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J. Biol. Chem., Vol. 276, Issue 3, 1677-1680, January 19, 2001

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ACCELERATED PUBLICATION
Rac/Cdc42 and p65PAK Regulate the Microtubule-destabilizing Protein Stathmin through Phosphorylation at Serine 16^*

Henrik Daub^§, Kris Gevaert^¶, Joel Vandekerckhove^¶, André Sobel^**, and Alan Hall^§§

From the  Medical Research Council Laboratory for Molecular Cell Biology, Cancer Research Campaign Oncogene and Signal Transduction Group, and the  Department of Biochemistry, University College London, Gower Street, London WC1E 6BT, United Kingdom, the ^¶ Department of Biochemistry and Medical Protein Research, Flanders Interuniversity Institute for Biotechnology, Ghent University, B-900 Belgium, and ^** INSERM U440, IFM, 17 rue du Fer à Moulin, 75005 Paris, France

Received for publication, September 12, 2000, and in revised form, October 27, 2000

	ABSTRACT

TOP ABSTRACT INTRODUCTION EXPERIMENTAL PROCEDURES RESULTS AND DISCUSSION REFERENCES

We have identified a rapid protein phosphorylation event at residue serine 16 of stathmin using two-dimensional gel electrophoresis coupled to matrix-assisted laser desorption/ionization mass spectrometry in combination with post-source decay analysis, which is induced by the epidermal growth factor receptor. Phosphorylation is specifically mediated by the small GTPases Rac and Cdc42 and their common downstream target, the serine/threonine kinase p65PAK. Both GTPases have previously been shown to regulate the dynamics of actin polymerization. Because stathmin destabilizes microtubules, and this process is inhibited by phosphorylation at residue 16, Rac and Cdc42 can potentially regulate both F-actin and microtubule dynamics.

	INTRODUCTION

TOP ABSTRACT INTRODUCTION EXPERIMENTAL PROCEDURES RESULTS AND DISCUSSION REFERENCES

Members of the Rho GTPase family, Rho, Rac, and Cdc42, control the assembly of filamentous actin structures in all mammalian cells (1). Their ability to link extracellular signals to the reorganization of the actin cytoskeleton suggests that they are likely to be important regulators of actin-driven cell processes, and Rac, for example, is crucial for growth cone guidance and cell migration both in tissue culture cells and in vivo in Drosophila and Caenorhabditis elegans (2-4). We report here that activation of Rac and to a lesser extent Cdc42 by EGF¹ leads to the phosphorylation of stathmin at residue 16. Phosphorylation at this site has been shown to inhibit stathmin-induced destabilization of microtubules, and our results suggest, therefore, that Rac and Cdc42 can regulate the dynamics of both the actin and the microtubule cytoskeletons (5, 6).

	EXPERIMENTAL PROCEDURES

TOP ABSTRACT INTRODUCTION EXPERIMENTAL PROCEDURES RESULTS AND DISCUSSION REFERENCES

Reagents, Antibodies, and Plasmids-- Human recombinant EGF was from Collaborative Biomedical Products. Toxins B1470 and B10463 were gifts from C. von Eichel-Streiber (Inst. for Microbial Medicine, Mainz, Germany). CNF-1 was a gift from G. Schmidt (University of Freiburg, Germany). Polyclonal antisera directed against stathmin and stathmin that has been phosphorylated at serines 16, 25, or 38 were described previously (7). Monoclonal phosphospecific anti-ERK1,2 antibody was from Sigma. The PAK1-(83-149) autoinhibitory fragment cloned into pRK5Myc was derived from murine PAK1 (8). All other expression plasmids encoded Myc-tagged proteins and have been described previously (7, 9).

