Correspondence to: Winfield S. Sale, Department of Cell Biology, Emory University School of Medicine, 1648 Pierce Dr., Atlanta, GA 30322. Tel:(404) 727-6265 Fax:(404) 727-6256 E-mail:win{at}cellbio.emory.edu.
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Abstract |
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Previous physiological and pharmacological experiments have demonstrated that the Chlamydomonas flagellar axoneme contains a cAMP-dependent protein kinase (PKA) that regulates axonemal motility and dynein activity. However, the mechanism for anchoring PKA in the axoneme is unknown. Here we test the hypothesis that the axoneme contains an A-kinase anchoring protein (AKAP). By performing RII blot overlays on motility mutants defective for specific axonemal structures, two axonemal AKAPs have been identified: a 240-kD AKAP associated with the central pair apparatus, and a 97-kD AKAP located in the radial spoke stalk. Based on a detailed analysis, we have shown that AKAP97 is radial spoke protein 3 (RSP3). By expressing truncated forms of RSP3, we have localized the RII-binding domain to a region between amino acids 144180. Amino acids 161180 are homologous with the RII-binding domains of other AKAPs and are predicted to form an amphipathic helix. Amino acid substitution of the central residues of this region (L to P or VL to AA) results in the complete loss of RII binding. RSP3 is located near the inner arm dyneins, where an anchored PKA would be in direct position to modify dynein activity and regulate flagellar motility.
Key Words: kinases, cell motility, flagella, dynein, AKAP
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Introduction |
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The goal of this study is to determine the mechanism for localization of the cAMP-dependent protein kinase (PKA)1 in ciliary and flagellar axonemes. In several experimental systems, cAMP or specific inhibitors of PKA alter motility of ATP-induced reactivated movement of ciliary and flagellar axonemes. For example, cAMP is required for activation of sperm tail axonemes from many species (for reviews see
We proposed that PKA is localized to the axoneme through association with A-kinase anchoring proteins (AKAPs). AKAPs are a family of proteins that target PKA to specific intracellular sites through interaction with type I (RI) or type II (RII) PKA regulatory subunits (for a review see
To test the hypothesis that axonemal PKA is anchored by AKAPs, we used isolated axonemes from Chlamydomonas. Chlamydomonas offers several experimental advantages including the ease of axoneme isolation and the availability of mutants that are immotile and defective in specific axonemal structures. Based on pharmacological analysis, Chlamydomonas axonemal PKA regulates reactivated motility as well as dynein-driven microtubule sliding activity (
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Materials and Methods |
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Chlamydomonas Strains and Growth Conditions
Chlamydomonas reinhardtii strains used include 137c and cc124 (wild-type), pf14 (lacks radial spoke, paralyzed flagella), pf17 (lacks radial spoke head, paralyzed flagella), pf27 (radial spoke phosphorylation defect, paralyzed flagella), pf15 (lacks central pair apparatus, paralyzed flagella), pf16 (lacks C1 microtubule of central pair apparatus, paralyzed flagella), pf18 (lacks central pair apparatus, paralyzed flagella), pf19 (lacks central pair apparatus, paralyzed flagella), pf20 (unstable central pair apparatus, paralyzed flagella), pf6 (lacks projection on C1 microtubule of central pair apparatus, paralyzed flagella), pf28pf30 (lacks outer dynein arms and I1 inner dynein arms, paralyzed flagella), and pf2 (defective in the dynein regulatory complex, paralyzed flagella). All strains were obtained from the Chlamydomonas Genetics Center (Duke University, Durham, NC) with the exception of pf28pf30 which was generated by crossing pf28 with pf30 (strains obtained from the Chlamydomonas Genetics Center). The double mutant was then isolated from a nonparental ditype tetrad. Cells were grown in liquid modified medium I, with aeration and a 14-h/10-h light/dark cycle (
Isolation of Axonemes
Axonemes were isolated as described previously (
RII Overlays
RII overlays were performed according to the method of -32P]ATP (BLU502A; NEN Life Science Products) were used to radiolabel either biochemically purified RII (Promega) or recombinant RII
(plasmid provided by Dr. J. Scott, Vollum Institute, Oregon Health Sciences University, Portland, OR). Radiolabeled RII was overlayed on the nitrocellulose blots. Blots were exposed on a phosphorimager screen, and data was recorded and analyzed using the Phosphorimager SI (Molecular Dynamics) and ImageQuant software (Molecular Dynamics). Ht31 (DLIEEAASRIVDAVIEQVKAAGAY) and Ht31-P (DLIEEAASRPVDAVIEQVKAAGAY) peptides were synthesized by the Emory University Microchemical Facility (peptide sequences from
Western Blot Analysis
Proteins were separated by SDS-PAGE and transferred onto nitrocellulose membranes. Membranes were treated with 3% nonfat milk in TBS to prevent nonspecific protein binding. Blots were incubated with primary antibodies: anti-RSP3 at 1:200 (
Identification of the RII-binding Site
RSP3 fragments were generated by PCR using the RSP3 cDNA ( (GIBCO BRL) or E. coli BL21(DE3) Gold cells (Stratagene) and expressed as GST fusion proteins. Expression was induced at mid-log phase by the addition of 0.1 mM IPTG for 12 h. After expression, the bacterial cells were pelleted, resuspended in SDS-PAGE sample buffer, and boiled in preparation for SDS-PAGE. Expression was monitored by Western blot analysis with an antibody to GST, and RII binding was tested by RII blot overlays.
