JOURNAL OF VIROLOGY, Dec. 1998, p. 10218–10221 0022-538X/98/$04.0010 Copyright © 1998, American Society for Microbiology. All Rights Reserved.
Vol. 72, No. 12
Equine Infectious Anemia Virus Gag Polyprotein Late Domain Specifically Recruits Cellular AP-2 Adapter Protein Complexes during Virion Assembly BRIDGET A. PUFFER,1 S. C. WATKINS,2
AND
RONALD C. MONTELARO1*
Department of Molecular Genetics and Biochemistry1 and Department of Cell Biology and Physiology,2 University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania 15261 Received 16 June 1998/Accepted 21 August 1998
We have identified an interaction between the equine infectious anemia virus (EIAV) late assembly domain and the cellular AP-2 clathrin-associated adapter protein complex. A YXXL motif within the EIAV Gag late assembly domain was previously characterized as a sequence critical for release of assembling virions. We now show that this YXXL sequence interacts in vitro with the AP-50 subunit of the AP-2 complex, while the functionally interchangeable late assembly domains carried by the Rous sarcoma virus p2b protein and human immunodeficiency virus type 1 p6 protein, which utilize PPPY and PTAPP L domains, respectively, do not bind AP-50 in vitro. In addition, EIAV late domain mutants containing mutations that have previously been shown to abrogate budding also exhibit marked decreases in AP-50 binding efficiencies. A role for AP-2 complex in viral assembly is supported by immunofluorescence analysis of EIAV-infected equine dermal cells demonstrating specific colocalization of the a adaptin subunit of AP-2 with the EIAV p9 protein at sites of virus budding on the plasma membrane. These data provide strong evidence that EIAV utilizes the cellular AP-2 complex to accomplish virion assembly and release. The assembly of retroviral particles is driven by the Gag polyprotein. Gag polyproteins are synthesized in the cytoplasm from genome-length mRNA and are targeted to the plasma membrane, where approximately 2,000 associate to form immature budding particles (21). Maturation of the virion to an infectious particle occurs at some point either late in budding or immediately after release of the virus particle when the Gag polyproteins are processed by the virus-encoded protease. The equine infectious anemia virus (EIAV) Gag polyprotein is processed to generate the matrix (MA; p15), capsid (CA; p26), nucleocapsid (NC; p11), and p9 proteins (8, 11). The p9 sequence at the C terminus of Gag has been shown to function at a point late in virus assembly and is critical for release of assembled virus particles (14, 16). The human immunodeficiency virus type 1 (HIV-1) p6, Rous sarcoma virus (RSV) p2b, and EIAV p9 proteins were identified to be functional homologs by construction of chimeric Gag polyproteins and analysis of particle assembly (14, 16). While HIV-1 p6, RSV p2b and EIAV p9 are functionally interchangeable, they share little amino acid sequence similarity. It has been determined that a PTAP sequence contained in HIV-1 p6 and conserved among all lentiviruses, with the exception of EIAV, was critical for late domain function (4, 7, 9). This sequence is suggestive of PXXP binding motifs which have been shown to interact with the SH3 domain protein structural module (15). Mutational analysis of RSV p2b and, recently, pp16 of MasonPfizer monkey virus identified a PPPY sequence, also highly conserved among the oncoviruses, as being critical for late domain function (22–25). This motif has been shown to interact with a semiconserved structural module referred to as a WW domain (3, 19). The interaction was verified in vitro;
* Corresponding author. Mailing address: Department of Molecular Genetics and Biochemistry, University of Pittsburgh School of Medicine, W1144 Biomedical Science Tower, Pittsburgh, PA 15261. Phone: (412) 648-8869. Fax: (412) 383-8859. E-mail:
[email protected].