Cell Culture and Transfections-- HEp-2 cells were cultured in Dulbecco's modified Eagle's medium supplemented with 10% fetal bovine serum. 1 × 10⁵ cells per well were seeded into 12-well dishes and serum-starved 24 h later. For preparative purposes, cells were seeded at 1 × 10⁶ cells per 10-cm dish and were serum-starved 2 days later for another 20 h prior to lysis. For [³²P] or [³³P]orthophosphate labeling, cells were incubated in phosphate-free medium in the presence of 12.5 µ Ci/ml ³²P_i for preparative or 25 µ Ci/ml ³²P_i or 100 µ Ci/ml ³³P_i for analytical purposes for 3 h prior to lysis. For transfection experiments, HEp-2 cells were seeded at 2 × 10⁵ per well into 6-well dishes 20 h before transfection. Cells were incubated for 4 h in 1.0 ml of serum-free medium containing 6 µl of LipofectAMINE (Life Technologies, Inc.) and 1.5 to 1.7 µg of total DNA per well. The transfection mixture was supplemented with 1 ml of medium containing 20% fetal bovine serum, and cells were lysed 20 h later.

Cell Lysis, Gel Electrophoresis, and Immunoblotting-- Cell lysis was performed in 50 mM Tris, pH 7.5, 100 mM dithiothreitol, 0.3% SDS, 5 mM sodium pyrophosphate, 150 units/ml benzonase plus additives (1 mM EDTA, 2 mM MgCl₂, 1 mM sodium fluoride, 1 mM orthovanadate, 10 µg/ml aprotinin, 10 µg/ml leupeptin, 1 mM phenylmethylsulfonyl fluoride). For immunoprecipitations, lysis buffer contained 50 mM Hepes pH 7.5, 150 mM NaCl, 0.4% Triton X-100, 10 mM sodium pyrophosphate, 10% glycerol plus additives. Both buffers solubilized stathmin to the same extent. For two-dimensional gel electrophoresis, isoelectric focusing was performed using linear 11-cm immobilized pH 4-7 gradient drystrips on an IPGphor (Amersham Pharmacia Biotech). For preparative purposes, the final focusing step was at 8000 V for 4 h.

Protein Purification-- Fourteen 10-cm dishes of serum-starved and ³²P_i-labeled HEp-2 cells were stimulated with 100 ng/ml EGF for 7 min and lysed (350 µl per dish). Lysates were precleared, supplemented with 80 µl of 20% SDS and 120 µl of 87% glycerol per ml, and the sample containing about 30 mg of total protein was loaded on the Model 491 Prep Cell (Bio-Rad). Preparative gel electrophoresis was performed utilizing a 11% separating gel. Aliquots containing proteins in the 19,000- to 23,000-dalton range were precipitated and analyzed by two-dimensional gel electrophoresis and autoradiography of silver-stained gels (10). The 21,000-dalton phosphoprotein of interest, which was found in two fractions, was dried to one-tenth of its original volume, precipitated, and subjected to preparative two-dimensional gel electrophoresis.

MALDI-MS Analysis-- The protein spot of interest was visualized by Coomassie staining and excised, and the gel fragment was washed in water. The gel piece was allowed to shrink in 100 µl of acetonitrile/water (1:1) (both Baker HPLC Analyzed, Mallinckrodt Baker B.V., Deventer, The Netherlands) for 30 min and dried in a centrifugal vacuum concentrator. 10 µl of 50 mM ammonium bicarbonate buffer containing 0.05 µg of sequencing grade modified trypsin (Promega) was added followed by ammonium bicarbonate buffer to submerge the gel piece. Digestion proceeded overnight at 37 °C and was stopped by acidification. The supernatant was removed from the gel pieces and a fraction (10%) was purified/concentrated on added Poros® 50 R2 beads (Roche Molecular Biochemicals GmbH, Mannheim, Germany) and used for direct MALDI-MS peptide mass fingerprint analysis (11). The remainder of the digestion mixture was separated on a RP-HPLC column; eluting peptides were automatically collected on Poros R2 beads and used for MALDI-PSD measurements (12). Measurements were performed on a Bruker Reflex III MALDI-TOF-MS (Bruker Daltonik GmbH, Bremen, Germany) operating in the reflectron mode. RP-HPLC fractions were first scanned in the reflectron mode, and candidate peptides were selected and their PSD spectra recorded (12). The information present in the PSD spectra was used by the SEQUEST algorithm (13) to identify the protein in a public nonredundant protein database.

