1 Department of Cell and Molecular Biology, Northwestern University Medical School, 303 E. Chicago Avenue, Chicago, IL 60611, USA
2 Department of Cell and Molecular Physiology, University of North Carolina, Chapel Hill, NC 27599, USA
*Corresponding author (e-mail: j-jones3{at}nwu.edu)
Accepted August 31, 2001
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SUMMARY |
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Key words: Human autoantigen, hemidesmosomes, actin cytoskeleton
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INTRODUCTION |
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BP180, also known as collagen XVII, was first identified using sera of patients suffering from bullous pemphigoid, a blistering skin disease (Giudice et al., 1992; Jones et al., 1998). It is a type 2 transmembrane protein whose N-terminus resides in the cytoplasmic plaque of the hemidesmosome (Hopkinson et al., 1992). The extracellular domain of BP180 is composed of a series of collagen-like repeats that are separated by linker regions (Giudice et al., 1992; Li et al., 1993). Like other collagens, BP180 exists as a trimer and it has now been established that the extracellular domain of BP180 contributes to the structure of anchoring filaments observed in the lamina lucida of the basement membrane zone (Balding et al., 1997; Bedane et al., 1997; Hirako et al., 1996).
Evidence from analyses of the skin of patients suffering from bullous pemphigoid and generalized atrophic epidermolysis bullosa (GABEB), a genetic blistering skin disease, indicate that BP180 plays an important role in hemidesmosome assembly and adhesion of the epidermis to the dermis (Borradori and Sonnenberg, 1999; Jonkman et al., 1995). In bullous pemphigoid, autoantibodies against a non-collagenous extracellular epitope in the BP180 molecule are believed to be pathogenic and induce dysadhesion of the epidermis (Giudice et al., 1993; Liu et al., 1993). Moreover, epidermal cells in GABEB patients carrying mutations in the gene that encodes for BP180 have a reduction in the number of hemidesmosomes, leading to tissue instability (Borradori and Sonnenberg, 1999; Jonkman et al., 1995).
BP180 has multiple binding partners in the hemidesmosome. The cytoplasmic domain of BP180 can interact with BP230 and the ß4 integrin subunit, whereas the extracellular domain can interact with the 6 integrin subunit (Aho and Uitto, 1998; Borradori et al., 1997; Hopkinson et al., 1998; Hopkinson and Jones, 2000). In this study, we focused our attention on identifying novel binding partners of the cytoplasmic domain of the BP180 molecule. Using yeast two-hybrid technology we have identified actinins as binding partners of BP180. This finding was surprising since BP180 had been thought to be present exclusively at hemidesmosomes, where it links to intermediate filaments. Conversely, the actinins are microfilament-associated proteins that are found in adherens junctions at sites of cell-cell interaction and in focal contacts at sites where cells abut their substrate (Borradori and Sonnenberg, 1999; Burridge and Chrzanowska-Wodnicka, 1996; Jones et al., 1998). However, in this study we show that BP180 is not only found at sites of cell-substrate interaction but also is concentrated at regions of cell-cell-contact, where it co-distributes with and can interact with a number of adherens junction proteins. Moreover, we present evidence that BP180 is involved in establishment of intercellular junctions and can play a role in cell-cell interactions.
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MATERIALS AND METHODS |
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Yeast two-hybrid assay
An MCF-10A cDNA library was prepared according to the Two-Hybrid cDNA Library Construction Kit protocol (Clontech Labs Inc., Palo Alto, CA). cDNAs encoding portions of BP180, actinin family members, calmodulin and 6-integrin were amplified using RT-PCR from MCF10A mRNA using specific forward and reverse primers containing engineered restriction sites. Murine myosin regulatory light chain (MRLC) was a gift from Rex Chisholm (Northwestern University). These fragments were digested with the appropriate enzymes, isolated from an agarose gel using the QIAquick gel extraction kit (Qiagen, Charworth, CA), and ligated in frame into digested yeast expression vectors pACT2 or pAS2-1 (Clontech Labs Inc., Palo Alto, CA). Individual clones containing these constructs were grown in selective media and DNA prepared from the clones using a Wizard mini prep kit (Promega, Madison, WI). All constructs were sequenced to ensure that the cDNAs were in frame and without error using Big Dye automated sequencing reagents (PE Applied Biosystems, Foster City, CA) on an ABI Prism DNA Sequencer (Foster City, CA).
