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Journal of Virology, November 2002, p. 11133-11138, Vol. 76, No. 21
0022-538X/02/$04.00+0 DOI: 10.1128/JVI.76.21.11133-11138.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.
Implication of the Lymphocyte-Specific Nuclear Body Protein Sp140 in an Innate Response to Human Immunodeficiency Virus Type 1
Navid Madani,1,
Robert Millette,1 Emily J. Platt,1 Mariana Marin,1 Susan L. Kozak,1 Donald B. Bloch,2 and David Kabat1*
Department of Biochemistry and Molecular Biology, Oregon Health and Science University, Portland, Oregon 97201-3098,1
Department of Medicine, Massachusetts General Hospital, Charlestown, Massachusetts 021292
Received 5 April 2002/
Accepted 18 July 2002

ABSTRACT
The viral infectivity factor (Vif) of human immunodeficiency
virus type 1 (HIV-1) neutralizes an unidentified antiviral pathway
that occurs only in nonpermissive (NP) cells. Using a yeast
two-hybrid screen of a human lymphocyte cDNA library, we identified
several potential Vif partners. One, the nuclear body protein
Sp140, was found specifically in all NP cells (
n = 12 cell lines
tested;
P 
0.001), and HIV-1 infection induced its partial dispersal
from nuclear bodies into cytosolic colocalization with Vif.
Our results implicate Sp140 in a response to HIV-1 that may
be related to or coordinated with the pathway that inactivates
HIV-1 lacking
vif.

