If you don't remember your password, you can reset it by entering your email address and clicking the Reset Password button. You will then receive an email that contains a secure link for resetting your password
If the address matches a valid account an email will be sent to __email__ with instructions for resetting your password
Molecular Virology Section, Laboratory of Molecular Microbiology, NIAID, National Institutes of Health, Bethesda, Maryland 20892-0460Centre de Recherche de Biochimie Macromoleculaire CNRS, ERS0155 Montpellier, France
* This work was supported in part by funds from the AIDS Targeted Antiviral Program from the Office of the Director, National Institutes of Health (to K.-T. J.) and by National Institutes of Health Grants AI35522 and AI42397 (to R. A. K.).The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. ** Elizabeth Glaser Scientist of the Pediatric AIDS Foundation.
Human chemokine receptor 5 (CCR5) functions as a co-receptor for Human immunodeficiency virus (HIV-1) infection. CCR5 is a seven-transmembrane cell surface receptor. Recently, a naturally occurring mutation of CCR5, ccr5Δ32, has been described. A small number of Caucasians are homozygouslyccr5Δ32/ccr5Δ32, while a larger number of individuals are heterozygously CCR5/ccr5Δ32. Theccr5Δ32/ccr5Δ32 genotype has been linked to a phenotype that is “highly” protected from HIV-1 infection. On the other hand, several studies have shown that the CCR5/ccr5Δ32 genotype confers “relative” protection from AIDS with onset of disease being delayed by 2–4 years. Although it is known that peripheral blood lymphocytes from heterozygous individuals (CCR5/ccr5Δ32) support ex vivo HIV-1 replication at a reduced level compared with CCR5/CCR5 cells, the molecular basis for this observation is unknown. Here we report on events that post-translationally modify CCR5. We show that CCR5 progresses through the endoplasmic reticulum prior to appearing on the cell surface. Mature CCR5 can be post-translationally modified by phosphorylation and/or co-translationally by multimerization. By contrast, mutant ccr5Δ32, although retaining the capacity for multimerization, was incapable of being phosphorylated. ccr5Δ32 heterocomplexes with CCR5, and this interaction retains CCR5 in the endoplasmic reticulum resulting in reduced cell surface expression. Thus, co-expression in cells of ccr5Δ32 with CCR5 produces a trans-inhibition by the former of ability by the latter to support HIV-1 infection. Taken together, our findings suggest CCR5/ccr5Δ32 heterodimerization as a molecular explanation for the delayed onset of AIDS inCCR5/ccr5Δ32 individuals.
). Chemokine receptors belong to the superfamily of G protein-coupled receptors that have seven transmembrane domains. Chemokines are a family of small proteins (7–16 kDa) that can be operationally divided in two subgroups. The α subfamily (CXC) is distinguished from the β subfamily (CC) by the insertion of a single amino acid between the first and the second cysteine residues. The binding of chemokines to their receptors induces a rapid calcium influx and inflammatory responses (
). Accordingly, further investigations have demonstrated the existence, in some ethnic groups (e.g. Caucasians), of a natural genetic mutation inCCR5 (ccr5Δ32, an internal 32-nucleotide deletion in the CCR5 open reading frame). Homozygousccr5Δ32/ccr5Δ32 genotype confers resistance to HIV-1 infection in vitro and in vivo (
). Currently, how heterozygocity (CCR5/ccr5Δ32) mechanistically impacts disease progression is unknown. It has, however, been observed thatCCR5/ccr5Δ32 PBMCs are less infectible in vitroby M-tropic HIV-1s than CCR5/CCR5 cells (
The mechanism through which ccr5Δ32 might affect CCR5-function remains to be clarified. To assess this issue we studied the effect of ccr5Δ32 on the processing, stability, and cell surface expression of CCR5. We found that 1) CCR5 is post-translationally phosphorylated upon MIP-1β stimulation of cells; 2) intracellularly, CCR5 exists as either CCR5/CCR5 or CCR5/ccr5Δ32 multimers; 3) CCR5 and ccr5Δ32 are found in different cellular locales (the former is predominantly on the cell surface while the latter is retained in the ER); and 4) co-expression of ccr5Δ32 inhibited surface expression of CCR5 and CCR5-mediated infection by M-tropic HIV-1 isolates.
