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Journal of Virology, May 2005, p. 5870-5874, Vol. 79, No. 9
0022-538X/05/$08.00+0 doi:10.1128/JVI.79.9.5870-5874.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.
Viral RNA Is Required for the Association of APOBEC3G with Human Immunodeficiency Virus Type 1 Nucleoprotein Complexes
Mohammad A. Khan,1
Sandra Kao,1
Eri Miyagi,1
Hiroaki Takeuchi,1
Ritu Goila-Gaur,1
Sandrine Opi,1
Clay L. Gipson,2
Tristram G. Parslow,2
Hinh Ly,2 and
Klaus Strebel1*
Laboratory of Molecular Microbiology, National Institute of Allergy and Infectious Diseases, NIH, Bethesda, Maryland 20892-0460,1
Emory University, Department of Pathology and Laboratory Medicine, Experimental Pathology Division, Atlanta, Georgia 303222
Received 10 December 2004/
Accepted 21 December 2004

ABSTRACT
APOBEC3G (APO3G) is a host cytidine deaminase that is incorporated
into human immunodeficiency virus type 1 (HIV-1) particles.
We report here that viral RNA promotes stable association of
APO3G with HIV-1 nucleoprotein complexes (NPC). A target sequence
located within the 5'-untranslated region of the HIV-1 RNA was
identified to be necessary and sufficient for efficient APO3G
packaging. Fine mapping revealed a sequence normally involved
in viral genomic RNA dimerization and Gag binding to be important
for APO3G packaging and association with viral NPC. Our data
suggest that packaging of APO3G into HIV-1 NPC is enhanced by
viral RNA.

TEXT
Replication of human immunodeficiency virus type 1 (HIV-1) in
primary cells is dependent on the expression of Vif protein,
which counteracts the activity of the host cytidine deaminases
APOBEC3G (APO3G) and APOBEC3F (
4,
25,
29,
32). In the absence
of Vif, APO3G is incorporated into virus particles (
11,
16,
19,
20,
26,
27,
30), resulting in hypermutation of the viral
genome (
19) or degradation of mutated cDNA (
14,
18,
31) via
a DNA repair mechanism (reviewed in references
3 and
12). Interestingly,
human APO3G is not only packaged into human immunodeficiency
viruses but also incorporated into simian immunodeficiency viruses
and murine leukemia virus (
9,
18,
19). Packaging of APO3G into
such diverse viruses suggests that it either is a relatively
nonspecific process or involves signals shared by these viruses.
APO3G can bind RNA in vitro (
10). Indeed, several reports have
noted that the presence of viral RNA enhanced APO3G encapsidation
(
28); however, others (
17,
23) suggested that viral RNA was
not essential for APO3G packaging (
2,
5,
8,
17,
23,
28).
To further study the role of viral RNA in the packaging of APO3G into HIV-1 virions, we first compared the packaging of APO3G into either the wild-type infectious NL4-3 virus or a helper virus (C-Help) whose RNA genome is not packaged due to a 33-base deletion in the putative RNA packaging signal (21). Virus stocks were prepared by transient cotransfection of HeLa cells with either the pNL4-3 plasmid (1) or the C-Help vector DNA and the APO3G-expressing plasmid pcDNA-APO3G (11). Viruses were collected 48 h after transfection and purified by two rounds of sucrose gradient centrifugation. Cell lysates and concentrated virus preparations were analyzed by immunoblotting (Fig. 1A). We found that packaging of APO3G into helper virus was reduced by >3.5-fold compared to packaging into NL4-3 virus (Fig. 1B). Thus, viral RNA contributes to the specific packaging of APO3G into HIV-1 virions, consistent with data reported by Svarovskaia et al. (28).
If encapsidation of APO3G and viral RNA are linked, the APO3G
packaging defect observed with the helper virus (Fig.
1A and B)
should be overcome by the coexpression of packaging-competent
vector-derived RNA. To test this hypothesis, several packaging
vectors were constructed based on the lentiviral packaging vector
pHR'CMVGFP (
15). This vector contains both HIV-1 long terminal
repeats (LTRs), the 5'-untranslated region, 350 nucleotides
of
gag, and 880 nucleotides of an
env segment encompassing the
Rev-responsive element (RRE) region. Vector

C (Fig.
1C) is similar
to pHR'CMVGFP except for a 650-bp deletion in the cytomegalovirus
promoter region, which prevents expression of the green fluorescent
protein (GFP) reporter gene. This deletion is present in all
other constructs tested here. Vector

L lacks the LTR regions
and is under the transcriptional control of an SV40 promoter.
Vector

R lacks the RRE-containing
env sequence and contains
a codon-optimized
gag coding sequence (
24). A vector containing
a deletion of both LTRs and RRE (

L/R) was also constructed (Fig.
1C). As illustrated in Fig.
1D, coexpression of the packaging
vector (

C) resulted in a sixfold increase in the packaging of
APO3G compared to the helper virus alone. Similarly, cotransfection
of the