DNA preparations were used to transform the yeast strain Y190, according to protocols outlined in the Matchmaker Two-Hybrid System 2 Manual (Clontech Labs). Transfected colonies were selected by growth in medium lacking leucine, tryptophan and histidine (Leu/Trp/His) but containing 25 mM 3-amino-1,2,4-triazole (3-AT). The latter was used to inhibit low levels of leaky expression of His3p in the reporter yeast strain. To monitor transfection efficiency of both plasmids, the transfected yeast were also plated onto Leu medium, Trp medium or Leu/Trp medium. At 7 days the number of colonies growing on both the Leu/Trp and Leu/Trp/His media were scored. Yeast colonies growing on both Leu/Trp/His and Leu/Trp media were also spotted onto nylon filters and flash frozen in liquid nitrogen. To detect activation of the reporter gene lacZ and resulting expression of ß-galactosidase, the filters were placed on Whatman paper soaked in a solution containing X-gal (5-bromo-4-chloro-3-indolyl-ß-D-galactopyranoside). A binding domain plasmid (pVA3-1), containing a cDNA encoding murine p53, and an activation domain plasmid (pTD1-1), containing the SV40 large T-antigen coding sequence, were used as part of our control studies (Clontech Labs).
Recombinant protein preparation
The cytoplasmic domain of BP180 (residues 1-461), actinin-4 (residues 1-884), actinin-1 (residues 1-892) and MRLC (residues 1-172) were prepared as 6x His-tagged recombinant fusion proteins in E. coli. The relevant portion of BP180 was generated by RT-PCR using MCF10A mRNA and the BP180 cDNA fragment was cloned in frame into the pET 32 vector (Novagen, Madison, WI). This construct was sequenced to confirm that the reading frame was maintained and that the sequence was correct. E. coli were transformed with the BP180 construct and were subsequently induced to produce recombinant protein by the addition of isopropyl-ßD-thiogalactopyranoside (IPTG) to the medium. The cells were lysed and extracts were incubated overnight in a 6 M urea buffer. Cell extracts were passed over a His.Bind Resin column (Novagen, Madison, WI) and bound fusion protein eluted in an imidazole elution buffer in the presence of 6 M urea. The eluant was dialyzed against 10 mM Tris buffer (pH 7.5) overnight at 4°C, concentrated by lyophilization and resuspended in sterile H2O. The purity of the recombinant polypeptides was assessed by visualizing the protein samples by SDS-PAGE and by Western blotting.
To generate HA-tagged proteins, actinin-4, actinin-1 and MRLC, pACT2 constructs were used as DNA templates since the pACT2 vector contains a hemagglutinin sequence (HA) upstream of the cloning site. Vector-specific forward and sequence-specific reverse primers were used to generate cDNAs for actinin-4, actinin-1 and MRLC containing the HA sequence at the 5' end. The PCR products were purified and cloned in frame into the pBAD-TOPO vector (Invitrogen, Carlsbad, CA). Constructs were sequenced as described above to ensure that sequences were correct and that inserts were in frame. E. coli were transformed with appropriate constructs and expression of recombinant protein was induced by adding 2% arabinose to the medium for 3 hours at 37°C. Cells were lysed, centrifuged and the supernatant was passed over a His Bind Resin column (Novagen). Proteins were eluted in an imidazole elution solution and eluates were dialyzed overnight and processed as described above.
Green fluorescent protein (GFP) and HA-tagged constructs
cDNAs encoding full-length actinin-4 (residues 1-884) and truncated actinin-4 (residues 1-813) with the HA tag incorporated at 5' end, were cloned into pCR3.1-Uni vector (Invitrogen). In addition, actinin-4 cDNA cloned into the pEGFP-NI vector (Clontech) was used in some transfection studies.
