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CUTTING EDGE |





*
Division of Immunology and
Research Group in Human Reproductive Immunobiology, Department of Pathology, University of Cambridge, Cambridge, United Kingdom;
Basel Institute for Immunology, Basel, Switzerland; and
§
Cell Biophysics Laboratory, Imperial Cancer Research Fund, London, United Kingdom
| Abstract |
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10-kDa protein that is encoded within 100 bp of the DAP12
locus on human chromosome 19. KAP10 is predominantly expressed in
immune cells, including NK cells, T cells, and monocytes. We show that
KAP10, unlike other transmembrane adapter proteins, binds
phosphatidylinositol-3 kinase following phosphorylation
of a cytoplasmic YINM motif, which results in activation of Akt. In
addition, we identify KAP10 as being able to bind the adapter protein
Grb2. Based on our data, we suggest that this molecule is involved in
stimulation and costimulation in cells of both myeloid and lymphoid
origin. | Introduction |
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-chain, which
associates with both Fc receptors and activating Ig-like transcript
(ILT) (1, 2, 3, 4). More recently, an adapter molecule called
DAP12 (KARAP) has been shown to associate with MHC class I-specific
killer cell receptors (5, 6, 7, 8, 9). TM adapter proteins usually
exist as disulfide-linked hetero- or homodimers and share several
features, including one or more immunoreceptor-based tyrosine
activation motifs (ITAM) (D/ExxYxxL/I x68
YxxL/I) in their cytoplasmic tails and the ability to recruit src
homology domain-2 (SH2)-containing signaling proteins, such as ZAP70
and Syk, following tyrosine phosphorylation. Other TM costimulatory molecules, such as CD28, provide a second "survival" signal to cells, such as T cells, by using a single SH2-binding site to recruit further signaling molecules such as phosphatidylinositol-3 kinase (PI-3 kinase), Grb2, and Shc (10). These receptor networks provide cells with a sophisticated repertoire of signals that determine survival, activation, or differentiation.
We have identified and cloned a novel cDNA encoding a protein that is
an outriding member of the TM adapter protein class. The corresponding
gene is closely linked to DAP12. This novel protein has a YINM motif
more reminiscent of that found in the family of costimulatory
molecules. We show that this molecule is expressed in immune cells and,
like CD28 and others, can bind both PI-3 kinase and Grb2 following
stimulation. We call this molecule PI-3 kinase-associated protein of
10 kDa (KAP10).
| Materials and Methods |
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Rabbit anti-human KAP10 was generated by immunizing with a peptide spanning the entire cytoplasmic tail of KAP10. Anti-CD4 mAb L3T4 are from PharMingen (San Diego, CA); anti-PI-3 kinase, anti-Grb2, and the phosphotyrosine mAb are from Transduction Laboratories (Lexington, KY). HRP-conjugated goat anti-mouse or -rabbit Ab were from Immunotech (Marseille, France). F(ab')2 goat anti-rat IgG was from Jackson ImmunoResearch (West Grove, PA). Abs specific for phosphorylated Akt (at Thr308) and Akt Ab were from New England Biolabs (Beverly, MA). The following cell lines were used: Jurkat, NKL, NK92, U937, THP-1, RAJI, LCL721, 293T, YT, COS, SY5Y, SK-N-BE, IMR32, and a polyclonal NK cell line (NK9449).
Cloning and Northern blot analysis
A genomic clone that contained the DAP12 sequence (gb: AD000864) was analyzed using the NIX program at the Human Genome Mapping Program (HGMP) resource center (http://www.hgmp.mrc.ac.uk/) to determine the presence of other genes. Expressed sequence tag (EST) clones (IMAGE clones; 955952, 462536, and 331055) were obtained from the HGMP and sequenced using a 377 cycle sequencer (Applied Biosystems, Foster City, CA). A human multiple tissue northern blot (Clontech, Palo Alto, CA) was probed with a radiolabeled 285-bp KAP10 DNA fragment derived by PCR amplification from a KAP10 EST clone using primers 5'-ATGATCCATCTGGGTCACATC-3' and 5'-TCAGCCCCTGCCTGGCAT-3' according to the manufacturers instructions. Membranes were washed under high stringency conditions and exposed to x-ray film for 24 days.
Constructs and transfection
A CD4-KAP10 chimeric construct encoding the extracellular and TM region of murine CD4 and the cytoplasmic region of human KAP10 was produced by PCR as previously described (11). The PCR product was cloned into the expression vector pCDNA-3 and transfected into Jurkat T cells. After 34 wk of selection, G418-resistant clones were expanded and maintained in medium without G418.
