From the Department of Biochemistry, Hiroshima University School of Medicine, 1-2-3 Kasumi, Minami-ku, Hiroshima 734, Japan
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ABSTRACT |
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Ral-binding protein 1 (RalBP1) is a putative effector protein of Ral and exhibits a GTPase activating activity for Rac and CDC42. To clarify the function of RalBP1, we isolated a novel protein that interacts with RalBP1 by yeast two-hybrid screening and designated it POB1 (partner of RalBP1). POB1 consists of 521 amino acids, shares a homology with Eps15, which has been identified as an epidermal growth factor (EGF) receptor substrate, and has two proline-rich motifs. The POB1 mRNA was expressed in cerebrum, cerebellum, lung, kidney, and testis. POB1 interacted with RalBP1 in COS cells and the C-terminal region of POB1 was responsible for this interaction. The binding domain of RalBP1 to POB1 was distinct from its binding domain to Ral. Ral and POB1 simultaneously interacted with RalBP1 in COS cells. The binding of POB1 to RalBP1 did not affect the GTPase activating activity of RalBP1. Furthermore, POB1 bound to Grb2 but not to Nck or Crk. POB1 was tyrosine-phosphorylated in COS cells upon stimulation with EGF and made a complex with EGF receptor. These results suggest that RalBP1 makes a complex with POB1 and that this complex may provide a link between tyrosine kinase, Src homology 3 (SH3)-containing protein, and Ral.
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INTRODUCTION |
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Ral is a member of small G protein1 superfamily and consists of RalA and RalB (1, 2). As well as other small G proteins, Ral has the GDP-bound inactive and the GTP-bound active forms. The GDP-bound form of Ral is converted to the GTP-bound form by RalGDS, and inversely the GTP-bound form is changed to the GDP-bound form by RalGAP (3, 4). We and other groups have found that RalGDS is a putative effector protein of Ras (5-7). Since RalGDS stimulates the GDP/GTP exchange of Ral (4), it is possible that there is a new signaling pathway from Ras to Ral through RalGDS. Indeed, it has been shown that RalGDS stimulates the GDP/GTP exchange of Ral in a Ras-dependent manner in COS cells and that a dominant negative form of Ral blocks a Ras-dependent transformation in NIH3T3 cells (8). It has been also demonstrated that Ral is required for Src- and Ras-dependent activation of phospholipase D and that it regulates the initiation of border cell migration induced by Ras in Drosophila oogenesis (9, 10). Furthermore, it has been shown that RalGDS and Raf synergistically stimulate cellular proliferation and gene expression (11, 12). These results indicate that RalGDS and Ral act downstream of Ras and mediate Ras functions. However, the mechanism by which Ral regulates cellular functions is not known.
One possible clue to clarify the mode of action of Ral is RalBP1 (13-15). RalBP1 has a Ral-binding domain in its C-terminal region and binds to the GTP-bound form of Ral but not to the GDP-bound form. A mutation in the effector loop of Ral impairs its interaction with RalBP1 and RalBP1 inhibits the RalGAP activity for Ral (13, 16). These results suggest that RalBP1 is an effector protein of Ral. RalBP1 also has a RhoGAP homology domain in its central region and exhibits the GAP activity for Rac1 and CDC42 but not for RhoA (13-15). Recently we have found that Ral localized to the membrane through its post-translational modification induces translocation of RalBP1 from the cytosol to the membrane and that the post-translational modifications of Rac1 and CDC42 enhance the GAP activity of RalBP1 (17). Therefore, RalBP1 may link Ral to Rac or CDC42 on the membrane. However, there is no evidence obtained so far that RalBP1 regulates the activities of Rac and CDC42 in intact cells. It is not known whether RalBP1 has additional activities.
To gain more insight into the action of RalBP1, we sought to isolate proteins which interact with RalBP1 by yeast two-hybrid screening. We describe here the isolation of a novel RalBP1-interacting protein, which is designated POB1 (partner of RalBP1). POB1 shares a homology with Eps15, which has been identified as an EGF receptor substrate (18), and has two proline-rich motifs. POB1 and Ral simultaneously interact with RalBP1 on the different sites and these proteins make a ternary complex. Furthermore, POB1 binds to Grb2, is tyrosine-phosphorylated in COS cells in response to EGF, and makes a complex with EGF receptor. These results suggest that RalBP1 and POB1 may provide a link between tyrosine kinase, SH3-containing protein, and Ral.