TEXT
The Vif protein encoded by human immunodeficiency virus type
1 (HIV-1) is a 23,000-
Mr phosphoprotein that has been detected
in the cytosol and nucleus in association with membranes, intermediate
filaments, the viral Gag polyprotein, and the viral genomic
RNA (
8,
10,
17,
23,
25,
27,
43,
55). It enhances the infectivity
but not the quantity of HIV-1 virions released from nonpermissive
(NP) cells, including T lymphocytes and macrophages and some
leukemic T-cell lines, by 20- to 100-fold, but it is unnecessary
in permissive (P) cells (
7,
13,
22,
39,
44,
53). Although the
HIV-1 lacking
vif that is made in P cells efficiently infects
NP cells, the NP cells release noninfectious HIV-1 virions that
appear to have a normal protein and RNA composition but that
are irreversibly altered in a manner that inhibits reverse transcription
in target cells (
9,
13,
17,
24,
36,
44,
53).
Two lines of evidence suggest that for Vif to function, the protein must interact with an unknown cellular factor that occurs in NP cells. Recently, we and others found that the NP phenotype is dominant in P x NP heterokaryons (31, 42). This implies that NP cells contain a pathway for inactivating HIV-1, that this pathway is absent or incomplete in P cells, and that Vif neutralizes this pathway. Additionally, it was reported that Vif proteins of primate lentiviruses function only in the lymphocytes of species closely related to the viral hosts (41). This finding implies that for Vif to function, rather than associating with a viral factor, the protein must bind to a cellular factor that changes during evolution (41). Consequently, we hypothesized that P cells lack this Vif-binding target factor and/or another component or components of the pathway that inactivates HIV-1 lacking vif.
We used the L40 yeast system (26) with full-length HXB2 strain Vif as bait in a two-hybrid screen of a human leukocyte Matchmaker cDNA library (Clontech, Palo Alto, Calif.). This system provides high-level expression of LexA-B domain (BD)-bait and Ga14-A domain (AD)-prey proteins, thereby enhancing weak bait-prey associations. Although several prey were of potential interest, we focused principally on Sp140 because it is lymphocyte and macrophage specific and is induced by gamma interferon (3, 4, 15). Additionally, Sp140 is closely related to the widely expressed protein Sp100, which has been implicated in defenses against other viruses and associates with the promyelocytic leukemia protein (PML) in nuclear-matrix-associated nuclear bodies (NBs), also called PML oncogenic domains or nuclear domains 10 (6, 11, 12, 16, 18, 20, 34, 48, 49, 54, 57). Ku70, which is a component of HIV-1 preintegration complexes (14, 29), was also selected in our screen. These Vif-prey interactions were specific, as indicated by the activation of multiple reporter genes that contain the LexA promoter and by negative results obtained with an unrelated bait derived from the Sin Nombre hantavirus nucleocapsid and with unrelated prey.
We analyzed small deletion mutations distributed throughout Vif that were previously shown to reduce its activity by greater than 93% (45). Mutations in the amino-terminal region (resulting in mutants
12-23,
43-59, and
73-87) and especially in the central region (resulting in mutant
97-112) strongly inhibited the protein's two-hybrid association with Sp140, whereas mutations further toward the carboxyl terminus (resulting in mutants
111-128,
140-148, and
147-158) had negligible effects (Fig. 1). The Vif domain involved in the interaction with Sp140 does not include the conserved SLYQXLA sequence that has been implicated in RNA binding (17). Despite considerable differences in sequence, Vif proteins encoded by the IIIB and HXB2 strains of HIV-1 were similarly active in associating with Sp140. The Ga14-AD-Sp140 prey cDNAs that we isolated encoded the common carboxyl-terminal region between amino acids 527 and 836, which begins with the sequence DGQVV and includes a SAND domain, a PHD-type zinc finger, and a bromodomain (15). Preliminary evidence suggested that the SAND and PHD domains interact and that one or both of these regions associate with Vif (data not shown).
Northern blot analyses of RNAs from P and NP leukemic cell lines
and from stimulated and unstimulated peripheral blood lymphocytes
(PBLs) were done as previously described (
50). As shown in Fig.
2, there was a precise correspondence between the NP phenotype
(
7,
21,
22,
38,
39,
44,
46,
53) and the presence of an approximately
3.0-kb component of Sp140 mRNA. Equal amounts of RNA were loaded
in the lanes, and identical results were obtained with independent
preparations of the RNAs. Thus, in an initial study (left panel),
we found this Sp140 mRNA in PBLs, especially after the PBLs
were stimulated with interleukin-2 plus phytohemagglutinin-P,
and in H9, HUT78, and MT-2 cells but not in Jurkat, JKE6.1,
Molt4/8, U87MB, SupT1, U937, CEM-SS, or A3.01 cells. Sp140 mRNA
is also expressed when HL60 and NB4 myeloid cells are induced
to differentiate, and it appears to be absent in nonhematopoietic
cells (
3,
4,
15). Subsequently, we confirmed the correspondence
by testing additional leukemic cell lines for their NP or P
phenotype and for the presence of Sp140 mRNA (Fig.
1, right
panel). Although C8166 cells initially appeared to be an exception
since they were reported to be semipermissive or P (
13,
47),
the C8166-45 clone that we received from a repository contains
Sp140 and is NP by our assay system (
30). If we consider only
the independently derived cell lines as described in the legend
to Fig.
2, these results demonstrate a correlation between the
presence of Sp140 mRNA and the NP phenotype that is extremely
unlikely to have occurred by chance (
n = 12 cell lines;
P 
0.001).
Although it could be postulated that Sp140 expression and the
NP phenotype are not functionally coupled but are only coincidentally
restricted to a subset of leukemic cells that are arrested at
a specific stage of differentiation, this idea is difficult
to reconcile with the fact that these NP cell lines are heteroploid
and derive from different hematopoietic lineages. Sp140 occurs
in T and B lymphocytes and in NB4 and HL60 myeloid cells (
3,
15). Nevertheless, it is possible that the NP phenotype develops
in the late stages of differentiation in several hematopoietic
lineages and that Sp140 is specifically expressed at that same
stage. In any case, Sp140 occurs in all natural cellular targets
of HIV-1 in close association with the antiviral NP phenotype.
Previous studies have shown that Sp140 occurs almost exclusively in NBs (3, 4, 15) but that Vif is mainly cytosolic (23, 25, 27, 43). We found, however, that HIV-1 induces significant relocalization of Sp140 between 16 and 20 h postinfection. Because the endogenous Sp140 in NP cell lines occurs in small amounts that were difficult to detect, and because recombinant Sp140 damages P cell lines and therefore has not been stably expressed (see below), we transiently transduced HeLa-CD4 cells with a previously described adenoviral-Sp140 (Ad-Sp140) vector (3-5). As expected, Sp140 localized in clusters that were randomly situated within the nuclei (Fig. 3, top panels). However, by 24 h following exposure to HIV-1, at a time when Vif became evident in approximately 3 to 5% of the cells, several changes occurred in the Sp140 distribution. First, in the cells that expressed Vif, a portion of Sp140 was dispersed throughout the cytosol (Fig. 3). Indeed, in these cells, Vif appeared to colocalize with this dispersed form of Sp140. In addition, in these and other HIV-1-infected cells, the Sp140-containing nuclear clusters often appeared to have partially or completely migrated toward the nuclear peripheries, where they eventually formed a layer underlying the nuclear membranes (Fig. 3, lower panels). Although Vif does not induce these changes in the nuclear clusters, our results suggest that it may enhance the cytosolic retention of Sp140.
We used the same cell culture system to determine whether Vif-Sp140
complexes could be immunoprecipitated from cytosolic extracts.
In this case, HeLa-CD4 cells that had been transduced with either
Ad-Sp140 or a negative control Ad-green fluorescent protein
(GFP) vector (
3-
5) were infected for 24 h with wild-type HIV-1
prior to lysis with a buffer containing 1.0% Triton X-100 at
0°C. After the removal of nuclei and debris by centrifugation,
the extracts were incubated with preimmune rabbit serum and
protein A-Sepharose beads (Sigma, St. Louis, Mo.) prior to recentrifugation.
Aliquots of the supernatants were incubated either with more
preimmune serum or with antiserum to Vif prior to the addition
of more protein A-Sepharose beads and centrifugation. Proteins
extracted from these beads were used for Western immunoblotting
as described previously (
33). Consistent with our immunofluorescence
microscopy results, small amounts of Vif were immunoprecipitated
by our Vif antiserum but not by preimmune serum, and the Vif
immunoprecipitates specifically contained a small quantity of
Sp140 (Fig.
4). Although the quantities of these proteins were
small compared to the background levels, this was expected because
only 3 to 5% of the cells expressed a detectable amount of Vif
and because only approximately 10% of the Sp140 in these few
cells was dispersed into the cytosol (Fig.
3). Control experiments
using cells lacking HIV-1 established that these extraction
procedures solubilized approximately 5 to 10% of the total cellular
Sp140 and that our Vif antiserum did not directly immunoprecipitate
Sp140 (results not shown). Based principally on the congruence
of these results with our immunofluorescence microscopy data,
we propose that Vif can at least weakly interact with a form
of Sp140 that disperses into the cytosol following HIV-1 infections.
Diverse viruses synthesize proteins that localize in NBs or
induce dispersal of NB proteins (
6,
11,
16,
18,
20,
35,
49,
54), consistent with evidence that PML and associated proteins,
including Sp100 and Mx1, participate in antiviral defenses that
the viruses must counteract (
12,
19,
52). Moreover, many NB
proteins, including PML and Sp100, are conjugated with the small
ubiquitin-related modifier protein SUMO-1 (
19,
34,
35,
48).
Sumoyl transferase occurs in NBs and contributes to NB integrity
(
19,
20,
34,
35,
48,
56), and several virus-encoded proteins
cause desumoylation and/or degradation of Sp100 and PML (
1,
11,
20,
34,
37). As shown by the SUMO-1-specific Western immunoblot
in Fig.
5, expression of Sp140 in HeLa cells caused several
changes in the pattern of sumoylation and the inductive effects
were all counteracted by coexpression of Vif. The major