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION EXPERIMENTAL PROCEDURES RESULTS AND DISCUSSION REFERENCES

Rac interacts with numerous cellular targets including at least three families of Ser/Thr kinases, p65PAK, MLK, and p70S6kinase. To identify phosphorylation events controlled by Rac, metabolically labeled HEp-2 cells were pretreated with either clostridial toxin B1470, which inactivates Rac, R-ras, Rap1, Ral and, to a lesser extent, Cdc42, or with toxin B10463, which inactivates Rho, Rac, and Cdc42 (14, 15). Cells were then stimulated with EGF, a known activator of Rac, for 7 min and cell lysates were analyzed by two-dimensional gel electrophoresis. The increase in intensity of only 2 of about 20 EGF-stimulated phosphoproteins was blocked by both toxins (Fig. 1A). One was isolated from a preparative two-dimensional gel and subjected to MALDI-MS post-source decay analysis. The excised protein was identified as stathmin.

View larger version (26K): [in this window] [in a new window]   Fig. 1.   EGF-induced and GTPase-dependent phosphorylation of stathmin. A, HEp-2 cells were serum-starved for 18 h and labeled with 100 µCi/ml 33Pi for 3 h. Cells were pretreated with 160 ng/ml toxin B1470 or 2 µg/ml toxin B10463 for 3 h prior to stimulation with 100 ng/ml EGF for 5 min. After cell lysis and two-dimensional gel electrophoresis, phosphoproteins were visualized by autoradiography. B, HEp-2 cells were serum-starved for 18 h and labeled with 25 µCi/ml 32Pi for 3 h before EGF stimulation and cell lysis. After two-dimensional gel electrophoresis and transfer to nitrocellulose membrane, phosphoproteins were visualized by autoradiography (upper panels) and protein by anti-stathmin antiserum C (lower panels). A and B, the position of the phosphoprotein purified for MALDI-MS analysis is indicated by arrows. Molecular mass and apparent pH are indicated on the left side and on the bottom of each panel, respectively. C, electrophoretic mobilities of unphosphorylated (N) and the various mono-(P1), di-(P2), and triphosphorylated (P3) stathmin phosphoforms on two-dimensional gels are shown (16). D, cell extracts (see B) were immunoblotted with antisera specific for stathmin phosphorylated at serines 16, 25, or 38. Molecular mass is on the left and the positions of unphosphorylated stathmin and its mono-, di-, and triphosphorylated phosphoforms are shown on the bottom of each panel.

As shown in Fig. 1B, EGF treatment resulted in the appearance of two new forms of stathmin and a significant increase in the intensity of at least one other form. Stathmin has been shown to be phosphorylated on four distinct serines: 16, 25, 38, and 63, resulting in a variety of migration patterns on two-dimensional gels (Fig. 1C and Refs. 7 and 16). To identify which residues are phosphorylated in response to EGF, antibodies specific for three phosphorylated forms (serines 16, 25, and 38) were used on Western blots (7). As seen in Fig. 1D, EGF induces a dramatic increase in phosphorylation at Ser-16 and Ser-25. Ser-38 is phosphorylated in control cells and does not increase significantly upon EGF addition. The lack of a significant change in the P3 isoform at 19 kDa (Refs. 25, 38, 63 and Fig. 1D) coupled with the strong increase in the P3 isoform at 23 kDa (16, 25, 38) suggests that very little phosphorylation is induced at Ser-63 by EGF.

