|
Advertisement | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
J. Biol. Chem., Vol. 281, Issue 35, 25006-25010, September 1, 2006
Structure of the N-terminal Ankyrin Repeat Domain of the TRPV2 Ion Channel*
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| ABSTRACT |
|---|
|
|
|---|
| INTRODUCTION |
|---|
|
|
|---|
The six mammalian TRPV channels are important in sensory and pain perception and in calcium homeostasis. At the molecular level, TRPV1 and TRPV2 are both activated at noxious temperatures (>43 °C and >52 °C, respectively) (2, 3). TRPV1 is also gated by exogenous ligands such as capsaicin and resiniferatoxin (reviewed in Ref. 4). TRPV3 is activated by warm temperatures (>3239 °C) and camphor (57), whereas TRPV4 senses osmotic stimuli, heat (>27 °C), and phorbol esters (810). The more distant TRPV homologs TRPV5 and TRPV6 are regulated by intracellular Ca2+ levels and mediate trans-epithelial Ca2+ transport in the kidney and intestine (11).
The N-terminal region of TRPV channels contains ankyrin repeats, which are likely to be essential for channel function as their deletion in TRPV1 impairs activation (12). Furthermore, the TRPV5 and TRPV6 ankyrin repeats were implicated in the tetrameric assembly of the channels (13, 14). Ankyrin repeats are 33-residue sequence motifs often involved in protein-protein interactions and present in many eukaryotic and prokaryotic proteins with functions that include signaling, cytoskeleton integrity, transcription, and cellular localization (15, 16).
Determining the three-dimensional structures of TRPV channel domains is an important step in understanding the molecular basis of gating and regulatory mechanisms. We report the structure of N-terminal cytosolic ankyrin repeat domain (ARD) of the TRPV2 ion channel. The TRPV2-ARD is monomeric in solution and its three-dimensional structure consist of six ankyrin repeat structural motifs, of which only four could be identified by sequence motif searches. Comparison with other ankyrin repeat structures highlights several unusual structural features of the TRPV2-ARD. The structure provides a context for data on the functional roles of the N-terminal regions in TRPV ion channel physiology.
| EXPERIMENTAL PROCEDURES |
|---|
|
|
|---|
-D-thiogalactopyranoside at room temperature for 816 h. Selenomethionine (Se-Met)-substituted protein was expressed with feedback inhibition of methionine synthesis in BL21(DE3) grown in fully supplemented M9 minimal medium with Se-Met.
Purification of TRPV2-ARDCell pellets were resuspended in lysis buffer (20 mM Tris-HCl, pH 8.0, 300 mM NaCl, 20 mM imidazole, 0.1 mM phenylmethylsulfonyl fluoride, 1 mM
-mercaptoethanol, and 0.1% Triton X-100) with 0.2 mg/ml lysozyme and lysed by sonication. The cleared lysate was loaded onto a nickel-nitrilotriacetic acid column (Qiagen), washed, and eluted using a step gradient (60, 100, and 250 mM imidazole). Fractions containing TRPV2-ARD were pooled, concentrated, and loaded on a Superdex 200 16/60 column (GE Healthcare) in 20 mM Tris-HCl, pH 8.0, 100 mM NaCl, 1 mM EDTA, 0.1 mM phenylmethylsulfonyl fluoride, and 1 mM DTT. Fractions containing TRPV2-ARD were pooled, diluted to 75 mM NaCl, loaded on a Resource-Q column (GE Healthcare), and eluted using a linear NaCl gradient. Pure fractions of TRPV2-ARD (75321) were pooled and dialyzed against 20 mM HEPES, pH 8.0, 100 mM NaCl, 10% glycerol, 1 mM DTT, and concentrated to
5 mg/ml and TRPV2-ARD (62326) dialyzed against 20 mM Tris-HCl, pH 8.0, 100 mM NaCl, 5% glycerol, and 1 mM DTT and concentrated to
10 mg/ml. Typical yield was 510 mg/liter of culture. Protein samples were judged adequate if >95% pure on Commassie-stained gels. Near 100% Se-Met incorporation was confirmed by mass spectroscopy.
|
Crystallization of TRPV2-ARDCrystals were grown by hanging drop vapor diffusion with a 1:1 ratio of protein and reservoir solution in each drop. Crystal form I (native and Se-Met-substituted) grew at room temperature from 1.2 M lithium acetate, 0.1 M MES, pH 6.5. Crystal form II grew at 4 °C from 1.2 M lithium acetate, 0.1 M MES, pH 6.0. Crystal form III (62326) grew at 4 °C from 9% PEG-8000, 0.1 M sodium citrate, pH 5.0, 57% xylitol. All crystals were flash-frozen after cryo-protection with 20% glycerol in the corresponding reservoir solution.