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EXPERIMENTAL PROCEDURES |
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Materials and Chemicals--
Yeast strain L40, plasmid vectors
for two-hybrid screening, pGAD10-derived human brain cDNA library,
and pEF-BOS were kindly supplied from Drs. Y. Takai, K. Tanaka, and S. Nagata (Osaka University, Suita, Japan) (19, 20). gt10 human brain
cDNA library was from Dr. K. Kaibuchi (Nara Institute Science and
Technology, Ikoma, Japan) (21). pGEX/Grb2 was a generous gift from Drs.
H. Miki and T. Takenawa (Institute of Medical Science, University of
Tokyo, Tokyo, Japan) (22, 23). pBSSK/Nck and pGFP/Crk were generous gifts from Drs. H. Hanafusa (Rockefeller University, New York) and M. Matsuda (National Institute of Health, Tokyo, Japan) (24), respectively. The anti-GST and Ral rabbit polyclonal antibodies were
made by a routine method and supplied from Dr. M. Nakata (Sumitomo
Electric Industries, Yokohama, Japan). The anti-HA antibody 12CA5 was
kindly provided by Dr. Q. Hu (Chiron Corp., Emeryville, CA). The
anti-Myc antibody was prepared from 9E10 cells. RalBP1 cDNA was
synthesized by reverse-transcriptase PCR as described (16).
[
-32P]dCTP, [35S]GTP
S, and
[
-32P]GTP were purchased from Amersham Inc.
(Buckinghamshire, United Kingdom). The anti-phosphotyrosine and EGF
receptor antibodies were from ICN Biomedicals, Inc. (Costa Mesa, CA)
and Transduction Laboratories (Lexington, KY), respectively. EGF was
purchased from Life Technologies, Inc. Other materials were from
commercial sources.
Plasmid Constructions--
To construct pBTM116HA/RalBP1,
pUC19/RalBP1 (16) was digested with BamHI, and the 1.9-kb
fragment encoding full-length RalBP1 was inserted into the
BamHI cut pBTM116HA. To construct pCGN/POB1 and
pCGN/POB1-(1-374) (amino acids 1-374), the 1.6- and 1.1-kb fragments
encoding full-length POB1 and POB1-(1-374) with PmaCI and
EcoRI sites were synthesized by PCR. These fragments were digested with PmaCI and EcoRI, blunted with
Klenow fragment, and inserted into the SmaI cut pCGN. To
construct pCGN/POB1-(375-521), pGAD10/POB1-(375-521) was digested
with BglII and inserted into the BamHI cut pCGN.
pBJ-Myc/RalBP1, pMAL/RalBP1, and pMAL/RalBP1-(364-647) were
constructed as described (16, 17). To construct pMAL/RalBP1-(1-210), pUC19/RalBP1 was digested with BamHI and PvuII.
The 0.6-kb fragment encoding RalBP1-(1-210) with BamHI and
PvuII sites was inserted into pMAL-c2, which was digested
with XbaI, blunted with Klenow fragment, and digested with
BamHI. pMAL-c2 was digested with EcoRI, blunted
with Klenow fragment, and self-ligated to generate
pMAL/EcoRI. To construct pMAL/RalBP1-(210-415), the
0.6-kb fragment encoding RalBP1-(210-415) with BamHI sites
synthesized by PCR was digested with BamHI and inserted into
the BamHI cut pMAL/
EcoRI. To construct pMAL/RalBP1-(391-499) and pMAL/RalBP1-(500-647), the 0.3- and 0.44-kb
fragments encoding RalBP1-(391-499) and RalBP1-(500-647) with
BamHI sites were synthesized by PCR and digested with
BamHI, then inserted into the BamHI cut pMAL-c2.