however, the “natural partner” in vivo remains to be defined (6). Alanine scanning mutagenesis of the EIAV late assembly (L) domain identified p9 amino acids Y23, P24, and L26 (YPXL) as critical for a functional late assembly domain, suggesting a YXXL motif (16). YXXL sequences in other proteins have been shown to bind the medium chain subunits of the AP-1 and AP-2 clathrin-associated adapter protein complexes by the yeast two-hybrid assay, and this interaction has been characterized in vitro (13, 20). Based upon this observation, we decided to examine possible interactions between retroviral late assembly domains and the medium chain (AP-50 subunit) of the plasma membrane-localized AP-2 clathrin-associated adapter protein complex. Glutathione S-transferase (GST) and a GST-p9 fusion protein (encoding the first 30 residues of p9) were purified and analyzed for binding to a truncated AP-50 subunit (residues 121 to 435) in a standard binding assay (13). Figure 1 shows that while GST alone did not bind to AP-50 (Fig. 1, lane 1), the GST-p9 protein was able to bind and precipitate AP-50 (Fig. 1, lane 2). We also tested the PTAPP sequence of HIV-1 (in p6) and the PPPPY sequence of RSV (in p2b) for binding to AP-50. The GST-p6 and GST-p2b proteins both failed to bind and precipitate AP-50 (Fig. 1, lanes 3 and 4, respectively). Thus, in vitro the EIAV YXXL sequence critical for release of budding virions does interact with the AP-50 subunit of AP-2, and this binding is specific for the EIAV p9 late domain. To further test our hypothesis that the EIAV L domain interacts with AP-50 to facilitate virion assembly, we constructed EIAV L domain GST fusion protein analogs containing late domain alanine scanning mutations that were shown to be functionally defective in a COS-1 cellular budding assay (16). GST-p9 fusion proteins containing substitution of residue Y23, P24, or L26 with alanine were tested with the in vitro AP-50 binding assay, and the resultant bands were quantitated by densitometry and normalized to the value obtained for GSTp9. We observed that alanine substitution for either Y23, P24,
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FIG. 1. Analysis of AP-50 binding to retroviral late domains in the context of GST fusion proteins. Truncated AP-50 was synthesized from the 3M9 cDNA in vitro by using the TNT Quick Coupled Master mix reticulocyte lysate with [35S]methionine. Binding to 10 mg of an eluted GST fusion protein was initiated in 600 ml of binding buffer (0.05% Triton X-100, 50 mM HEPES [pH 7.3], 10% glycerol, 0.1% bovine serum albumin), and protein complexes were precipitated with glutathione-Sepharose beads, washed twice with binding buffer, washed once with binding buffer supplemented with 100 mM NaCl, resolved by sodium dodecyl sulfate–12% polyacrylamide gel electrophoresis, and detected by autoradiography.
or L26 resulted in a decrease in the ability to bind AP-50 (Fig. 2, lanes 3, 4, and 5, respectively) when compared to GST-p9 binding (Fig. 2, lane 2), reflecting the activity levels obtained in a COS-1 cell budding assay (16). Quantitation of AP-50 bound by GST-p9 analogs demonstrated an average 75% decrease in AP-50 binding compared to the level observed for the GST-p9 wild-type sequence (Fig. 2B), in spite of the high concentrations of protein contained in the reaction mixtures. Having identified an in vitro interaction between AP-50 and EIAV p9, we next used immunofluorescence analysis to determine whether these proteins colocalize in EIAV-infected cells. To determine if the AP-2 complex associates with EIAV p9, we obtained a commercially available monoclonal antibody directed against the a adaptin subunit, a component of adapter protein complexes that is specific for the plasma membranelocalized complex AP-2 (1, 17, 18). Infected and uninfected equine dermal (ED) cells were fixed, permeabilized, labeled with anti-a adaptin and anti-EIAV p9, and examined by fluorescence microscopy. Examination of uninfected ED cells for a
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adaptin demonstrated a diffuse staining throughout the cell, typical of a adaptin staining (Fig. 3E) (1, 17, 18). Visualization of adaptin staining in EIAV-infected cells (Fig. 3B) revealed normal diffuse staining, in addition to specific sites of concentrated adaptin staining at the plasma membrane of the cell and intense apparently intracellular staining corresponding to a perinuclear localization (Fig. 3B). Visualization of the anti-p9 staining in uninfected cells demonstrated the absence of background antibody reactivity (Fig. 3D). In marked contrast to uninfected cells, infected ED cells showed intense p9 staining of virions at the plasma membrane, as well as an apparently perinuclear staining (Fig. 3A). To identify sites of colocalized adaptin and p9, the Cy3 and fluorescein isothiocyanate staining patterns were merged (Fig. 3C). Importantly, the pattern of dual staining (in orange) indicated specific concentrations of EIAV p9 and adaptin at apparent sites of virus budding. Examination of cells producing lower levels of viral protein also indicated colocalization of adaptin and p9 at cellular membranes (Fig. 3F [see inset]); however, these cells showed more typical diffuse adaptin staining and did not have intense perinuclear staining, as observed above. These results demonstrated that a adaptin, and thus the AP-2 complex, specifically colocalizes with sites of virus assembly in ED cells. YXXL sequences in other lentiviral proteins have been identified as also functioning in replication. The simian immunodeficiency virus transmembrane protein contains a YXXF sequence critical for the regulation of surface envelope protein levels (10). Specific interactions between the HIV-1 and simian immunodeficiency virus envelope proteins and adapter protein medium chains have also recently been demonstrated (2, 12). Examination of the EIAV transmembrane protein and accessory gene sequences fails to identify YXXF motifs; thus, we believe that the demonstrated colocalization of EIAV p9 and AP-2 is the result of the p9 gene-encoded YXXL sequence. We report here for the first time the characterization of an interaction between the late assembly domain of EIAV p9 and the cellular clathrin-associated adapter protein complex AP-2. We used an in vitro AP-50 binding assay to demonstrate for the first time a specific interaction between that subunit of the AP2 complex and the EIAV late assembly domain YXXL motif. While the EIAV p9 late assembly domain bound to AP-50, the functionally homologous late domains of HIV-1 and RSV that utilize PTAPP and PPPPY motifs, respectively, did not bind
FIG. 2. Analysis of AP-50 binding by EIAV late assembly domain wild-type and mutant sequences. (A) Binding activity was determined as described in the legend to Fig. 1. (B) Quantitation of binding efficiencies relative to that of the native p9 late domain sequence.