Mutagenesis
Mutagenesis was performed using the RSP3 construct containing amino acids 104180, generated as described above. Amino acid substitutions were made according to the protocol of the GeneEditorTM in vitro Site-Directed Mutagenesis System (Promega) with the modification of using 50 µl of GeneEditorTM antibiotic selection reagent. After mutagenesis, constructs were expressed and tested for RII binding as described above.
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Results and Discussion |
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The Axoneme Contains Two AKAPs
To test the hypothesis that the Chlamydomonas axoneme contains an AKAP, we performed RII blot overlays. As illustrated in Fig 1, the overlay resulted in the identification of several axonemal proteins that bind to RII (purified RII and recombinant RII yielded identical results). To determine which of these proteins bind to RII in a manner specific to AKAPs, the RII overlay was performed in the presence of the inhibitor peptide Ht31 or the control peptide Ht31-P. Ht31 is a synthetic peptide derived from the RII-binding domain of a known AKAP and contains an amphipathic helix (
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Ht31 inhibited RII binding to two particularly prominent axonemal proteins with masses of 240 kD (AKAP240) and 97 kD (AKAP97) (Fig 1, compare panels). AKAP240 is partially extractable with 0.6 M NaCl, whereas AKAP97 is not extractable with 0.6 M NaCl, indicating that AKAP97 is very tightly associated with the axoneme (Fig 1). In addition to AKAP240 and AKAP97, other proteins were identified in the RII overlay (Fig 1). However, these proteins were either not consistently detected, or RII binding to the proteins was not inhibited by the Ht31 peptide. Although it is possible that these other RII-binding proteins may include additional AKAPs, they were not considered further in this study.
To localize AKAP240 and AKAP97 within the axoneme, we performed RII overlays on axonemal protein from four different classes of Chlamydomonas motility mutants that are defective for specific axonemal structures (Fig 2 A). By doing so, we localized the AKAPs within the axoneme based on whether or not the AKAPs are present in the structural mutants. The analysis revealed that AKAP240 is absent in axonemes from a mutant defective for the central pair apparatus (pf18), and that AKAP97 is absent in axonemes from a mutant lacking the radial spokes (pf14) (Fig 2 B). Both AKAPs are present in axonemes from a mutant lacking the outer dynein arms and the I1 inner dynein arm (pf28pf30) as well as in axonemes from a mutant defective for the dynein regulatory complex (pf2) (Fig 2 B). Coomassie blue staining of a corresponding SDS-PAGE gel was performed to verify equivalent protein load. RII binding to the AKAPs was inhibited by addition of the Ht31 peptide (data not shown). The results of the mutant analysis suggest a model in which AKAP240 is associated with the central pair apparatus and AKAP97 is a component of the radial spoke.
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AKAP240 Is Localized to a Substructure of the Central Pair Apparatus
To further define the location of AKAP240, RII overlays were performed on axonemes from mutants defective in various components of the central pair apparatus (for a review see
AKAP97 Is RSP3
To further localize AKAP97 within the radial spoke, we analyzed axonemes from several different radial spoke mutants by performing RII overlays. The results indicated that although AKAP97 is absent in pf14 (lacking the entire radial spoke), AKAP97 is present in pf17, which is defective for the radial spoke head only (Fig 3a and Fig B). This suggests that AKAP97 is associated with the radial spoke stalk. We also examined pf27, a radial spoke mutant in which five radial spoke components that are normally phosphorylated are underphosphorylated in the mutant (
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Because RSP3 has been previously characterized to migrate at 97 kD on an SDS-PAGE gel, we hypothesized that AKAP97 may be identical to RSP3. To test this hypothesis, we performed an RII overlay of radial spoke mutant axonemes, and then probed the same blot with an antibody to RSP3. An identical staining pattern of AKAP97 was observed for both the RII overlay and analysis with anti-RSP3 (Fig 3 C), indicating that AKAP97 corresponds to RSP3. Interestingly, although the predicted size of RSP3 is only 57 kD, RSP3 migrates on an SDS-PAGE gel to a position nearly two times its predicted molecular weight. This anomalous migration is a common feature among members of the AKAP family (
RSP3 Contains a Single RII-binding Site
To confirm that AKAP97 is RSP3, we expressed recombinant RSP3 as a GST fusion protein in E. coli and tested for RII binding using an RII blot overlay. The overlay demonstrated that recombinant RSP3 binds to RII (Fig 4 A). RII binding was specifically inhibited by addition of the Ht31 peptide (data not shown). These results provided verification that RSP3 is an AKAP.