To construct pCGN/RalBP1, pBJ-Myc/RalBP1 was digested with
XbaI and BamHI, and the 1.9-kb fragment encoding
full-length RalBP1 was inserted into the XbaI and
BamHI cut pCGN. To generate pUC19-Myc, pBJ-Myc was digested with XbaI and EcoRI and the fragment encoding Myc
epitope was inserted into the EcoRI-XbaI cut
pUC19. To construct pUC19-Myc/RalBG23V, the 0.6-kb fragment
encoding RalBG23V with XbaI sites synthesized by
PCR was digested with XbaI and inserted into the
XbaI cut pUC19-Myc. To construct
pEF-BOS-Myc/RalBG23V, pUC19-Myc/RalBG23V was
digested with KpnI and HindIII and blunted with
T4 DNA polymerase. The 0.7-kb fragment encoding
Myc-RalBG23V was inserted into pEF-BOS, which was digested
with XbaI and blunted with Klenow fragment. To construct
pGEX/POB1-(375-521), pGAD10/POB1-(375-521) was digested with
NdeI, blunted with Klenow fragment, and digested with
BamHI. The 0.6-kb fragment encoding POB1-(375-521) was
inserted into the SmaI and BamHI cut pGEX2T. To
construct pGEX/Nck, pBSSK/Nck was digested with BamHI and
the 1.1-kb fragment encoding Nck was inserted into the BamHI
cut pGEX2T. To construct pGEX/Crk, pGFP/Crk was digested with
BamHI and the 0.9-kb fragment encoding Crk was inserted into
the BamHI cut pGEX2T. pCGN/RalBG23V,
pCGN/RalBS28N, pGEX/RalB, and pV-IKS/Rac1 were constructed
as described (17, 25, 26).
Yeast Two-hybrid Screening--
Yeast strain L40
(MATa trp1 leu2 his3 ade2 LYS2::lexA-HIS3
URA3::lexA-lacZ) was used as a host for the two-hybrid
screening (27). Yeast cells were grown on rich medium (YAPD) containing 2% glucose, 2% Bacto-peptone, 1% Bacto-yeast extracts, and 0.002% adenine sulfate. Yeast transformations were performed by the lithium acetate method. Transformants were selected on SD medium containing 2%
glucose, 0.67% yeast nitrogen base without amino acids, and appropriate amino acids, which were supplemented to SD medium when
required. A strain L40 carrying pBTM116HA/RalBP1 was transformed with a
human brain cDNA library constructed in pGAD10. pBTM116HA/RalBP1 directs the expression of a fusion between the DNA-binding domain of
LexA and the entire RalBP1 from an ADH promoter.
Approximately 6 × 106 transformants were screened for
the growth on SD plate medium lacking tryptophan, leucine, and
histidine as evidenced by transactivation of a LexA-HIS3
reporter gene and histidine prototrophy. His+ colonies were
scored for -galactosidase activity. Plasmids harboring cDNAs
were recovered from positive colonies and introduced by electroporation
into E. coli HB101 on the M9 plate lacking leucine. HB101 is
leuB
, and this defect can be complemented by
the LEU2 gene in the library plasmid. Then library plasmids
were recovered from HB101 and transformed again into L40 containing
pBTM116HA/RalBP1. The nucleotide sequences of plasmid DNAs, which
conferred the LacZ+ phenotype on L40 containing
pBTM116HA/RalBP1, were determined.
Molecular Cloning of POB1--
The clone identified by the yeast
two-hybrid method contained a region that interacted with RalBP1 and
the 3-end but did not contain the 5
-end of an open reading frame. To
obtain full-length cDNA, the clone was labeled with random primers
and [
-32P]dCTP and used to screen
gt10 human brain
cDNA library. However, the initial screening did not identify the
5
-end of the gene. To obtain more 5
sequence information, the 5
-most
650 nucleotides of the largest clone was labeled with
[
-32P]dCTP to screen the same library, and a number of
positive clones were isolated. All clones, collectively spanning 3.2 kb, were sequenced using double-strand templates and Thermo Sequenase
premixed cycle sequencing kit (Amersham, Buckinghamshire, UK) and
Hitachi DNA sequencer SQ-5500 (Hitachi Ltd., Tokyo, Japan). To
construct full-length POB1 cDNA clone, the cDNA fragment
containing POB1 codons 10-521 (amino acid numbers) from the initial
screening was subcloned into pUC19. The cDNA fragment containing
POB1 codons 1-253 (amino acid numbers) from the second screening was
jointed to this sequence to obtain pUC19/POB1.
Northern Blot Analysis--
Total RNA was extracted from various
rat tissues as described (28). Northern blot analysis was performed as
described (29). Twenty µg of total RNA was then resolved by agarose
gel electrophoresis and transferred to nitrocellulose filters. A 1.6-kb
cDNA probe corresponding to full-length POB1 was labeled with
random primers and [-32P]dCTP and hybridized to the
membrane. The membrane was washed and exposed to Kodak X-Omat film.
