitis B antigen and in inhibiting viral replication. HdAg-L can
also form "empty" virus like particles (VLPs) without HDV RNA or HBAg.
HDAg-L is expressed when cellular processes (cellular enzyme ADAR1) mutate
the UAG stop codon on HDAg-S to a UGG tryptophan codon, adding a 19 amino
acid carboxyl terminal extension which includes a CXXQ farnesylation
segment which is involved in assembly and inhibition. While the functions
of this added segment in the HDAg-L are known, the mechanisms by which
they work have not been fully discovered.
O'Malley, B. and Lazinski, D. 2005. Roles of Carboxyl-Terminal and
Farnesylated Residues in the Functions of the Large Hepatitis Delta
Antigen. J. Virology. 79(2):1142-1153.Pubmed
The nature of the structure of the HdAg-L contributes to its main
functions. Genotypes of HdAg are typically classified into three
categories, 1, 2, and 3. The three genotypes have shown differences in
disease phenotype as well. By examining the efficiency of different
genotypes of HdAg-L of forming virus like particles (VLPs), O<92>Malley and
Lazinski examined how the amino acid sequence particularly of the unique
portion of the HDAg-L contributes to its particular functions, assembly
and inhibition of replication. From this they drew several conclusions:
- certain specific positions in the HDAg-L carboxyl terminus
interact with the HBAg
- the presence of HDAg-S can stimulate farnesylation of HDAg-L (and
in nature, HDAg-L is never expressed without HDAg-S), perhaps by
stabilizing HDAg-L in its optimal conformation
- inhibition of HDV replication derives from association with
membranes
- myristoylation of the amino terminus (amino acids 1 through 195
that is part of the HDAg-S as well) will allow association with membranes
(and thus inhibition of replication) and will allow HDAg-S to interact
with HBAg and form VLPs
- however, farnesylation at the carboxyl terminus allows HDAg-L to
perform these functions (membrane association and thus inhibition and
association with HBAg to form VLPs) far more efficiently (roughly
ten-fold more efficient)
- association with HBAg is brought about both by association with
membranes and by induction of conformational change in the region that
interacts with HBAg
- There is no direct association between efficiency of assembly and
disease virulence (as previously thought)
Wang YH, Chang SC, Huang C, Li YP, Lee CH, Chang MF. 2005. Novel nuclear
export signal-interacting protein, NESI, critical for the assembly of
hepatitis delta virus. J of Virology. 79(13):8113-20.
Previous studies by Wang et al demonstrated that a Nuclear Export
Signal (NES) on HDAg-L is involved in HDV assembly. Wang et al further
explored the potential proteins interacting with the NES(HDAg-L) site. A
product of an mRNA found mainly in liver tissue was discovered to
interact
with the site, and was named NESI (NES(HDAg-L)-interacting protein).
NESI
is 469 amino acids long and includes an actin binding site and a nuclear
localization signal. It interacts with HDAg-L and is involved in HDV
assembly.