Data Collection and Structure DeterminationX-ray diffraction data were collected at 100 K. Native and Se-Met derivative data of TRPV2-ARD crystal form I were collected at the Advanced Photon Source Structural Biology Center ID-19 beamline with the SBC CCD2 detector and data on TRPV2-ARD crystal form III were collected at the Advanced Photon Source BM8 beamline using an ADSC Q315 CCD detector. Both were processed with HKL2000 (17). Data on TRPV2-ARD crystal form II were collected on a MicroMax007 x-ray generator equipped with an R-AXIS IV++ detector and processed with CrystalClear (Rigaku/MSC Inc.). The TRPV2-ARD structure (crystal form I; 2.2 Å resolution) was determined using Se-Met single-wavelength anomalous diffraction phasing with SOLVE (18) and DM (19). The TRPV2-ARD crystal form II (to 1.6 Å) and III (to 3.1 Å) structures were determined by molecular replacement with the TRPV2-ARD crystal form I structure as a search model using MOLREP (20). Structures were refined with TLS in REFMAC (21). Data, phasing, and refinement statistics are provided in supplemental Table 1.
| RESULTS |
|---|
|
|
|---|
-helices are predicted to precede and link the ankyrin repeat sequence motifs. We therefore postulated that the ARD extended further toward the N terminus, from residues 75 to 321 (Fig. 1A). A similar prediction was recently published for TRPV6 (14). This N-terminal boundary coincides with the beginning of the high sequence identity region within the TRPV subfamily. Hence, a TRPV2-ARD protein construct spanning residues 75321 was expressed in E. coli and purified (Fig. 1B). Solution Characterization of Oligomeric StateTRPV proteins are tetrameric ion channels (24, 25), and the ankyrin repeats of TRPV5 and TRPV6 have been shown to be important for channel tetramerization (13, 14). We therefore investigated the oligomeric state of the isolated TRPV2-ARD. Both size exclusion chromatography and multi-angle laser light scattering (26) indicate that the TRPV2-ARD is monodisperse and monomeric in solution at protein concentrations up to 350µM (size exclusion chromatography apparent molecular mass of 29 kDa and light scattering molecular mass of 27.3 kDa, versus calculated molecular mass = 28 628 Da for construct 75321; Fig. 1B and data not shown).
Overall Structure of TRPV2-ARDWe determined the structure of the TRPV2-ARD in three crystal forms (supplemental Table 1), and except where noted, the 1.6-Å TRPV2-ARD structure from crystal form II was used for analyses (Fig. 1C). The structure comprises six structural repeats consisting of a pair of anti-parallel
-helices with an intervening "finger" loop. All six repeats therefore resemble the ankyrin repeat structural motif, with two short anti-parallel helices followed by a finger loop projecting outward from the helical axes at a
90° angle, resulting in a characteristic L-shaped cross-section. The helical hairpins stack together such that the inner helices and fingers form a concave surface. This assembled structure has been likened to a cupped hand: the loops form the fingers, hence the name, and the exposed surface of the inner helices forms the palm. This concave palm surface is often a site of protein-protein interactions in other ankyrin repeat proteins (15, 16).
Conformational Differences of TRPV2-ARD in Three Different Crystal FormsWithin the three crystal forms of the TRPV2-ARD, five independent observations of the structure are available for comparison. The overall structures are similar, with pairwise root mean squared deviation values ranging from 0.31 to 2.0 Å for all C
atoms, although several local conformational differences were observed. The most different conformations are superimposed in Fig. 1D, displaying marked differences within the long fingers 13 and the N terminus of outer helix 5, indicating additional flexibility afforded by this unusually long helix. The flexibility suggested by this conformational variability supports the idea that these three fingers may play important roles in interacting with other proteins. A number of bulky aromatic residues, Phe161, Tyr162, Phe198, Phe199, Phe207, Tyr208, and Phe209, are located on the concave face of fingers 2 and 3 (Fig. 1E). This extended patch of solvent-exposed aromatic residues forms a potential hydrophobic interacting surface.