To identify which phosphorylation event is mediated by Rho GTPases, phosphospecific antibodies were used to analyze Western blots of one-dimensional gels of lysates from cells treated with the two clostridial toxins. As can be seen in Fig. 2A, 160 ng/ml toxin B1470 results in a complete inhibition of EGF-induced phosphorylation of Ser-16 although at even 10-fold higher concentrations, the toxin does not inhibit EGF-induced phosphorylation at Ser-25. Ser-25 has been reported to be phosphorylated by ERK MAP kinases and the EGF-stimulated activation of ERK1 and ERK2 was not changed by toxin treatment (Fig. 2A, bottom left and Ref. 17). Toxin B10463 gave similar results (data not shown). The basal levels of phosphorylated Ser-38 were unaffected by both toxins. Cytotoxic necrotizing factor 1 (CNF1) from Escherichia coli activates all members of the Rho family and when added to HEp-2 cells results in increased phosphorylation of stathmin only on Ser-16 (Fig. 2B and Ref. 18). We conclude that EGF-induced phosphorylation at Ser-16 of stathmin is mediated by Rac and/or Cdc42.

View larger version (28K): [in this window] [in a new window]   Fig. 2.   Effects of GTPase-modifying toxins on EGF-stimulated stathmin phosphorylation. A, serum-starved HEp-2 cells were incubated with toxin B1470 for 3 h and stimulated with 100 ng/ml EGF for 7 min. Total cell lysates were analyzed by immunoblotting with antisera specific for stathmin phosphorylated at serines 16, 25, or 38 or with an anti-phospho-ERK antibody. B, serum-starved HEp-2 cells were treated with 2 µg/ml CNF-1 for the indicated times, and stathmin phosphorylation was assessed as described above.

To confirm that Rac and Cdc42 can induce phosphorylation at Ser-16, HEp-2 cells were transfected with a Myc-tagged stathmin expression vector either alone or with dominant-negative Rac (N17Rac1) or Cdc42 (N17Cdc42) and treated 24 h later with EGF. Inhibition of Rac almost completely prevented phosphorylation on Ser-16, whereas inhibition of Cdc42 inhibited phosphorylation by around 50% (Fig. 3A). Next, stathmin was cotransfected with constitutively activated GTPases. L61Rac and L61Cdc42, but not L63Rho, induced phosphorylation specifically at Ser-16 (Fig. 3B).

View larger version (33K): [in this window] [in a new window]   Fig. 3.   Analysis of Rac and Cdc42-mediated stathmin phosphorylation at Ser-16. A, HEp-2 cells were transiently transfected with Myc-tagged stathmin (0.5 µg/well) plus either control vector or Myc-tagged N17Rac1 or N17Cdc42 (1.0 µg/well each). After 24 h, cells were stimulated with 100 ng/ml EGF for 7 min and lysed. Myc-tagged proteins were immunoprecipitated from cell extracts and immunoblotted with either anti-[phospho-Ser-16]stathmin (upper) or anti-Myc (lower) antibodies. B, HEp-2 cells were transfected with Myc-tagged stathmin (0.5 µg/well) plus either control vector or Myc-tagged L63RhoA, L61Rac1, or L61Cdc42 (0.5 µg/well each). After 24 h, cells were lysed, and anti-Myc immunoprecipitates were analyzed as described above.

Three isoforms (1, 2, and 3) of the Ser/Thr kinase p65PAK interact directly with Rac and Cdc42 leading to a variety of cellular effects (8, 19). To determine whether p65PAK mediates Ser-16 phosphorylation, HEp-2 cells were transfected with stathmin along with an autoinhibitory fragment (residues 83-149) derived from p65PAK (8). As seen in Fig. 4, A and B, inhibition of p65PAK completely prevented EGF-, L61Rac-, and L61Cdc42-induced phosphorylation on Ser-16.