L,

R, and

L/R variant constructs increased packaging
of APO3G by six- to sevenfold. These results suggest that the
5'-untranslated region and the first 350 nucleotides of
gag were sufficient for APO3G encapsidation into HIV-1 virions.
The 5'-untranslated region contains numerous secondary structures required for virus replication (Fig. 2A) (reviewed in reference 22). To further map the sequences and RNA secondary structures required for APO3G packaging, we tested two stem-loop mutants, mS.1 and mS.3 (Fig. 2A), located in a region that has been shown to be important for viral RNA dimerization and encapsidation. The sequences located on both strands of the individual helical structures of stem-loop 1 (SL1) or stem-loop 3 (SL3) were swapped in mS.1 and mS.3 mutants, respectively, thus changing the primary sequences of the stems without perturbing formation of the hairpin structures. These mutants were selected because they package viral RNA at or near wild-type levels and were found to produce fully infectious viruses (7). Reverse transcription (RT)-PCR analysis showed similar levels of virus-associated RNA in the mS.1 and mS.3 viruses while C-Help virus did not contain detectable levels of viral RNA (Fig. 2B). Packaging of APO3G into mS.1 and mS.3 particles was analyzed by immunoblotting (Fig. 2C). While APO3G was expressed at similar levels intracellularly (Fig. 2C, lanes 1 and 2), APO3G packaging into mS.1 was reduced by >3-fold compared to mS.3 virus (Fig. 2C, compare lanes 3 and 4). While our data do not rule out the possible involvement of other regions in the 5'-untranslated region, they clearly identify SL1 as a region important for packaging of APO3G into HIV-1 virions and hence strongly argue for a specific involvement of viral RNA in APO3G packaging.
We have previously established that components of the viral
core are resistant to detergent extraction, whereas other viral
components, such as matrix (MA) or envelope protein, are detergent
sensitive and can be separated from core-associated proteins
by sucrose step gradient centrifugation (
13). In this assay,
intact viruses accumulate at the 20%/60% interphase of the step
gradient column (Fig.
3B, S3) as evidenced by the enrichment
of CA and MA proteins in the S3 fraction (Fig.
3A, lane 3).
In contrast, detergent treatment resulted in the partitioning
of MA into the soluble S1 fraction (Fig.
3A, lane 4). As observed
previously (
13), CA remained partially resistant to detergent
treatment. Overall, the detergent sensitivity of these viral
components was very similar for all samples tested. Interestingly,
NL4-3-associated APO3G was largely resistant to detergent extraction
(Fig.
3A). In fact, >70% of the virus-associated APO3G copurified
with the viral core fraction S3 (Fig.
3C). These results suggest
for the first time that APO3G is packaged into virus particles
as a stable complex with the viral core. Similar results were
observed for Vif-defective NL4-3 (Fig.
3D). Thus, the presence
or absence of Vif did not affect the relative affinity of APO3G
to the viral core but merely altered the absolute amounts of
virus-associated APO3G. In contrast, detergent treatment of
C-Help virus extracted about two-thirds of virus-associated
APO3G from the core fraction (Fig.
3E). Similarly, APO3G packaged
into mS.1 particles was highly detergent sensitive (Fig.
3F).
Despite the presence of viral RNA, APO3G association with the
mS.1 core was significantly reduced, suggesting an important
role for the sequences surrounding stem-loop 1 in core association
of APO3G. These biochemical studies therefore support the genetic
data provided in Fig.
2 and strongly argue for a role of viral
RNA in APO3G packaging.
While our data clearly demonstrate that APO3G packaging into
virus particles is enhanced by viral RNA packaging, the presence
of viral RNA per se is not absolutely essential as low levels
of APO3G are packaged even in the absence of viral RNA (Fig.
1). These results are consistent with previous studies demonstrating
that APO3G can be packaged into viruslike particles in the absence
of viral RNA through an interaction with the viral NC protein
(
5,
8,
17,
23,
28). Although APO3G has RNA binding properties,
it is conceivable that APO3G recognizes a unique RNA-protein
complex that forms during viral assembly as part of the RNA
dimerization/encapsidation process that has been shown to involve
the SL1 hairpin as well as other hairpin structures and sequences
(
7). Such a mechanism could explain the efficient packaging
of APO3G into diverse viruses, such as HIV-2, simian immunodeficiency
virus, or murine leukemia virus and the sensitivity of APO3G
packaging to mutations in NC protein, which has been shown to
participate in both dimerization and encapsidation of viral
RNA (
22).

ACKNOWLEDGMENTS
We thank George Pavlakis and Antonio Valentin for the codon-optimized
Gag expression vector p55BM1-10PO-2SD+. We thank Xiao-Fang Yu
and Bryan Cullen for sharing unpublished data. We gratefully
acknowledge Alicia Buckler-White and Ron Plishka for oligonucleotide
synthesis and sequence analysis.
Part of this work was supported by a grant from the NIH Intramural AIDS Targeted Antiviral Program to K.S.

FOOTNOTES
* Corresponding author. Mailing address: NIH, NIAID, 4/312, 4 Center Drive, MSC 0460, Bethesda, MD 20892-0460. Phone: (301) 496-3132. Fax: (301) 402-0226. E-mail:
kstrebel{at}nih.gov.


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Journal of Virology, May 2005, p. 5870-5874, Vol. 79, No. 9
0022-538X/05/$08.00+0 doi:10.1128/JVI.79.9.5870-5874.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.
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