). CCR5 mutants, cloned into pcDNA3 (Invitrogen), include pCMV/CCR5Δcyt (aa 1–303), pCMV/CCR5/6TM (aa 1–279), pCMV/CCR5/5TM (aa 1–235), and pCMV/CCR5Δ32 (aa 1–187). Each mutant was generated by polymerase chain reaction with the HA epitope fused to each cDNA at the 3′ terminus. pCMVCCR5-Flag was a gift from Ron Willey (National Institutes of Health).
Cell Culture, Infection, and Transfection
HeLa cells were propagated in Dulbecco's modified Eagle's medium (DMEM) with 10% fetal bovine serum. PBMCs were activated for 3 days in RPMI with 10% fetal bovine serum containing PHA and then washed and resuspended into the same medium without PHA and with 100 units/ml interleukin-2 (Boehringer Mannheim). PBMCs were exposed to M-tropic HIV-1 isolate AD8 (500 units of TCID50) for 1 h at 37 °C, washed, and then resuspended into fresh medium. Virus replication was monitored by reverse transcriptase assay as described previously (
). Transfection of HeLa cells was performed using calcium phosphate.
24 h after transfection, cells were washed twice in DMEM without methionine and cysteine starved for 30 min in the same medium at 37 °C. [35S]Methionine + cysteine (translabel ICN) at 1 mCi/ml final concentration was added to the cells. The cells were then incubated at 37 °C for the indicated amount of pulse time and then washed and resuspended in DMEM with methionine and cysteine for the indicated chase times.
For immunoprecipitation, identical protein amounts were suspended into 1 ml of Triton lysis buffer (0.5% Triton X-100, 300 mm NaCl, 50 mm Tris, pH 7.4, 0.2 mm phenylmethylsulfonyl fluoride) and incubated for 2 h at 4 °C with either anti-Flag M2 (Eastman Kodak Co.) or anti-HA (12CA5, Boehringer Mannheim). A mixture of protein A- and protein G-Sepharose (Pharmacia Biotech Inc.) was added to each sample followed by a 1-h incubation at 4 °C. Three washes were performed in Triton wash buffer (0.1% Triton X-100, 300 mm NaCl, 50 mm Tris, pH 7.4, 0.2 mm phenylmethylsulfonyl fluoride) and a final wash in SDS/deoxycholate buffer (300 mm NaCl, 50 mm Tris, pH 7.4, 0.1% SDS, 0.1% deoxycholate). The immunoprecipitated products were solubilized in 1 × loading buffer (125 mm Tris, pH 6.8, 20% glycerol, 2% SDS, 2% β-mercaptoethanol, 0.01% bromphenol blue) and resolved by SDS-PAGE.
HeLa cells were seeded onto coverslips and transfected. 24 h later, cells were fixed with fresh 4% paraformaldehyde, pH 7.0, for 10 min at room temperature. Fixed cells were permeabilized with a 2-min wash in 100% methanol at room temperature followed by several washes in PBS with 4% bovine serum albumin (PBS/BSA). Appropriately diluted primary antibody was incubated with coverslips overnight at 4 °C. Excess antibody was removed with four washes in PBS/BSA. Species-specific second antibody conjugated to Texas Red (Cappel) was then reacted with the coverslips for 1 h at room temperature followed with four washes in PBS/BSA. The final samples were mounted onto slides and visualized using a Ziess Axiophot confocal microscope.
Yeast Two-hybrid Assay
Two-hybrid assays were performed according to manufacturer's protocols (CLONTECH).
RESULTS AND DISCUSSION
To ask whether CCR5 forms an oligomer, HeLa cells were transfected with pCMV/CCR5-Flag, which contains the Flag epitope fused to the C terminus of CCR5. 24 h later, cells were pulsed with [35S]methionine + cysteine for 15 min, washed, and then chased in complete medium for 1 h. Extracts prepared from pulsed and pulsed + chased cells were immunoprecipitated using anti-Flag M2. The immunoprecipitates were resuspended into either 1 × loading buffer (Fig. 1A, lanes 1 and2) or “native” loading buffer without β-mercaptoethanol with a reduced amount of SDS (0.25%) (Fig.1A, lanes 3 and 4). The samples were resolved by SDS-PAGE. The reduced mobility of CCR5 in the pulsed + chased sample (Fig. 1A, lane 2) when compared with the pulsed sample (Fig.1A, lane 1) is consistent with a post-translational modification.