View larger version (28K): [in this window] [in a new window]   Fig. 4.   PAK mediates stathmin phosphorylation at Ser-16. A, HEp-2 cells were transiently transfected in 6-well dishes with Myc-tagged stathmin (0.5 µg/well) plus either control vector or Myc-tagged PAK1-(83-149) inhibitory fragment (1.0 µg/well). After 24 h, cells were stimulated with 100 ng/ml EGF for 7 min and lysed. Myc-tagged proteins were immunoprecipitated from cell extracts and immunoblotted with either anti-[phospho-Ser-16]stathmin (upper) or anti-Myc (lower) antibodies. B, HEp-2 cells were transfected with Myc-tagged stathmin (0.5 µg/well) plus either control vector or Myc-tagged L61Rac1 or L61 Cdc42 (0.2 µg/well each). Where indicated, PAK1-(83-149) expression plasmid was cotransfected (1.0 µg/well). After 24 h, cells were lysed, and anti-Myc immunoprecipitates were analyzed as described in A.

In conclusion, we have show that the addition of EGF to HEp-2 cells leads to a Rac/Cdc42 and p65PAK-dependent phosphorylation of stathmin at Ser-16. Whether p65PAK directly phosphorylates stathmin or whether it activates another downstream kinase is not currently known. Two kinases have been reported to phosphorylate stathmin at Ser-16, the cAMP-dependent kinase A and the Ca²⁺/calmodulin-dependent kinase isoforms, CaMK IV/Gr and CaMKII (5, 16, 20). The cAMP-dependent kinase A is unlikely to be involved because it preferentially phosphorylates Ser-63, and pretreatment of HEp-2 cells with EGTA to block Ca²⁺ influx had no effect, suggesting that CaM kinases are not involved (data not shown).

Stathmin plays an important role in controlling microtubule dynamic either by sequestering / tubulin heterodimers or by increasing catastrophe frequency at the plus ends of microtubules or both (21-23). As a result, stathmin causes destabilization of growing microtubules and phosphorylation at Ser-16 appears to block this activity (5, 6). Rac and Cdc42 regulate actin polymerization and form membrane protrusions at the leading edge of migrating cells and neuronal growth cones. The results described here suggest, therefore, that Rac and Cdc42 might control both F-actin and microtubule dynamics in localized regions associated with cell protrusions (24, 25).

	ACKNOWLEDGEMENTS

We thank Drs. C. von Eichel-Streiber, G. Schmidt, and K. Aktories for their kind gifts of bacterial toxins. We also thank Karen Knox for stathmin DNA isolation, P. Curmi and O. Gavet for stathmin mutant constructions, V. Manceau for stathmin phosphospecific antisera, and Richard Lamb and Sandrine Etienne- Manneville for stimulating discussions.

	FOOTNOTES

^* This work was supported in part by the Cancer Research Campaign, United Kingdom (to A. H. and H. D). The work in Ghent was supported by Grant IUAP P4/23 from the Interuniversity Attraction Poles of the Prime Minister's Office.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.

^§ Recipient of an EMBO long term fellowship. Present address: Axxima Pharmaceuticals AG, Am Klopferspitz 19, 82152 Martinsried, Germany.

Postdoctoral Fellow of the Fund for Scientific Research, Flanders, Belgium (F. W. O.-Vlaanderen).

^§§ To whom correspondence should be addressed. E-mail: alan. hall@ucl.ac.uk.

Published, JBC Papers in Press, October 31, 2000, DOI 10.1074/jbc.C000635200

	ABBREVIATIONS

The abbreviations used are: EGF, epidermal growth factor; MALDI-TOF-MS, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry; PSD, post-source decay analysis; MAP, mitogen-activated protein; RP-HPLC, reversed phase-high performance liquid chromatography; ERK, extracellular receptor kinase.

	REFERENCES

TOP ABSTRACT INTRODUCTION EXPERIMENTAL PROCEDURES RESULTS AND DISCUSSION REFERENCES

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				P. Amayed, D. Pantaloni, and M.-F. Carlier The Effect of Stathmin Phosphorylation on Microtubule Assembly Depends on Tubulin Critical Concentration J. Biol. Chem., June 14, 2002; 277(25): 22718 - 22724. [Abstract] [Full Text] [PDF]