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J. Biol. Chem., Vol. 282, Issue 30, 21583-21587, July 27, 2007
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From the Cellular Neurology Unit, NINDS, National Institutes of Health, Bethesda, Maryland 20892
Received for publication, April 27, 2007 Accepted for publication May 31, 2007.
| ABSTRACT |
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| INTRODUCTION |
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The dynamin-related GTPase Drp1 is an evolutionarily conserved protein that mediates mitochondrial division, and its functional impairment results in aggregates of large, interconnected mitochondria within cells. Drp1 is also involved in the mitochondrial scissioning that occurs during apoptosis and cell division (5, 7). However, mechanisms by which Drp1 function is regulated during such critical cellular events are less well understood. Frequently, cellular responses to changing conditions are mediated by reversible covalent modifications of existing molecules, and Drp1 is modified post-translationally by protein phosphorylation, ubiquitylation, and sumoylation (7–10). However, in most cases, neither the direct impact of these modifications on Drp1 functions nor the sites of modification have been thoroughly investigated.
Like many dynamin-related proteins, Drp1 has an N-terminal GTP-binding domain, a middle assembly domain, a short insert (insert B), and a C-terminal interaction domain dubbed the GTPase effector domain (GED);2 the GED domain folds back to interact in cis with the GTP-binding and middle domains and also interacts intermolecularly in trans with GED domains in other Drp1 proteins (11–15). Here, we demonstrate phosphorylation of Drp1 by cAMP-dependent protein kinase (protein kinase A; PKA) at a specific serine residue within the C-terminal GED domain of Drp1. We further show that this phosphorylation alters the intramolecular domain associations of the Drp1 GTPase, resulting in decreased GTPase activity and impairment of mitochondrial fission.
| EXPERIMENTAL PROCEDURES |
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In Vitro Phosphorylation—Glutathione S-transferase (GST)-Drp1 fusion proteins were purified using glutathione-Sepharose 4B resin (GE Healthcare) and then incubated with 50 ng of purified PKA catalytic subunit (Promega, Madison, WI) in 10 mM HEPES (pH 7.0) with 20 mM MgCl2, 50 mM ATP, and 1 pmol of [
-32P]ATP EasyTides (3000 Ci/mmol; PerkinElmer) at 30 °C for 30 min. Reactions were terminated by adding Laemmli sample buffer, and samples were resolved by SDS-PAGE and visualized by autoradiography.
Immunoblotting—HeLa cells were transfected as described previously (15), washed twice with phosphatebuffered saline (PBS; pH 7.4), and then solubilized with radioimmunoprecipitation buffer. Where indicated, alkaline phosphatase (1 unit/µl; New England Biolabs, Beverly, MA) or
protein phosphatase (1 unit/µl; New England Biolabs) were added to cell lysates and incubated at 37 °C for 1 h. Proteins were resolved by SDS-PAGE and then immunoblotted (15). Affinity-purified rabbit polyclonal anti-peptide antibodies against Drp1 (number 2457) were described previously (15). Rabbit polyclonal antibodies that specifically recognize Drp1 phosphorylated at Ser637 were generated commercially (Quality Controlled Biochemicals, Hopkinton, MA) with a synthetic phosphopeptide Ac-CPVARKL(pS)AREQRD-amide corresponding to Drp1 residues 631–643 (splice variant 1; GenBank Accession Number NP_036192
[GenBank]
). Mouse monoclonal anti-actin ascites was obtained from Sigma-Aldrich (1:2000; clone AC-40; IgG2a).
Yeast Two-hybrid Analyses—Full-length Drp1 and Drp1 truncation fragments were cloned into pGAD10 and pBHA vectors as described previously (15). Yeast two-hybrid tests were performed in the L40 strain, which contains LexA operators upstream of HIS3 and lacZ reporter genes. Interactions were semiquantified based on growth on His/Leu/Trp dropout plates supplemented with 10 mM 3-amino-1,2,4-triazole (Sigma-Aldrich) and time for colonies to turn blue in X-gal filter lift assays from Leu/Trp dropout plates (15), with similar results obtained in at least three experiments.