When the same analysis was repeated using native loading buffer (Fig. 1A, lanes 3 and 4), the mobility difference observed for pulsed (lane 3) and pulsed + chased (lane 4) samples was replicated. The native buffer-PAGE revealed additional CCR5-specific bands with sizes consistent with dimeric moieties (d; Fig.1A, lanes 3 and 4).
To characterize better multimerization potentials, we analyzed CCR5 using the yeast two-hybrid approach (TableI). In this analysis, C-terminal truncation mutants of CCR5 were found to interact with wild type CCR5. However, a mutant deleted in the first 58 amino acids failed to interact with intact CCR5. Thus, in yeasts, the CCR5-CCR5 interactive domain resides in the N-terminal portion of the protein, which encompasses the first transmembrane region.
Table IYeast two-hybrid analysis of CCR5/CCR5 interaction
CCR5/4TM (ccr5Δ32) (1–187)
Yeast were transformed by PIDCCR5wt and the different mutants using LiAc method of Gietz et al. (
) represents a positive control. A qualitative colony-lift filter assay was performed using 5-bromo-4-chloro-3-indolyl-β-d-galactopyranoside as substrate. Strongly positive colonies (++++) appeared as dark blue within 20 min. Positives (+++) were blue within 1 h.
The biological implications of dimerization were further examined. We co-expressed CCR5 and ccr5Δ32 in HeLa CD4+/LTR-βgal cells and assayed for cell fusion engendered by an M-tropic HIV-1 isolate, HIV-1AD8 (
). A prediction of protein dimerization is that ccr5Δ32 could be a dominant negative inhibitor of CCR5 function. We found that transfection of HeLa CD4+ LTR-CAT with wild type CCR5 (Fig.2A) did render, as expected, HeLa cells susceptible to HIV-1AD8 infection (Fig.2A, compare lane 8 with 9). Co-transfection of CCR5 with ccr5Δcyt (CCR5 lacking the cytoplasmic tail) did not affect HIV-1AD8 infection (Fig. 2A, lane 10). However, co-expression of CCR5 with either ccr5/6TM, or ccr5/5TM, or ccr5/4TM (ccr5Δ32) or ccr5/3TM (i.e.C-terminal mutants deleted for six, five, four, or three transmembrane regions, respectively) dramatically reduced the ability of cells to support HIV-1AD8-induced fusion. No effect was seen when pNL4–3 (T-tropic HIV-1) infections were conducted under parallel conditions (Fig. 2A, lanes 1–7), confirming M-tropic specificity of CCR5 in these experiments. Co-expression of ccrΔ32 and CCR5 in monolayer cells can be regarded as an artificial approach modeling in vivo infection. To construct a more physiological test, we conducted infections comparing PBMCs from oneCCR5/CCR5 and two CCR5/ccr5Δ32 individuals. These results (Fig. 2B) are consistent with the above findings (Fig. 2A) and previously published results (
), confirming that simultaneous presence of ccr5Δ32 with CCR5 in the same cell reduces susceptibility to infection by M-tropic HIV-1.
The infection results led us to consider how ccr5Δ32 might affect thein vivo presentation of CCR5. To address this question, we used confocal immunofluorescence to visualize potential intracellular influences of ccr5Δ32 on CCR5. Upon staining with specific antiserum, steady-state CCR5 and ccr5Δ32 were found in distinctly different subcellular locales. While CCR5 was found on cell surfaces (Fig.3A), ccr5Δ32 stained in the ER (Fig. 3B). ER localization was observed for all ccr5 mutants (ccr5/6TM, ccr5/5TM, ccr5/4TM, and ccr5/3TM), except ccr5Δcyt, which was found on the cell surface (data not shown). When CCR5 and ccr5Δ32 were co-expressed in the same cell, significantly reduced surface staining of the former was seen (Fig.3C).