Antibodies
A mouse IgM monoclonal antibody (1804b) against the N-terminal domain of BP180 was prepared as described (Hopkinson et al., 1992; Riddelle et al., 1992). J17 rabbit antiserum was generated against the same BP180 domain (Hopkinson et al., 1992). The rabbit C-terminal antiserum generated against the C-terminal domain of BP180 was obtained from Kim Yancey (NIH, Bethesda, MD). Monoclonal antibody HA.11 against the HA epitope tag was obtained from BAbCO. The actinin-4 polyclonal serum was a generous gift from Setsuo Hirohashi (Honda et al., 1998). The mouse monoclonal antibody against actinin-1 was purchased from Sigma Chemical Co. The p120 antibody was obtained from BD Transduction Laboratories. The E-cadherin monoclonal antibody was generously provided by Kathy Green (Northwestern University).
Gel electrophoresis, immunoblotting and immunoprecipitation
Recombinant proteins, bacterial and mammalian cell extracts were solubilized in sample buffer (8 M urea, 10% ß-mercaptoethanol, 1% SDS, 10% glycerol in 10 mM Tris-HCl, pH 6.8) and were subjected to SDS-PAGE using 7.5-10% acrylamide gels (Laemmli, 1970). For western immunoblotting, proteins separated on gels were transferred to nitrocellulose membranes that were then processed with antibody as described elsewhere (Harlow and Lane, 1988). S-tag peptide fusion proteins were visualized with S protein conjugated to alkaline phosphatase (Novagen).
For immunoprecipitation studies using recombinant proteins, approximately 1 µg of actinin-4, actinin-1 or MRLC protein and BP180 fragments were mixed together in 500 µl of 1x immunoprecipitation buffer (25 mM Hepes pH 7.5, 1% Brij 97, 150 mM NaCl, 5 mM MgCl2, 0.2% SDS and protease inhibitors). After a 2 hour incubation at 4°C, J17 antiserum against BP180 was added to a 1:100 dilution and incubated at 4°C for a further 2 hours. Subsequently, 50 µl of protein G agarose (Gibco/BRL, Gaithersburg, MD) were added to the mix for an additional 1 hour. The protein G agarose was collected by centrifugation, washed four times in buffer and then solubilized in sample buffer. The resulting protein solution was processed for western immunoblotting as detailed above.
In the case of pp126 cells, extracts were prepared using immunoprecipitation buffer, which was then processed as described above, with one exception, the extracts were passed through a 22 gauge needle prior to addition of antibody.
Immunofluorescence microscopy
Cells, grown on glass coverslips, were fixed for 1 minute in 3.7% formaldehyde and then extracted in 0.5% Triton X-100 at 4°C for 7 minutes. Single and double-label immunofluorescence were performed as detailed previously (Riddelle et al., 1992). After mounting, coverslips were viewed on a Zeiss LSM510 confocal microscope, fitted with appropriate filters for visualization of GFP as well as fluorescein and rhodamine-conjugated probes (Carl Zeiss, Thornwood, NY). Controls for immunocytochemistry included omission of primary antibodies or use of irrelevant immunoglobulin (IgG) to determine non-specific binding of secondary antibodies.
Oral tissues were rapidly frozen in liquid nitrogen and cryosections 5-6 µm thick, were prepared on a Tissue-Trek cryostat (Miles Laboratories Inc., Elkhart, IN). Sections were then placed on microscope slides and fixed for 5 minutes in acetone at 20°C. Samples were air-dried and processed for double-label immunofluorescence as described above.
Antibody blocking assays
A polyclonal antibody against the C-terminal extracellular domain of BP180 was added to pp126 cells maintained in low calcium medium at a final concentration of 50 µg/ml for 24 hours. Rabbit IgG serum was used as a control. Calcium from a 100X stock was added to each dish to a final calcium concentration of 2.0 mM in the presence of BP180 antibody or control IgG. After 4 hours, cells were processed for immunofluorescence as described above. Rabbit IgG was visualized using a fluorochrome-conjugated secondary anti-rabbit goat antibody.