PI-3 kinase association
Peptides were synthesized by MWG Biotec (Ebersberg, Germany) (KAP10-pY71 and KAP10-Y71) and Zinsser Analytic (Frankfurt, Germany) (pY740/pY751) and were of the following sequences: KAP10-Y71- CPAQEDGKYINMPGRG, KAP10-pY71-KSPAQEDGKpYINMPGRG, and pY740/pY751-GGpYMDMSKDESVDpYVPML, where pY represents a phosphotyrosine residue. Phosphatidylinositol was purchased from Sigma (St. Louis, MO). Synthetic peptides were covalently coupled to Actigel ALD-superflow beads (Sterogene Bioseparations, Carlsbad, CA) as described by the manufacturer, at a peptide concentration of 1 mg/ml. Jurkat and NKL cells were lysed using Triton X-100 lysis buffer (1% Triton X-100, 20 mM Tris (pH 7.4), 50 mM NaCl, 1 mM EDTA, 5 mM 2-ME, and protease inhibitors (Complete, Boehringer Mannheim, Indianapolis, IN)), the nuclei were removed by centrifugation, and cell lysates were precleared with protein A-Sepharose beads. Cell lysates were incubated with 20 ml (20 mg of peptide) of peptide coupled to Actigel.
PI-3 kinase assays were conducted as previously described (12). Extracted phospholipids were analyzed by TLC. TLC plates were exposed to a phosphor screen and images analyzed using ImageQuant software (Molecular Dynamics, Sunnyvale, CA).
Immunoprecipitations and Western blotting
Cells (2 x 107) were washed in PBS and treated with pervanadate for 10 min at room temperature. Cell lysates were incubated for 2 h at 4°C with 2 µg of anti-CD4 mAb and then incubated with protein G-Sepharose beads for 1.5 h. After precipitation, the beads were washed five times with lysis buffer before elution of the proteins by boiling for 5 min in SDS-PAGE sample buffer. Samples were electrophoresed in 12% polyacrylamide gels and proteins were transferred to polyvinylidene difluoride (PVDF)-membranes (Hybond-P, Amersham Laboratories, Amersham, U.K.) by semidry blotting. Membranes were blocked in 4% nonfat milk in PBS containing 0.1% Tween 20 for 1 h. PI-3 kinase, Grb2, or tyrosine phosphorylated proteins were detected by appropriate Abs followed by HRP secondary reagents. Bound HRP-conjugated goat anti-mouse or -rabbit Ab was visualized using enhanced chemiluminescence (ECL, Amersham).
CD4-KAP10 stimulation
Cells were incubated for 1 h in the presence 1 µM PI-3 kinase inhibitor wortmannin (Calbiochem, La Jolla, CA) or in medium. Thereafter, cells were stimulated with anti-CD4 mAb (L3T4) and cross-linker (F(ab')2 goat anti-rat IgG for the indicated time periods. Cells were then lysed in sample buffer and boiled for 5 min. Following SDS-PAGE and transfer to PVDF membrane (Hybond-P, Amersham Laboratories), phosphorylated Akt was detected with a phospho-Akt Ab (New England Biolabs) followed by an HRP-conjugated goat anti-rabbit Ab (Immunotech). Membranes were stripped at 55°C for 1 h and reprobed with anti-Akt Ab.
| Results and Discussion |
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There are many instances in the mammalian genome of linked genes
with related functions, such as the MHC and the CD3 gene complex
(13, 14). We searched in the vicinity of the human DAP12
gene for related sequences. A novel 2.0-kb gene was identified on a
genomic clone containing the DAP12 gene. We obtained corresponding EST
clones. Comparison of the composite cDNA with the human genomic
sequence revealed that the gene, which we termed KAP10, comprises four
exons and is located in a tail-to-tail orientation within 100-bp of
DAP12. As shown in Fig. 1
, within the
504-bp cDNA clone is an open reading frame (ORF) of 282 bp, encoding a
putative type I membrane protein of 94 amino acids with a molecular
mass of
9.5 kDa. The amino acid sequence of KAP10 included a
putative 19 amino acid signal peptide. The molecular mass of the mature
protein is 7.5 kDa. The predicted extracellular domain spans 25 amino
acids and contains two cysteine residues proximal to the TM. An
aspartate residue is found in the middle of the TM domain, a feature
similar to DAP12. Comparison with either CTLA-4 or CD28 reveals
homology surrounding the cytoplasmic tyrosine residue and provides some
insight as to the function of KAP10 (Fig. 1
B). The
cytoplasmic tail contains a tyrosine-containing motif (YINM)
corresponding to a putative PI-3 kinase SH2 domain-binding motif
(pYxxM) (15), a Grb2 site (pYxN), and an Shc site
(pY-Y[E/I]x[I/L/M]) (16).