|
23° along the stacking axis (27). In contrast, the overall structure of the TRPV2-ARD can be seen as two adjacent stacks of repeats. Repeats 14 stack with an average twist of
2°, similar to ankyrin, but a large twist of
21° between repeats 4 and 5 leads to a distortion of the concave palm-finger surface often used for protein-protein interactions by ankyrin repeat-containing proteins (Fig. 2B). The length of the repeats is quite variable and generally longer than the 33 residues of canonical ankyrin repeats, 43, 47, 45, 36, 49, and 33 residues, respectively, in TRPV2 (note that the last repeat does not have an associated finger; Fig. 2A). Insertions within ankyrin repeat domains have been previously observed in other structures and are most often found in the finger region, although other insertion locations have also been observed (15, 16). In TRPV2-ARD, insertions are found not only in fingers 13 but also in outer helices 5 and 6. Although the lengths of the inner helices are typical of a canonical ankyrin repeat, 7 residues on average (15), outer helices 5 and 6 are significantly longer than the 9-residue average for ankyrin repeats (15), with outer helix 5 exceptionally long at 21 residues (Fig. 2).
|
-hairpin hydrogen bonding pattern often seen in other ankyrin repeat proteins (27). While the C-terminal fingers 4 and 5 are short and similar in size and shape to fingers of canonical ankyrin repeats, fingers 13 form long extended loops. In addition, finger 3 folds over on top of inner helices 3 and 4, thus creating a bulge in the concave surface of the domain.
Sequence Conservation and Implications for Other TRPV Ion ChannelsThe aligned sequences of all six mammalian TRPV ion channels show a high level of similarity within the ARD (Fig. 3A). In fact, the N-terminal boundary of the ARD (residue
70 in TRPV2) also represents a boundary within the sequence alignment marking the beginning of the region of highest sequence similarity. We therefore believe that the six-repeat ARD will be conserved throughout the TRPV subfamily and that the domain boundaries identified in this study encompass the entire ARD of TRPV channels. TRPV5 and TRPV6 are more distantly related to the TRPV14 subgroup, and this divergence is reflected in the ARD, where many of the putative finger loops are shorter in TRPV56 than in TRPV14 (Fig. 3). Fig. 3B shows the distribution of conserved and variable residues within the TRPV family on the TRPV2-ARD structure. Conservation of sequence in the inner helices and the hydrophobic core of the palm supports our proposal that all TRPV channels have a six-repeat ARD. Also, the aromatic residues on the surface of fingers 2 and 3 are well conserved, particularly within the TRPV14 channels (Fig. 3). In contrast, the residues with the highest degree of sequence variability are located within the first ankyrin repeat, the solvent-exposed surface of the outer helices and the fingers of the ARD. The tip of finger 3, which seems very flexible according to our structural studies, also has particularly high sequence variability. Thus this finger is likely to adopt different conformations in the other TRPV channels and could be a major determinant of specificity in the regulatory interactions of TRPV channels.
| DISCUSSION |
|---|
|
|
|---|
Implications for the Regulation of TRPV ChannelsFunctional and biochemical studies have implicated the N-terminal domain of TRPV channels in several regulatory mechanisms involving both phosphorylation events and the binding of regulatory proteins. Phosphorylation and dephosphorylation are common mechanisms by which ion channel function is regulated. Protein kinase A phosphorylates several residues within TRPV1, including Ser116 and Thr370, and thereby prevents desensitization of the channel (28, 29), and Src kinase phosphorylates Tyr200 in human TRPV1 (Tyr199 in rat (30)). Based on the TRPV2-ARD structure, TRPV1 S116 is predicted to be on inner helix 1 and Tyr199 on the concave face of finger 2, and either phosphorylation event could therefore regulate the binding of regulatory factors to the concave face of the TRPV1-ARD. Thr370 is just past the proposed C-terminal boundary of the TRPV1-ARD, likely in a linker region between the ARD and the following domain. Thus Thr370 phosphorylation could possibly regulate the spatial relationship of the two domains.