Purification of MBP Fused to and GST Fused to Proteins from
Escherichia coli--
MBP fused to RalBP1 (MBP-RalBP1),
MBP-RalBP1-(364-647), GST fused to RalB (GST-RalB), and GST-Grb2 were
purified from E. coli as described (16, 17, 22, 23). The
E. coli expressing MBP-RalBP1-(1-210),
MBP-RalBP1-(210-415), MBP-RalBP1-(391-499), MBP-RalBP1-(500-647),
GST-POB1-(375-521), GST-Crk, and GST-Nck were grown at 37 °C to an
absorbance of 0.5 (absorbance at 600 nm), then
isopropyl--D-thiogalactopyranoside was added at a final concentration of 0.3 mM. After further incubation was
carried out for 2 h at 37 °C, the expressed proteins were
purified in accordance with the manufacturer's instructions.
Approximately 70 amino acids from pMAL-c2 were attached to the C
terminus of RalBP1-(1-210), RalBP1-(210-415), and RalBP1-(391-499).
The post-translationally modified form of Rac1 was purified from
Spodoptera frugiperda 9 cells as described (17, 26)
Interaction of POB1 with RalBP1 in COS Cells-- COS cells (10-cm-diameter plate) transfected with pCGN-, pBJ-, and pEF-BOS-derived plasmids were lysed in 0.5 ml of lysis buffer (20 mM Tris/HCl (pH 7.5), 1% nonidet P-40, 137 mM NaCl, 10% glycerol, 1 mM phenylmethylsulfonyl fluoride, 20 µg/ml aprotinin, and 10 µg/ml leupeptin) as described (30). POB1 and its deletion mutants were tagged with HA epitope at their N termini. RalBP1 was tagged with Myc epitope at its N terminus. The lysates (620 µg of protein) were immunoprecipitated with the anti-Myc or HA antibody. The precipitates were washed once with 20 mM Tris/HCl (pH 7.5), 1% nonidet P-40, 137 mM NaCl, and 10% glycerol, twice with 100 mM Tris/HCl (pH 7.5) and 0.5 M LiCl, and once with 10 mM Tris/HCl (pH 7.5). The precipitates were subjected to SDS-polyacrylamide gel electrophoresis (31), transferred to nitrocellulose filters, and probed with the anti-HA or Myc antibody as described (16, 26, 30). Where specified, the lysates expressing RalBP1 and POB1 with RalG23V or RalS28N were used.
Interaction of POB1 and Ral with RalBP1 in Vitro--
Various
MBP fused to RalBP1 deletion mutants (20 pmol each) were incubated with
GST-POB1-(375-521) (44 pmol) or the GTPS-bound form of GST-Ral (44 pmol) in 40 µl of reaction mixture (20 mM Tris/HCl (pH
7.5), 1 mM dithiothreitol, and 0.05% CHAPS) for 1 h
at 4 °C. The GTP
S-bound form of Ral was made as described (16). The amylose resin was added to this mixture and further incubation was
performed. After 30 min, the resin was precipitated by centrifugation. The precipitates were probed with the anti-GST antibody.
Interaction of POB1 with Grb2, Nck, and Crk in Vitro-- The lysates (465 µg of protein) of COS cells expressing HA-POB1 were incubated with 1 µM GST-Grb2, GST-Nck, GST-Crk, or GST in 170 µl of the lysis buffer for 1 h at 4 °C. GST fused to proteins was precipitated with glutathione-Sepharose 4B. The precipitates were probed with the anti-HA and GST antibodies.
Complex Formation of POB1 with EGF Receptor--
COS cells
expressing HA-POB1 with or without Myc-RalBP1 were serum-starved for
24 h before stimulation with 100 ng/ml EGF for 15 min. The lysates
(620 µg of protein) were prepared as described above except that 5 mM sodium orthovanadate and 50 mM
-glycerophosphate were added to the lysis buffer and
immunoprecipitated with the anti-HA antibody. The precipitates were
probed with the anti-HA, Myc, phosphotyrosine, or EGF receptor
antibody.
Other Methods--
The assay for the interaction of the
[35S]GTPS-bound form of Ral with immobilized
MBP-RalBP1 in the presence of GST-POB1-(375-521) was carried out as
described (17). The GAP activity of RalBP1 for the post-translationally
modified Rac1 was measured in the presence of GST-POB1-(375-521) as
described (17). Protein concentrations were determined using bovine
serum albumin as a standard (32).