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RESULTS |
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The bait and the MCF-10A cDNA library were co-transfected into the Y190 yeast strain and the transfected yeast were plated onto restrictive media. Yeast colonies able to grow on the media were analyzed further for ß-galactosidase activity. Approximately 20 clones that turn blue were characterized at the molecular level. One such clone contained a cDNA insert encoding residues 708-813 of an actinin isoform termed actinin-4 (Honda et al., 1998) (Table 1). We decided to focus on this clone because we were intrigued by the possibility that BP180 might interact with the actin cytoskeleton. To confirm that BP180 and actinin-4 specifically interact in the yeast two-hybrid assay, we prepared an actinin-4 cDNA encoding the same residues (703-813) by RT-PCR using mRNA prepared from MCF-10A cells. This cDNA was cloned into a pACT2 yeast expression vector. Following co-transfection with a pAS2-1 vector containing a BP180 cDNA, the transfected yeast grow on restrictive medium and turn blue in the ß-galactosidase assay (Table 1). Control transfectants including those co-expressing either actinin-4 or BP180 in combination with 6 integrin, p53 or large T-antigen do not grow on Leu/Trp/His medium and fail to turn blue under the same conditions (Table 1).
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To provide additional evidence for interaction between actinin family members and BP180, we prepared recombinant proteins of full-length actinin-4, actinin-1, MRLC and the cytoplasmic fragment (residues 1-461) of BP180 in bacteria. All proteins were His-tagged and were purified by column chromatography. In addition to the His tag, the recombinant actinin-4, actinin-1 and MRLC were tagged with a hemagglutinin (HA) epitope that is recognized by monoclonal antibody HA.11. The BP180 fragment was fused to an S-tag peptide that binds ribonuclease S-protein. The actinin-4, actinin-1 or MRLC and BP180 recombinant proteins were mixed in Hepes-buffered saline containing a cocktail of protease inhibitors. Proteins were precipitated by the addition of a 1:100 dilution of antiserum J17, which was generated against the N-terminus of BP180 (Hopkinson et al., 1992). Precipitated proteins were separated by SDS-PAGE, and transferred to nitrocellulose. The filter was probed with HA.11 antibodies, which recognize the HA epitope on the actinin-4, actinin-1 or MRLC polypeptides. Actinin-4 is co-precipitated by the J17 antiserum when mixed with purified BP180 cytoplasmic domain but is not co-precipitated by J17 antibodies when incubated in the absence of BP180 protein (Fig. 2Ai, compare lanes 1 and 2). Similar results were obtained when actinin-1 was mixed with the BP180 fragment (Fig. 2Aii). By contrast, MRLC is not co-precipitated by the J17 antiserum when mixed with purified BP180 cytoplasmic domain and is not co-precipitated by J17 antibody when incubated in the absence of BP180 protein (Fig. 2Aiii). S-protein binding was used to confirm successful precipitation of BP180 polypeptide (Fig. 2B).
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DISCUSSION |
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One surprise of our study was that actinin-4, like actinin-1, is present at cell-cell contact sites both in MCF-10A cells and the switched oral keratinocytes. This finding is inconsistent with a previous study by Honda et al. in which it was reported that actinin-4 is a nuclear protein (Honda et al., 1998). To address this apparent discrepancy we expressed full-length actinin-4 in our epithelial cell populations. This confirmed our antibody labeling experiments in that the protein product of the actinin-4 transgene primarily targets to the cell surface and is absent from the nucleus.
Interestingly, as part of our analyses of actinin-4 distribution in various cells, we discovered that a truncated version of actinin-4 lacking the last 71 amino acids at the C-terminus of the molecule localizes exclusively to the nucleus. In light of these observations, we suggest that certain cancer cells, including those in which actinin-4 was first described, may process actinin-4, causing it to move to the nucleus (Honda et al., 1998). This would explain the localization data presented by the later study, although, to date, we have been unable to confirm that cancer cells express such a truncated actinin-4 molecule (data not shown).