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-32P]GTP EasyTides (3000 Ci/mmol; PerkinElmer) in assay buffer. For assay of in vitro phosphorylated CBP-Drp1 fusion proteins, 3.0 µg of CBP-Drp1 was first incubated with 50 ng of purified PKA catalytic subunit in 10 mM HEPES (pH 7.0) with 20 mM MgCl2 and 500 mM ATP for 30 min at 30 °C before diluting into GTPase assay buffer. Samples of the reaction mixtures were spotted onto polyethyleneimine cellulose on polyester thin layer chromatography (TLC) plates (Sigma-Aldrich) at various times. Guanine nucleotides were separated by ascending chromatography in 1.0 M formic acid: 1.2 M LiCl. [32P]GDP and [32P]GTP spots were identified, and intensities were quantified using a Storm 860 PhosphorImager with ImageQuant software (GE Healthcare). GTPase activity was expressed as the ratio of GDP to total guanine nucleotides (GDP + GTP) at each time point. All data points represent the averages of at least three independent experiments. Immunostaining—HeLa cells were grown on glass coverslips and transfected with Myc-Drp1 as described previously (15). Drp1 mutants were produced using the QuikChange method (Stratagene, La Jolla, CA). After 36–48 h of transfection, cells were washed with PBS and fixed with 4% paraformaldehyde in PBS for 20 min. After three washes with PBS, cells were incubated in blocking buffer (5% horse serum, 0.1% bovine serum albumin, and 0.1% Triton X-100 in PBS) for 1 h. Goat anti-Myc epitope (1:500; Bethyl Laboratories, Montgomery, TX) and mouse monoclonal anti-cytochrome c (1:500; Pharmingen) antibodies were incubated in blocking buffer overnight at 4 °C. After three washes with PBS, Alexa Fluor 488 and/or Alexa Fluor 568 secondary antibodies (1:500; Invitrogen) were added for 1 h. After three washes with PBS, coverslips were mounted using Gel/Mount (Biomeda, Burlingame, CA). Images were acquired using a Zeiss LSM-510 confocal microscope. For determination of mitochondrial morphology, three trials were performed, with 100 transfected HeLa cells in each trial classified into normal, elongated and fragmented mitochondrial groups. Where indicated, HeLa cells were incubated with 20 µM forskolin and 10 µM IBMX for 10, 20, or 30 min before fixation and subsequent immunostaining for cytochrome c.
| RESULTS AND DISCUSSION |
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82 kDa. We tested multiple different kinase activators and found that forskolin and IBMX, activators of PKA, significantly increased phosphorylation levels (Fig. 1A). To determine whether this stimulated phosphorylation occurred at the same or different sites from the basal phosphorylation, we performed two-dimensional tryptic phosphopeptide mapping. This revealed that there was a single novel site of phosphorylation in response to PKA activation that was distinct from sites of basal phosphorylation (Fig. 1B).
To confirm that the phosphorylation was directly mediated by PKA, we performed in vitro phosphorylation assays with [
-32P]ATP using purified PKA in combination with GST-Drp1 fusion proteins, and we observed a prominent increase in Drp1 phosphorylation. A number of fusion proteins comprising Drp1 truncation constructs were tested, narrowing down the region sufficient for PKA phosphorylation to residues 490–646; phosphoamino acid analysis identified the phosphorylated residue(s) as serine (data not shown). One of the sites within this region, Ser637, comprised a consensus sequence for PKA phosphorylation (-R(R/K)X(S/T)-). Site-directed mutagenesis of this site, S637A, resulted in loss of PKA-mediated phosphorylation in vitro, both with full-length GST-Drp1S637A and with GST-Drp1(490–646)S637A (Fig. 1C and data not shown). After two-dimensional tryptic phosphopeptide mapping, the PKA-stimulated phosphopeptide from wild-type GST-Drp1WT migrated essentially the same as that identified in intact cells. It was selectively abolished in the GST-Drp1S637A mutant, confirming the PKA phosphorylation site as Ser637 (Fig. 1D). Finally, we developed anti-phosphopeptide antibodies against phospho-Ser637 in Drp1; the phosphorylation site was detected in Drp1WT only after in vitro phosphorylation with PKA, and not at all with Drp1S637A, confirming the specificity of the antibodies for phospho-Ser637. Also, the immunoreactive signal was markedly diminished after treatment with broad spectrum protein phosphatases (Fig. 1E). Similarly, in extracts from HeLa cells treated with activators of PKA, there was appearance of a phospho-Ser637 band at
82 kDa, representing endogenous phosphorylated Drp1, that was similarly sensitive to protein phosphatase treatment (Fig. 1F).
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Phosphorylation at Ser637 Impairs Drp1 Intramolecular Associations and GTPase Activity—Since the Ser637 phosphorylation site is within the GED domain that has been shown to mediate both intramolecular and intermolecular interactions (15), we examined whether this modification changed Drp1 interactions using a phosphomimetic substitution, S637D, in yeast two-hybrid tests. We found that this mutation significantly inhibited the intramolecular interactions between GED and GTP-binding/middle domains (Fig. 2A) but not intermolecular interactions between full-length Drp1 proteins (Fig. 2B).