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RESULTS |
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Isolation of RalBP1-interacting Protein-- To identify proteins that physically interact with RalBP1, we conducted a human brain cDNA library screening with the yeast two-hybrid method. From 6 × 106 initial transformants, we identified five positive clones (His+ and LacZ+) and the library plasmids were recovered from the clones. Among these five plasmids, three clones were found to confer both the His+ and LacZ+ phenotypes on L40 containing pBTM116HA/RalBP1. The cDNA inserts from these three clones were 1 kb, and all encoded a single sequence containing an open reading frame of 147 amino acids and the consensus sequence for a stop codon.
To identify full-length cDNA of this RalBP1-interacting protein, a human brain cDNA library was screened using the cDNA isolated by the yeast two-hybrid method and a number of overlapping positive clones were isolated. The overlapping clones spanned a distance of 3.2 kb and contained an uninterrupted open reading frame of 1563 base pairs, encoding a predicted protein of 521 amino acids with a calculated Mr of 57,901 (Fig. 1A). We designated this protein POB1 for partner of RalBP1. The first ATG was preceded by stop codons in all three reading frames and the 5
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Interaction of POB1 with RalBP1 in COS Cells-- To examine whether POB1 interacts with RalBP1 in intact cells, we coexpressed HA-POB1 with Myc-RalBP1 in COS cells (Fig. 3A, lanes 1-3). The lower bands, which were seen under POB1, might be its degradation products. When the lysates coexpressing HA-POB1 with Myc-RalBP1 were immunoprecipitated with the anti-Myc antibody, both HA-POB1 and Myc-RalBP1 were detected in the RalBP1 immune complex (Fig. 3B, lanes 1-3). Similarly when the same lysates were immunoprecipitated with the anti-HA antibody, both HA-POB1 and Myc-RalBP1 were detected in the POB1 immune complex (Fig. 3B, lane 6). Neither HA-POB1 nor Myc-RalBP1 was immunoprecipitated from the lysates coexpressing HA-POB1 and Myc-RalBP1 with non-immune immunoglobulin (data not shown). These results indicate that POB1 interacts with RalBP1 in intact cells. By the yeast two-hybrid screening, POB1-(375-521) was identified as a RalBP1-binding domain. We examined whether this domain is essential for the binding of POB1 to RalBP1. Myc-RalBP1 was coexpressed with HA-POB1-(1-374) or HA-POB1-(375-521) in COS cells (Fig. 3A, lanes 4 and 5). When the lysates coexpressing Myc-RalBP1 with HA-POB1-(1-374) or HA-POB1-(375-521) were immunoprecipitated with the anti-Myc antibody, HA-POB1-(375-521), but not HA-POB1-(1-374), was detected in the RalBP1 immune complex (Fig. 3B, lanes 4 and 5). Similarly when the same lysates were immunoprecipitated with the anti-HA antibody, Myc-RalBP1 was detected in the POB1-(375-521) immune complex but not in the POB1-(1-374) immune complex (Fig. 3B, lanes 7 and 8). These results indicate that POB1-(375-521) is necessary and sufficient for the binding of POB1 to RalBP1.
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Interaction of POB1 with RalBP1 in Vitro--
To examine which
region of RalBP1 interacts with POB1, various RalBP1 deletion mutants
were purified as MBP fused to proteins and POB1-(375-521) was purified
as a GST fused to protein from E. coli (Fig.
4A). After MBP-RalBP1 deletion
mutants were incubated with GST-POB1-(375-521), the amylose resin was
added. The resin was precipitated by centrifugation, and the
precipitates were probed with the anti-GST antibody. POB1-(375-521)
interacted with full-length RalBP1, RalBP1-(364-647), and
RalBP1-(500-647) but not with RalBP1-(1-210), RalBP1-(210-415), or
RalBP1-(391-499) (Fig. 4B). These results indicate that the
binding of POB1 to RalBP1 is direct and that the C-terminal region of
RalBP1 has the POB1-binding domain. It has been shown that the
GTP-bound form of Ral binds to the C-terminal region of RalBP1
(RalBP1-(364-647)) (13-15). To examine whether Ral and POB1 bind to
the same site of RalBP1, MBP-RalBP1 mutants were incubated with the
GTPS-bound form of GST-Ral, then the amylose resin was added. The
resin was precipitated by centrifugation and the precipitates were
probed with the anti-GST antibody. As consistent with the previous
observations (13-15), Ral bound to full-length RalBP1 and
RalBP1-(364-647), but not to RalBP1-(1-210) or
RalBP1-(210-415) (Fig. 4C, lanes 1-4). In the region of
amino acids 364-647 of RalBP1, Ral interacted with RalBP1-(391-499)
but not with RalBP1-(500-647) (Fig. 4C, lanes 5 and
6). These results clearly show that the POB1-binding domain
of RalBP1 is distinct from the Ral-binding domain of RalBP1.