The above findings are consistent with CCR5-CCR5 dimers. The biochemical evidence provided by mobility shifts in PAGE (Fig. 1, compare lanes 1 and 2) could also be explained by other modifying events such as phosphorylation. Because the C terminus of CCR5 is rich in serine and threonine residues, which are potential substrate sites for G protein-coupled receptor-kinase(s), and because phosphorylation could conceivably influence overall receptor function, we queried for this possibility. We, thus, transfected HeLa cells with pCMV/CCR5-Flag and labeled the cells in parallel with either [35S]methionine + cysteine (Fig.4A, lanes 1–4) or [32P]orthophosphate (Fig. 4A, lanes 5–8) for 2 h in the absence (Fig. 4A, lanes 1, 3, 5, and7) or presence (Fig. 4A, lanes 2, 4, 6, and8) of 100 ng/ml of MIP-1β. Cell extracts normalized for radioactivity were then immunoprecipitated with anti-Flag antibody. In the immunoprecipitates, the amount of 35S-labeled CCR5 was found to be equivalent between mock-treated and cells treated with MIP-1β (Fig. 4A, compare lane 1 with lane 2 and lane 3 with lane 4). However, in the same experiment, a large difference in 32P-labeled CCR5 in the presence of MIP-1β was observed (Fig. 4A, comparelane 5 with lane 6 and lane 7 withlane 8). These results suggest that MIP-1β does not affect the ambient expression of CCR5, but does affect the phosphorylation state of this protein.
The cytoplasmic tail of CCR5 is rich in potential phosphate acceptor sites. To examine the contribution of this portion of the protein to phosphorylation, HeLa cells were transfected with pCMV/CCR5Δcyt. This construction encodes a truncated form of CCR5 lacking the cytoplasmic tail fused at its C terminus to a HA epitope. In this instance, the experiment was performed as that described in the legend to Fig.4A except that anti-HA was used for immunoprecipitation. As shown in Fig. 4B, similar amounts of 35S-labeled CCR5Δcyt was found in mock-treated and cells treated with MIP-1β (compare lane 1 with lane 2). However, we failed to detect any 32P-labeled CCR5Δcyt (Fig. 4B, lanes 3 and 4). These results suggest that an intact cytoplasmic tail of CCR5 is essential for phosphorylation.
In summary, our study raises three points: 1) functional CCR5 molecule exists as a multimer; 2) expression of ccr5Δ32 reduces cell surface expression of wild type CCR5; and 3) MIP-1β induces the phosphorylation of CCR5 in its cytoplasmic tail. Taken together, these points suggest that there are complex post-translational regulatory events for CCR5. Relevant to the observation of receptor multimerization, it is clear that CCR5 mutants such as ccr5Δ32 are defective in some specific aspects of post-translational processing that render them aberrantly trapped in the ER (see Fig. 3). However, these defective mutants, nevertheless, maintain an ability to multimerize with wild type CCR5 and through this heterocomplexing retains the wild type moiety into the ER. We believe that this mutant-wild type tethering mechanism explains, in part, why PBMCs from CCR5/ccr5Δ32 individuals have lower levels of cell surface CCR5 when compared with CCR5/CCR5 counterparts (
The functional role of phosphorylation on CCR5 is less clear. Likely, this modification governs a step in transducing the MIP-1β signal from the cell surface into the cytoplasm/nucleus. Recently, Amaraet al. (
) reported that internalization of chemokine receptors (CXCR4 and CCR5), induced by their ligand, contribute to chemokines-mediated inhibition of HIV-1 replication. Interestingly, the cytoplasmic tail of CXCR4 is required for SDF-1-mediated internalization of CXCR4 (
). Whether phosphorylation of chemokine receptors influences ligand-induced internalization is unknown. Another possibility is that chemokine-stimulated CCR5 phosphorylation can exert entry-unrelated effects that ameliorate HIV-1 infection. Indeed, MIP-1β, MIP-1α, and RANTES have been shown to have unexpected effects on HIV-1 replication in macrophages (
). Whether the CCR2 mutation shares functional similarities with the CCR5 Δ32 mutation (e.g. producing nonproductive transdominant multimerization or affecting phosphorylation) remains to be explored. However, the natural existence of transdominant negative chemokine receptor mutants might be a general principle that explains differential resistance of certain subpopulations for AIDS and that might help guide future intervention strategies for HIV-1.
We thank E. Berger for gift of CCR5 plasmid and R. Willey for gift of pCMV/CCR5-Flag construct and for helpful discussions.