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To map the location of the RII-binding domain within the amino acid sequence of RSP3, we expressed truncated forms of RSP3 as GST fusion proteins in E. coli. Expression of the protein fragments was monitored by Coomassie staining and Western blot analysis with an antibody to GST. Using RII blot overlays, we then tested the various RSP3 truncations for RII binding (results shown in Fig 4 A). A COOH-terminal deletion of amino acids 221516 had no effect on RII binding. However, an NH2-terminal deletion of amino acids 1193 completely abolished RII binding, indicating that the RII-binding site is contained in the NH2-terminal portion of RSP3. By testing smaller protein fragments, we determined that RSP3 contains a single RII-binding domain located between amino acids 144180. Curiously, RII binding to the 144180 fragment is weaker than to larger fragments. One possible explanation for this observation is that the close proximity of the GST protein to the RII-binding site in the 144180 fragment results in steric hindrance by GST. However, it cannot be ruled out that residues NH2-terminal of amino acid 144 enhance the binding of RII to RSP3.
RSP3 Contains an RII-binding Motif Similar to Other AKAPs
Recently the AKAP domain for RII binding has been refined, and conserved residues within the amphipathic helix have been proposed (
To provide definitive evidence that amino acids 161178 comprise the RII-binding domain of RSP3, we performed a point mutation at amino acid 170. Using the construct containing amino acids 104180, the central, conserved residue, leucine 170, was replaced by a proline residue. We predicted that this substitution would disrupt the amphipathic helix and abolish RII binding. We expressed the construct as a GST fusion protein in bacteria, and tested its ability to bind to RII using an RII blot overlay. The analysis revealed that the leucine to proline substitution completely abolished RII binding to RSP3 (Fig 4 D). To ensure that the loss of RII binding in the leucine to proline substitution did not occur as a result of nonspecific secondary structural alterations, we performed a more structurally conservative amino acid substitution in which valine 169 and leucine 170 were replaced by alanines. The substitutions again resulted in the complete loss of RII binding, indicating that these residues are essential for RII to bind to RSP3 (Fig 4 D). Furthermore, the substitutions correspond to mutations in other AKAPs that disrupt RII binding (
The RII-binding Site in RSP3 Is Well Conserved in RSP3 Orthologs
Orthologs to RSP3 have been identified in human, mouse, and Drosophila databases. Alignment of the RII-binding site of RSP3 with the corresponding residues in these orthologs reveals that the RII-binding site is well conserved (Fig 5). The locations of the orthologs in their respective organisms is not known, but since the axoneme is a highly conserved structure, they most likely will be found in the radial spokes of flagellar and perhaps ciliary axonemes. It is important to test whether any of the orthologs is also an AKAP. If so, it suggests that the AKAP is an important component of the axoneme, and therefore may also be important in the regulation of motility. There is some evidence that AKAPs indeed play an important role in the control of flagellar motility. For example, cell-permeable analogues of Ht31 have been shown to inhibit sperm motility (
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The Role of RSP3 in Chlamydomonas Flagellar Axonemes
Genetic and biochemical experiments in Chlamydomonas have demonstrated that the radial spokes, and in particular RSP3, are important in the regulation of flagellar motility (for reviews see
Genetic and molecular analyses have demonstrated that RSP3 is located at the base of the radial spoke stalk, adjacent to the outer doublet microtubules and near the inner dynein arms (Fig 3 A). A basic region of RSP3, consisting of amino acids 4280, is necessary and sufficient for RSP3 to be targeted to the outer doublet microtubules (
One disadvantage for our study of RSP3 is that, along with other proteins of the radial spoke, RSP3 is not easily solubilized and forms a strong association with the axoneme (
In summary, we have identified an AKAP that is associated with the central pair apparatus, and an AKAP that is a component of the radial spoke. Recent studies have shown that in addition to an AKAP, the radial spoke also contains a kinase, as well as calmodulin (Yang, P., D.R. Diener, J.L. Rosenbaum, and W.S. Sale, submitted for publication). We propose that RSP3 localizes PKA to a position that, in conjunction with other components of the radial spoke, regulates dynein activity and controls flagellar motility. Future experiments will test this prediction and examine the physiological role of the AKAP in the flagellar axoneme.
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Footnotes |
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1 Abbreviations used in this paper: AKAP, A-kinase anchoring protein; GST, glutathione-S-transferase; PKA, cAMP-dependent protein kinase; RSP, radial spoke protein.
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Acknowledgements |
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The authors would like to thank John Scott (Vollum Institute, Oregon Health Sciences University) for helpful discussion, experimental advice, and the generous gift of the RII plasmid and Ht31 peptide. We would like to gratefully acknowledge Katrina Waymire and Pinfen Yang for experimental advice, and Lynne Quarmby, Grant MacGregor, and Pinfen Yang for thoughtful discussion of the manuscript.
Submitted: 30 January 2001
Revised: 27 February 2001
Accepted: 12 March 2001
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References |
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