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Ternary Complex Formation of Ral, RalBP1, and POB1--
To examine
whether the interaction of RalBP1 with POB1 affects the Ral binding
activity of RalBP1, POB1-(375-521) and the [35S]GTPS-bound form of Ral were incubated with
immobilized RalBP1. One µM
[35S]GTP
S-bound form of Ral was used in this
experiment. In the absence of POB1-(375-521), Ral bound to RalBP1 at a
molar ratio 0.8:1 (17). POB1-(375-521) did not affect the interaction
of Ral with RalBP1, even though 9-fold excess of POB1-(375-521) was added (Fig. 5A). Under these
conditions, POB1-(375-521) bound to RalBP1 in a
dose-dependent manner (Fig. 5A, inset). To
clarify the interaction of these three proteins in intact cells,
Myc-RalBP1 and HA-POB1 were expressed with HA-RalG23V, an
active form, or HA-RalS28N, a dominant negative form, in
COS cells (Fig. 5B, lanes 1 and 2). When
Myc-RalBP1 was immunoprecipitated with the anti-Myc antibody, both
HA-POB1 and HA-RalG23V were coprecipitated with Myc-RalBP1
(Fig. 5B, lane 3). HA-POB1 but not HA-RalS28N
was coprecipitated with Myc-RalBP1 (Fig. 5B, lane 4). The
binding of Ral to RalBP1 was not found to affect that of POB1 to
RalBP1. RalBP1 and RalG23V tagged with HA and Myc epitopes,
respectively, were constructed, and further experiments to show the
ternary complex formation of Ral, RalBP1, and POB1 were carried out.
When Myc-RalG23V was immunoprecipitated with the anti-Myc
antibody from the lysates expressing HA-RalBP1, HA-POB1, and
Myc-RalG23V, both HA-RalBP1 and HA-POB1 were detected in
the Ral immune complex (Fig. 5B, lanes 5 and
8). However, when the lysates expressing HA-RalBP1 and
HA-POB1 without Myc-RalG23V or the lysates expressing
HA-POB1 and Myc-RalG23V without HA-RalBP1 were
immunoprecipitated with the anti-Myc antibody, HA-POB1 was not
precipitated in these immune complexes (Fig. 5B, lanes 6, 7, 9, and 10). These results indicate that the binding of
POB1 to RalBP1 does not affect the Ral-binding activity of RalBP1, that
activated Ral and POB1 can simultaneously interact with RalBP1, and
that these proteins make a ternary complex in intact cells.
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Effect of the Interaction of POB1 with RalBP1 on the GAP Activity of RalBP1-- It has been shown that RalBP1 possesses the GAP activity for Rac and CDC42 (13-15) and that the binding of Ral to RalBP1 does not affect the GAP activity (15, 17). The effect of the interaction of POB1 with RalBP1 on the GAP activity was examined. RalBP1 stimulated the GTPase activity of Rac in a dose-dependent manner (Fig. 6). The GAP activity of RalBP1 for Rac was not affected by the interaction of POB1-(375-521) (Fig. 6).
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Interaction of POB1 with Grb2-- POB1 has two proline-rich motifs, PPTPPPRP345 and PPPPALPPRP383 (Fig. 1A). These amino acid sequences belong to class II ligands of proline-rich motifs, which are known to associate with SH3 domains of adaptor proteins such as Grb2, Nck, and Crk (34, 35). To examine the possibility of the interaction of POB1 with these adaptor proteins, Grb2, Nck, and Crk were purified as GST fused to proteins. The lysates of COS cells expressing HA-POB1 were incubated with GST-Grb2, GST-Nck, GST-Crk, or GST. POB1 bound to Grb2 specifically among these proteins (Fig. 7).