Since this intramolecular interaction has previously been shown to be important for GTPase activity (15), we examined the effects of Ser637 phosphorylation on Drp1 GTPase activity. Using in vitro GTPase assays with purified CBP-Drp1 fusion proteins, we found that PKA phosphorylation caused a significant reduction of GTPase activity (Fig. 3A). Similarly, the phosphomimetic GST-Drp1S637D exhibited decreased GTPase activity as well (Fig. 3B), although not so robust an effect as that seen with the more highly negatively charged phospho-Ser637 modification. This loss of activity likely occurs through effects of the phospho-Ser637 modification and S637D mutation on the intramolecular GED domain interaction with GTP-binding/middle domains since there are no effects on oligomerization as assessed by yeast two-hybrid assays (Fig. 2B) and chemical cross-linking studies (data not shown). Indeed, recent studies examining dynamin have suggested that the middle domain is critical for both tetramerization and higher-order self-assembly (16).
Importantly, our data identify a key regulatory mechanism for the GED domain interaction with middle and GTP-binding domains. Although this interaction has been intensively studied for other dynamin superfamily members, phosphorylation at Ser637 in Drp1 represents the first demonstration of dynamic regulation of this intramolecular interaction with functional changes in GTPase activity.
Phosphorylation of Drp1 at Ser637 Inhibits Mitochondrial Division—We examined the effects of IBMX and forskolin, which increase cAMP levels and activated PKA, on mitochondrial morphology in HeLa cells 10–30 min after application but saw no consistent morphology changes. Since cAMP has pleiotropic effects in cells that may mask any specific effects of Drp1 phosphorylation on mitochondrial morphology, we next evaluated the effects of the phosphomimetic S637D mutation on mitochondrial morphology. Overexpression of this mutant form resulted in mitochondria that were more elongated (Fig. 4). Interestingly, another GED mutation in Drp1 that decreases GTPase activity, K679A, inhibited mitochondrial fission as well (15), reminiscent of our results with Drp1S637D. Importantly, these effects on mitochondrial morphology are less prominent than those seen upon overexpression of a well characterized Drp1 mutation, K38A, which lacks detectable GTPase activity (15). Thus, effects on mitochondrial fission seem to mirror the extent of changes in GTPase activity for these mutations. Over-expression of Drp1S637A that cannot be phosphorylated by PKA showed no clear differences in mitochondrial morphology, consistent with a number of reports that found no increase in fission upon overexpression of wild-type Drp1.
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Functional Implications—In this study, we have shown that PKA phosphorylates Drp1 on Ser637 and inhibits its GTPase activity, likely through changes in the intramolecular association among the GED and GTP-binding/middle domains of Drp1, although higher-order oligomerization may also be affected. This represents the first reported Drp1 modification to have a direct effect on Drp1 GTPase activity.
Furthermore, in keeping with the impaired Drp1 GTPase activity, we have found that a phosphomimetic substitution, S637D, inhibits mitochondrial division. However, the functional effects of GED domain mutations in general have been controversial in the dynamin superfamily since some studies have seen facilitation of fission in response to changes in conserved amino acid residues for both dynamin and Drp1 (17, 18), whereas others have not (15, 19). On the other hand, since very recent studies support a two-step model for dynamin function (20), with an early regulatory GTPase-like function preceding a late, assembly-dependent step during which GTPase activity is required for fission, different GED mutations may give different results. In addition, the phosphomimetic substitution is a permanent one that lacks the dynamic aspects of phosphorylation in vivo, and thus, we suspect that the timing of phosphorylation would be critical. Further identification of the temporal aspects of the phosphorylation/dephosphorylation cycle will help unravel the cellular role of this modification.
| FOOTNOTES |
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The on-line version of this article (available at http://www.jbc.org) contains a supplemental figure. ![]()
1 To whom correspondence should be addressed: Cellular Neurology Unit, NINDS, National Institutes of Health, Bldg. 35, Rm. 2C-913, 9000 Rockville Pike, Bethesda, MD 20892-3704. Tel.: 301-451-9680; Fax: 301-480-4888; E-mail: blackstc{at}ninds.nih.gov.
2 The abbreviations used are: GED, GTPase-effector domain; PKA, cAMP-dependent protein kinase; IBMX, 3-isobutyl-1-methylxanthine; GST, glutathione S-transferase; CBP, calmodulin-binding peptide; TLC, thin layer chromatography; PBS, phosphate-buffered saline; X-gal, 5-bromo-4-chloro-3-indolyl-
-D-galactopyranoside; WT, wild type. ![]()
| ACKNOWLEDGMENTS |
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| REFERENCES |
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