A binding site for the SH3 domain of PACSIN3 was recently identified at residues 132144 of TRPV4, which is a prolinerich region immediately preceding the predicted ARD (Fig. 3 (31)). The TRPV1 N terminus, in particular residues 200414, encompassing the last three ankyrin repeats and
50 additional residues preceding the channel domain, was found to associate with synaptotagmin IX and snapin, two components of SNARE (soluble N-ethylmaleimide-sensitive factor (NSF) attachment protein receptor)-dependent exocytosis in excitable cells (32). Two-hybrid screens have also identified proteins that interact with the N terminus of TRPV2, including the recombinase gene activator and acyl-CoA binding domain protein 3 (ACBD3) (3335). The identification of the ARD boundaries can help design further mutagenesis or structural studies of the TRPV-ARD interactions with these identified ligands. The TRPV2-ARD structure reported here will therefore serve as a scaffold for future investigations of the molecular mechanisms regulating TRPV ion channels.
| FOOTNOTES |
|---|
* This work was supported by American Heart Association Grant SDG 0335134N (to R. G.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. ![]()
The online version of this article (available at http://www.jbc.org) contains supplemental Table 1.
The atomic coordinates and structure factors (codes 2ETA, 2ETB, and 2ETC) have been deposited in the Protein Data Bank, Research Collaboratory for Structural Bioinformatics, Rutgers University, New Brunswick, NJ (http://www.rcsb.org/). ![]()
1 Present address: Dept. of Biochemistry and Molecular Biophysics, Columbia University, 630 West 168th St., New York, NY 10032. ![]()
2 A McKnight Scholar. To whom correspondence should be addressed. Tel.: 617-495-5616; Fax: 617-496-9684; E-mail: gaudet{at}mcb.harvard.edu.
3 The abbreviations used are: TRP, transient receptor potential; ARD, ankyrin repeat domain; Se-Met, selenomethionine; DTT, dithiothreitol; MES, 4-morpholineethanesulfonic acid. ![]()
| ACKNOWLEDGMENTS |
|---|
| REFERENCES |
|---|
|
|
|---|
This article has been cited by other articles:
![]() |
R. Gaudet TRP channels entering the structural era J. Physiol., August 1, 2008; 586(15): 3565 - 3575. [Abstract] [Full Text] [PDF] |
||||
![]() |
Y.-F. Guan, X.-Y. Huang, J. Zhu, J.-F. Gao, H.-X. Zhang, and Z.-N. Yang RUPTURED POLLEN GRAIN1, a Member of the MtN3/saliva Gene Family, Is Crucial for Exine Pattern Formation and Cell Integrity of Microspores in Arabidopsis Plant Physiology, June 1, 2008; 147(2): 852 - 863. [Abstract] [Full Text] [PDF] |
||||
![]() |
D. D'hoedt, G. Owsianik, J. Prenen, M. P. Cuajungco, C. Grimm, S. Heller, T. Voets, and B. Nilius Stimulus-specific Modulation of the Cation Channel TRPV4 by PACSIN 3 J. Biol. Chem., March 7, 2008; 283(10): 6272 - 6280. [Abstract] [Full Text] [PDF] |
||||
![]() |
N. Garcia-Sanz, P. Valente, A. Gomis, A. Fernandez-Carvajal, G. Fernandez-Ballester, F. Viana, C. Belmonte, and A. Ferrer-Montiel A Role of the Transient Receptor Potential Domain of Vanilloid Receptor I in Channel Gating J. Neurosci., October 24, 2007; 27(43): 11641 - 11650. [Abstract] [Full Text] [PDF] |
||||
![]() |
D. Kim and E. J. Cavanaugh Requirement of a Soluble Intracellular Factor for Activation of Transient Receptor Potential A1 by Pungent Chemicals: Role of Inorganic Polyphosphates J. Neurosci., June 13, 2007; 27(24): 6500 - 6509. [Abstract] [Full Text] [PDF] |
||||
![]() |
W. Cheng, F. Yang, C. L. Takanishi, and J. Zheng Thermosensitive TRPV Channel Subunits Coassemble into Heteromeric Channels with Intermediate Conductance and Gating Properties J. Gen. Physiol., March 26, 2007; 129(3): 191 - 207. [Abstract] [Full Text] [PDF] |
||||
![]() |
I. Erler, D. M. M. Al-Ansary, U. Wissenbach, T. F. J. Wagner, V. Flockerzi, and B. A. Niemeyer Trafficking and Assembly of the Cold-sensitive TRPM8 Channel J. Biol. Chem., December 15, 2006; 281(50): 38396 - 38404. [Abstract] [Full Text] [PDF] |
||||
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| All ASBMB Journals | Molecular and Cellular Proteomics |
| Journal of Lipid Research | ASBMB Today |