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Complex Formation of POB1 with EGF Receptor-- Besides proline-rich motifs, POB1 has a homologous region with Eps15, which has been identified as an EGF receptor substrate (Fig. 1). Therefore, it was examined whether POB1 is involved in tyrosine kinase signaling through EGF receptor. When COS cells expressing HA-POB1 were stimulated with EGF, HA-POB1 was tyrosine-phosphorylated (Fig. 8A). This tyrosine phosphorylation did not affect the interaction of HA-POB1 with Myc-RalBP1 in intact cells (Fig. 8B). Furthermore, HA-POB1 was immunoprecipitated with a tyrosine-phosphorylated 170-kDa protein in response to EGF, and this protein was recognized by the anti-EGF receptor antibody (Fig. 8C). Taken together, these results indicate that POB1 is tyrosine-phosphorylated in response to EGF and that it may make a complex with EGF receptor through an adaptor protein such as Grb2.
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DISCUSSION |
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We have isolated here a protein interacting with RalBP1 and
determined its primary structure. This protein is a novel protein and
named POB1. We have found that the C-terminal region (amino acids
375-521) of POB1 is important for its binding to RalBP1 using intact
cell and cell-free systems. Furthermore, we have shown that the
POB1-binding domain of RalBP1 is distinct from the Ral-binding domain,
that POB1 does not compete with the GTPS-bound form of Ral for
binding to RalBP1, and that RalBP1 and POB1 make a complex with an
activated form of Ral but not with a dominant negative form of Ral.
These results suggest that POB1 and RalBP1 make a binary complex in
resting conditions and that POB1-RalBP1 complex binds to Ral when Ral
is activated. Sequence analysis reveals that the RalBP1-binding domain
of POB1 and the POB1-binding domain of RalBP1 have high probabilities
of forming coiled coil (14, 36). Therefore, the interaction between
POB1 and RalBP1 may be mediated by these coiled-coil structures.
Although we do not know the physiological functions of POB1 at present,
POB1 could be involved in the actions of Ral and RalBP1. RalBP1 is known to possess the GAP activity for Rac and CDC42 (13-15). The interaction of POB1 with RalBP1 does not affect its GAP activity for
Rac1. It has been also shown that the binding of Ral to RalBP1 does not
affect its GAP activity (15, 17). Therefore, the mechanism by which
RalBP1 GAP activity is regulated is still unclear. Northern blot
analysis has revealed that the RalBP1 mRNA is present in various
rat tissues (13, 15), including not only tissues where the POB1
mRNA is detectable but also tissues where it is not detectable. For
instance, the RalBP1 mRNA, but not the POB1 mRNA, is present in
heart, liver, and spleen. These results raise the possibility that
RalBP1 interacts with another protein in the tissues where POB1 is
absent.
POB1 has two proline-rich motifs, PPTPPPRP (amino acids 338-345) and PPPPALPPRP (amino acids 374-383). Proline-rich motifs have been described as general cognate ligands for numerous SH3-containing proteins (37). The ligands can bind to SH3 domains in either an amino-carboxyl or carboxyl-amino orientation and are therefore classified as class I and class II (34, 35). Class I ligands contain (R/K/X)PXPPXP and bind to SH3 domains of Src, Abl, Fyn, and Lyn. Proline-rich motifs of POB1 belong to class II ligands, whose general form is PXXPXR. The class II ligands are the sites for SH3 domains of adaptor proteins such as Grb2, Nck, and Crk. We have found that among these adaptor proteins POB1 interacts with Grb2 specifically. Grb2 plays a central role in signaling by receptor tyrosine kinase, where its SH2 domain binds to the receptor and its two SH3 domains link to other signaling proteins such as SOS, Vav, c-Abl, and dynamin (38). These results raise the possibility that POB1 may make a complex with the receptor tyrosine kinase. Besides proline-rich motifs, POB1 has a homologous region with Eps15, which has been identified as an EGF receptor substrate (18). As to the relationship between POB1 and EGF receptor, we have found that POB1 is tyrosine-phosphorylated in response to EGF and that it makes a complex with EGF receptor. Taken together, these results suggest that POB1 may be recruited to EGF receptor through an adaptor protein (Grb2 is a candidate). We have not yet determined here the tyrosine phosphorylation site of POB1. Although POB1 has two consensus tyrosine sites, Tyr146 and Tyr229, for the phosphorylation by EGF receptor (39), it remains to be clarified whether EGF receptor phosphorylates POB1 directly or other tyrosine kinase does it. This tyrosine phosphorylation of POB1 does not affect its interaction with RalBP1. Therefore, the physiological significance of this phosphorylation also remains to be elucidated.