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FIG. 3. Immunofluorescence colocalization of EIAV p9 and AP-2. EIAV-infected (A, B, C, and F) and uninfected (D and E) ED cells were grown on glass coverslips, fixed, permeabilized, and stained for EIAV p9 (A and D) and a adaptin (B and E). EIAV p9 and a adaptin were detected by indirect immunofluorescence microscopy with a rabbit polyclonal p9 antiserum with Cy3-conjugated antirabbit secondary antibody and monoclonal mouse anti-a adaptin with fluorescein isothiocyanate-conjugated antimouse antibody. Nuclei (blue) were stained with Hoechst stain (D, E, and F). The p9 (red) and a adaptin (green) colocalization (orange) is shown in panels C and F. Bar, 10 mm.
AP-50 in this in vitro assay. We also demonstrated that proteins containing L domain mutations previously shown to abrogate function in a COS-1 cell budding assay were also defective in AP-50 binding. Finally, we demonstrated colocalization of AP-2 complexes with EIAV p9 at sites of virus
assembly within the cell, suggesting a role for AP-2 complex in virion assembly and release. The mechanisms through which retroviral late domains facilitate budding remain to be determined. The identification here of the involvement of clathrin-associated adapter proteins
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indicates that endocytosis machinery may be necessary for a step in virus assembly, possibly by recruiting other cellular proteins to the site of assembly to facilitate a membrane fission event, or by interacting with the Gag polyprotein and membrane phospholipids to facilitate assembly of the immature virus particle in a manner similar to the native function of adapter protein complexes in clathrin-coated pit formation (5). The cellular partners of the PTAP- and PPPY-type late domains remain to be identified. We have shown here that despite the interchangeable nature of the YXXL and PTAP or PPPY late domains to facilitate Gag budding in vitro, the PTAP and PPPY domains failed to bind with the AP-50, in contrast to the EIAV YXXL late domain. This difference indicates that different retroviruses may utilize different sites in cellular processing pathways to accomplish a common assembly step in virus replication in their respective host cells. For example, the various late domains may interact either with different subunits of a particular complex or with different proteins involved in the same cellular pathway. Alternatively, the cellular protein partners of late domains could be involved in entirely different cellular pathways as dictated by their host cell. While further experiments are necessary to elucidate the cellular machinery involved in late assembly and release of retroviral particles, the results of this study define a cellular process that has been adopted by EIAV and possibly other retroviruses to facilitate a critical step during virus assembly. We thank John Wills and Laurence Garnier for helpful conversations and assistance in preparing the manuscript and for providing the wild-type L domain GST fusion proteins. We acknowledge Markus Thali for assistance with AP-50 binding experiments and Juan Bonifacino for kindly providing the 3M2 and 3M9 cDNAs for AP-50. Finally, we recognize John Gibbs, Sean Alber, and Ciprian Almonte for assistance with immunofluorescence staining. This work was supported by National Institutes of Health grant 5R01CA49296 (R.C.M.). REFERENCES 1. Ball, C. L., S. P. Hunt, and M. S. Robinson. 1995. Expression and localization of a-adaptin isoforms. J. Cell Sci. 108:2865–2875. 2. Boge, M., S. Wyss, J. Bonifacino, and M. Thali. 1998. A membrane-proximal tyrosine-based signal mediates internalization of the HIV-1 envelope glycoprotein via interaction with the AP-2 clathrin adaptor. J. Biol. Chem. 273: 15773–15778. 3. Bork, P., and M. Sudol. 1994. The WW domain: a signalling site in dystrophin? Trends Biochem. Sci. 19:531–533. 4. Checroune, F., X. Yao, H. G. Gottlinger, D. Bergeron, and E. A. Cohen. 1995. Incorporation of Vpr into human immunodeficiency virus type 1: role of conserved regions within the p6 domain of Pr55gag. J. Acquired Immune Defic. Syndr. Hum. Retrovirol. 10:1–7. 5. DeCamilli, P., S. D. Emr, P. S. McPherson, and P. Novick. 1996. Phosphoinositides as regulators in membrane traffic. Science 271:1533–1538. 6. Garnier, L., J. W. Wills, M. F. Verderame, and M. Sudol. 1996. WW domains
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