80,000-
Mr sumoylated protein in mammalian cells is RanGAP1 (
2,
32), which
is absent from NBs and was unaffected by Sp140 or Vif.
It has not been possible to stably express Sp140 in P cell lines.
Indeed, we were unable to stably transfect HeLa-tTA cells with
a tetracycline-repressible Sp140 expression vector (
30) when
the cells were maintained throughout selection in a high concentration
of tetracycline, implying that even trace levels of Sp140 are
damaging to cells. This made it very difficult to determine
whether its expression in P cells would convert them to NP cells.
Efficient transient expression of Sp140 in a large proportion
of cells was achieved in HeLa-CD4 cells by using our Ad-Sp140
vector and in HEK293T cells by transfection with a pcDNA3.1-Sp140
vector but was not achieved by these same vectors in leukemic
T-cell lines. Under these conditions, we did not detect conversion
of HeLa-CD4 or HEK293T cells to NP cells (results not shown).
However, HeLa cells are unrelated to T lymphocytes and may lack
multiple components of the NP cell-specific antiviral pathway.
These results implicate Sp140 in an innate response to HIV-1 that occurs specifically in the natural cellular targets. These cells are termed NP because they contain a potent antiviral pathway that is neutralized by Vif (31, 42). Our results are compatible with evidence that NB proteins may be involved in defenses against diverse viruses (6, 11, 16, 18, 20, 34, 49, 54) and confirm the hypothesis that NP cells contain systems for response to HIV-1 that are absent in P cells (31, 42). However, we also emphasize that our results do not establish a direct role for Sp140 in the pathway that inactivates HIV-1 lacking vif. The interaction of Sp140 with Vif appears to be weak, and it is uncertain whether it is a true Vif target. Conceivably, Vif might bind to a motif that occurs in several proteins, and like Nef or Vpu (51), it might influence multiple cellular pathways.
During final revision of this paper, Sheehy et al. (40) reported that the protein CEM-15 occurs only in NP cells and confers the NP phenotype on P cell lines, including 293T cells. In the model suggested by their results, Vif binds to HIV-1 genomic RNA and shields it in some manner from the antiviral activity of CEM-15. Their model implies that Vif function may not involve direct interaction with CEM-15, and it would be difficult to reconcile with earlier evidence from the same group that Vif proteins of primate lentiviruses function only in the NP cells of species closely related to the viral host (41). Resolution of these issues will require additional investigations.

ACKNOWLEDGMENTS
This research was supported by NIH grant AI49729 and by grant
02754-RGT from the American Foundation for AIDS Research.
A rabbit antiserum to Vif (courtesy of D. Gabuzda) was generously provided by the AIDS Research and Reference Reagent Program of the NIH, and plasmids encoding Vif mutants were kindly donated by Michael Malim. We are grateful to Stanley Hollenberg for advice concerning the L40 yeast system, to John Scarborough for assistance with the Northern blotting, and to Kristine Rose for help with the testing of leukemic cell lines.

FOOTNOTES
* Corresponding author. Mailing address: Department of Biochemistry and Molecular Biology, Oregon Health and Science University, Portland, OR 97201-3098. Phone: (503) 494-8442. Fax: (503) 494-8393. E-mail:
kabat{at}ohsu.edu.

Present address: Department of Cancer Immunology and AIDS, Dana-Farber Cancer Institute, Harvard Medical School, Boston, MA 02115. 

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Journal of Virology, November 2002, p. 11133-11138, Vol. 76, No. 21
0022-538X/02/$04.00+0 DOI: 10.1128/JVI.76.21.11133-11138.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.
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