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Northern blot analysis using a KAP10-specific probe identified a
500-bp transcript (Fig. 2
A). KAP10 was shown to be
expressed in peripheral blood, spleen, and thymus while KAP10
transcripts were undetectable in prostate, testis, ovary, small
intestine, or colon. RT-PCR revealed expression in monocyte (U937,
THP1), T (Jurkat, H9), B (RAJI, LCL721) and NK (NK92, NKL) cell lines
and in decidual NK and macrophage cDNA libraries but not the fibroblast
cell line 293T, nor the neural cell lines IMR32, SK-N-BE, and
SY5Y. This expression pattern shows that KAP10 transcripts are
predominantly expressed in cells of a hemopoietic lineage. KAP10
protein was detected by Western blotting. Under reducing conditions,
KAP10 ran at
10 kDa and could be identified in extracts of primary
NK cells, CD8 T cells, and myeloid cell lines, and to a lesser extent
in Jurkat T cells (Fig. 2
B). Although KAP10 was also
observed in some EBV-transformed B cell lines (e.g., C1R), its
expression in normal B cells has yet to be determined. Under
nonreducing conditions, KAP10 migrated as a 20-kDa protein in CD8 T
cells (Fig. 2
C) and in KAP10-transfected COS cells (data not
shown), suggesting that KAP10 may be present as a homodimer or a
heterodimer, possibly via disulfide linkage between either of the
cysteines in the extracellular domain.
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The YINM motif in the cytoplasmic domain of KAP10 is reminiscent
of the PI-3 kinase binding motifs found in CD28 and CTLA-4. We showed
that the KAP10 sequence is also a binding site for PI-3 kinase.
Initially, synthetic peptides corresponding to this motif were used.
Peptides KAP10-pY71 and KAP10-Y71, representing the
phosphorylated and nonphosphorylated versions
of the amino acid sequence, were coupled to an affinity matrix Actigel
and incubated with whole cell extracts of Jurkat and NKL
cells. PI-3 kinase activity was associated specifically with the
tyrosine-phosphorylated KAP10 peptide and not its
unphosphorylated counterpart (Fig. 3
A). A peptide corresponding
to the PI-3 kinase binding site upon the platelet-derived growth factor
receptor (PDGFR-pY740/pY751), precipitated a relatively greater
quantity of PI-3 kinase activity, most likely due to differences in the
activation potential of this peptide, sicne this site consists of two
phosphotyrosines capable of dimerizing PI-3 kinase (17),
as compared with the single phosphotyrosine site present in the KAP10
sequence. These results demonstrate that phosphorylated
KAP10 can bind native PI-3 kinase from NKL and Jurkat T cells. To
confirm that the endogenous KAP10 protein also associated with PI-3
kinase, we used an anti-KAP10 antiserum for immunoprecipitation
experiments from pervanadate-treated NKL cells (Fig. 3
B). As
predicted by our in vitro peptide binding data, native KAP10 was found
to have associated PI-3 kinase activity.
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To investigate KAP10-mediated signaling, we generated murine
CD4-human KAP10 chimeras. The chimera was expressed in Jurkat T
cells, and expression was determined by FACS using an anti-murine
CD4 mAb (Fig. 4
A). Cells were
then treated with the tyrosine phosphatase inhibitor pervanadate to
induce tyrosine phosphorylation. Immunoprecipitation of
CD4-KAP10 followed by immunoblotting with a phosphotyrosine-specific
mAb showed that KAP10 was phosphorylated following
pervanadate treatment (Fig. 4
B). Additionally, probing
immunoprecipitates with appropriate mAbs showed a constitutive
association of both PI-3 kinase and Grb2 with KAP10, which was
significantly increased upon phosphorylation of KAP10
(Fig. 4
, C and D, respectively). Thus, KAP10
contains a binding motif for both Grb2 and PI-3 kinase and recruits
both these proteins in a phosphorylation-dependent
manner.
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It has been reported that CD28 stimulation triggers recruitment of
PI-3 kinase, leading to Akt (PKB/RAC; protein kinase B/related to A and
C protein kinases) activation and ultimately to cell survival (reviewed
in Ref. 18). Since KAP10 also recruits PI-3 kinase, we
tested whether stimulation of KAP10 leads to a similar activation of
Akt. The phosphorylation status of Akt was assessed
using a phospho-Akt-specific Ab. We found that KAP10
cross-linking led to phosphorylation of Akt (Fig. 5
A). Akt
phosphorylation was dependent upon the activity of PI-3
kinase, since it was abolished in the presence of the PI-3 kinase
inhibitor wortmannin. (Fig. 5
B). By influencing the
activation status of Akt, KAP10 may be involved in transducing an
important cell survival signal.
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Note. KAP10 was recently described as DAP10 by Wu et al., who showed that one of its partner chains is the C-type lectin NKG2D (19).
| Acknowledgments |
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| Footnotes |
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2 C.C., J.D., and A.G.H. contributed equally. ![]()
3 Address correspondence and reprint requests to Dr. Michael Wilson, Department of Pathology, Division of Immunology, University of Cambridge, Cambridge, CB2 1QP, U.K. E-mail address: ![]()
4 Abbreviations used in this paper: TM, transmembrane; PI-3 kinase, phosphatidylinositol-3 kinase; DAP12, DNAX-activating protein of 12 kDa; SH2, src homology domain-2; KAP10, PI-3 kinase-associated protein of
10 kDa; EST, expressed sequence tag; wort, wortmannin. ![]()
Received for publication August 3, 1999. Accepted for publication September 3, 1999.
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