One question that our study clearly raises relates to why BP180 is found at sites of cell-cell interaction. One possibility is that BP180 molecules at cell-cell borders may be in transit to the basal surface of cells where they become incorporated into hemidesmosomes. In this scenario, the association of BP180 with microfilament-associated protein at lateral cell surfaces may reflect the involvement of cellular motility machinery in BP180 movement between cell membrane domains following its synthesis and processing. At lateral cell surfaces, actinins may provide part of an anchorage site or act as a sink for newly synthesized BP180 protein. In other words, actinins may be playing a role as general scaffolding proteins for molecules such as BP180, as previously suggested (Parast et al., 2000).
We also speculate that p120 catenin may play an important role in the mechanism via which BP180 becomes directed to lateral cell surfaces. Previous workers have provided evidence from a yeast two-hybrid screen that BP180 and p120 catenin interact (Aho et al., 1999). Here we show that p120 catenin and BP180 co-distribute at cell-cell contact sites. Since p120 catenin binds to BP180 at a domain that is necessary for the targeting of BP180 to hemidesmosomes, we propose that by binding to BP180, p120 catenin inhibits BP180 incorporation into hemidesmosomes and, together with the actinins, tethers BP180 at cell borders. However, if this is the case, this is not a generalized phenomenon for hemidesmosomal proteins since BP230 or plectin, two cytoplasmic proteins of the hemidesmosomal plaque, are not found at sites of cell-cell contact in pp126 cells that are maintained in medium containing 2.0 mM calcium under which conditions cells assemble extensive intracellular junctions (not shown). Instead, both plectin and BP230 remain concentrated at sites of cell-substratum interaction in such cells.
Of course, in addition to transiting through lateral cell surfaces, BP180 may actually contribute to the structure and/or stability of cell-cell interactions. There is certainly precedent for this from the studies of other transmembrane, collagen-related proteins. For example, collagen type XIII, which, like BP180, is oriented with a type 2 topography in the plasma membrane, is believed to play a role in stabilizing cell-cell junctions (Hagg et al., 1998; Peltonen et al., 1999; Snellman et al., 2000). Moreover, there is evidence that three other type 2 membrane proteins that possess collagen-like extracellular domains, namely, the type 1 macrophage scavenger protein receptor (MSPR), MARCO (a macrophage receptor with collagenous structure) and ectodysplasin are involved in cell-cell binding (Elomaa et al., 1995; Ezer et al., 1999; Kodama et al., 1990). In particular, MSPR and MARCO mediate cell-bacterial interactions whereas ectodysplasin appears to distribute along cell-cell contact sites, where it interacts with the microfilament cytoskeleton (Elomaa et al., 1995; Ezer et al., 1999; Kodama et al., 1990). Indeed, in the studies we present here we provide evidence that BP180 plays an active role in establishing cell-cell interactions. In particular, an antibody against the C-terminal domain of BP180 appears to inhibit incorporation of adherens junction proteins into assembling junctional complexes.
In summary, we have identified new potential binding partners of a major structural protein of the hemidesmosome. That these partners are members of a family of actin binding proteins that fail to localize to hemidesmosomes provides an indication that BP180 is somewhat promiscuous with regard to its cytoskeleton connections. Remarkably, this mirrors the apparent promiscuity of the 6ß4 integrin heterodimer which, in the hemidesmosome, is involved in tethering the keratin cytoskeleton to the cell surface whereas, in certain cancer cells, it associates with the microfilament cytoskeleton network (Rabinovitz and Mercurio, 1997). Indeed, BP180 is one of an increasing number of proteins whose cytoskeleton allegiance may be modulated in quite a precise fashion and that may play a role in crosstalk of cell-matrix and cell-cell junctions.
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ACKNOWLEDGMENTS |
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