We have previously demonstrated that EGF induces the interaction of Ras with RalGDS and that activated Ras translocates RalGDS from the cytosol to the membrane (16, 30). We have also shown that EGF activates Ral through RalGDS and that activated Ral induces the translocation of RalBP1 (17, 40). It is not known whether POB1 acts upstream or downstream of RalBP1. If POB1 works as an upstream molecule of RalBP1, it may transmit the signal from EGF receptor to RalBP1 and collaborate with Ral to regulate the RalBP1 functions. If POB1 works as a downstream molecule of RalBP1, the Ral-RalBP1 complex may regulate the POB1 functions in synergy with the EGF receptor signaling. In either case, the ternary complex of Ral, RalBP1, and POB1 could mediate the tyrosine kinase and Ras signals to downstream events.
Eps15 consists of three domains, I, II, and III. Domain I (amino acids
9-314) is composed of three nonidentical repeats of about 70 amino
acids each. The individual repeats are named EH domain (for Eps15
homology) 1, 2, and 3 (18, 41). POB1-(143-208) shares 30, 39, and 33%
identity with EH domains 1, 2, and 3, respectively. EH domain of Eps15
has been suggested to be tyrosine-phosphorylated by EGF receptor, and
overexpression of Eps15 can transform NIH3T3 cells (18). EH domain has
been shown to bind to cytosolic proteins in normal and malignant cells,
although they have not yet been identified (41). EH domain is conserved
in Eps15r, yeast PAN1, yeast END3, and Caenorhabditis
elegans YNJ6 (41). PAN1 in yeast is required for normal
organization of the actin cytoskeleton and associates with the actin
patches on the cell cortex (42). END3 is required for the
internalization step of endocytosis and for actin cytoskeleton
organization (43). It has been shown that Ral is localized to synaptic
vesicles in neuron and -granules in platelets (44, 45) and that
RalBP1 exhibits GAP activity for Rac and CDC42, which control
cytoskeleton (46). Therefore, it is intriguing to speculate that Ral
and RalBP1 regulate endocytosis and cytoskeleton organization through
POB1. The study to isolate the protein interacting with EH domain of
POB1 is under way. Further studies are necessary to understand the
whole picture of the functions of a Ras-signaling pathway through
RalGDS, Ral, and RalBP1.
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ACKNOWLEDGEMENTS |
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We are grateful to Drs. Y. Takai, K. Tanaka, S. Nagata, K. Kaibuchi, H. Miki, T. Takenawa, M. Matsuda, H. Hanafusa, M. Nakata, and Q. Hu for their plasmids, cDNA libraries, and antibodies. We thank N. Kamiryo and M. Kodama for excellent technical assistance. We thank the Research Center for Molecular Medicine, Hiroshima University School of Medicine, for the use of their facilities.
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FOOTNOTES |
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* This work was supported by grants-in-aid for scientific research (B) and for scientific research on priority areas from the Ministry of Education, Science, and Culture, Japan (1996, 1997) and by grants from Yamanouchi Foundation for Research on Metabolic Disorders (1996), Fukuyama Transporting Shibuya Longevity and Health Foundation (1996), and Kato Memorial Bioscience Foundation (1997).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.
The nucleotide sequence(s) reported in this paper has been submitted to the GenBankTM/EMBL Data Bank with accession number(s) AF010233.
To whom all correspondence should be addressed: Dept. of
Biochemistry, Hiroshima University School of Medicine, 1-2-3 Kasumi, Minami-ku, Hiroshima 734, Japan. Tel.: 81-82-257-5130; Fax:
81-82-257-5134; E-mail: akikuchi{at}mcai.med.hiroshima-u.ac.jp.
1
The abbreviations used are: G protein,
GTP-binding protein; RalGDS, Ral GDP dissociation stimulator; GAP,
GTPase-activating protein; RalBP1, Ral-binding protein 1; EGF,
epidermal growth factor; SH3, Src homology 3; GST, glutathione
S-transferase; HA, hemagglutinin 1; PCR, polymerase chain
reaction; GTPS, guanosine-5
-0-(3-thiotriphosphate); MBP,
maltose-binding protein; CHAPS,
3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonic acid;
kb, kilobase(s).
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REFERENCES |
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