Rab26 Modulates the Cell Surface Transport of α2-Adrenergic Receptors from the Golgi*

Background: Mechanisms underlying the cell surface export of G protein-coupled receptors (GPCRs) are poorly understood. Results: Rab26 modulates the Golgi to cell surface transport of α2-adrenergic receptors (α2-ARs) and interacts with the receptors. Conclusion: Rab26 controls the post-Golgi traffic of α2-ARs via a physical interaction. Significance: Rab26 functions as an important regulator for anterograde transport of GPCRs. The molecular mechanisms underlying the transport from the Golgi to the cell surface of G protein-coupled receptors remain poorly elucidated. Here we determined the role of Rab26, a Ras-like small GTPase involved in vesicle-mediated secretion, in the cell surface export of α2-adrenergic receptors. We found that transient expression of Rab26 mutants and siRNA-mediated depletion of Rab26 significantly attenuated the cell surface numbers of α2A-AR and α2B-AR, as well as ERK1/2 activation by α2B-AR. Furthermore, the receptors were extensively arrested in the Golgi by Rab26 mutants and siRNA. Moreover, Rab26 directly and activation-dependently interacted with α2B-AR, specifically the third intracellular loop. These data demonstrate that the small GTPase Rab26 regulates the Golgi to cell surface traffic of α2-adrenergic receptors, likely through a physical interaction. These data also provide the first evidence implicating an important function of Rab26 in coordinating plasma membrane protein transport.

G protein-coupled receptors (GPCRs) 2 constitute the largest superfamily of the cell surface receptors and modulate a variety of cell functions largely via coupling to heterotrimeric G proteins, which in turn activate downstream effectors (1)(2)(3). One important factor that determines the magnitude of signaling initiated by a given receptor in cell is the amount of the receptor expressed at the cell surface available for binding to its cognate ligands. The level of receptor expression at the cell surface at the steady state is controlled by a number of elaborately regulated trafficking processes such as anterograde export of newly synthesized receptors to the cell surface, the endocytosis of the cell surface receptors to endosomes upon agonist stimulation, the recycling of the internalized receptor from endosomes to the cell surface, and the transport of receptors to lysosome for degradation (4). However, compared with the extensive studies on the events involved in the endocytosis, recycling, and degradation pathways (5)(6)(7)(8)(9)(10)(11), the molecular mechanisms governing the cell surface transport of GPCRs, especially from the Golgi, are poorly understood.
It has become apparent that GPCR transport from the Golgi to the cell surface is a regulated process. Similar to export from the ER, GPCR exit from the Golgi is controlled by the structural features embedded within the receptors. We have previously demonstrated that the N-terminal YS motif specifically regulates the exit of ␣ 2 -adrenergic receptors (␣ 2 -ARs) from the Golgi (12). There are three ␣ 2 -AR subtypes, designated as ␣ 2A -AR, ␣ 2B -AR, and ␣ 2C -AR. Both ␣ 2A -AR and ␣ 2B -AR nicely express at the cell surface, whereas ␣ 2C -AR cell surface expression depends on the cell types (13). Because the YS motif is highly conserved among the three ␣ 2 -AR subtypes in different species, it may provide a common signal directing the exit from the Golgi of this subgroup of adrenergic receptors. In addition, G protein-coupled olfactory and chemokine receptors are released from the ER but accumulated in the Golgi (14,15), and the opsin mutant E150K is accumulated in the cis and medial Golgi (16). Recent studies have also revealed that Ras-like small GTPases may play important roles in the Golgi to cell surface transport of GPCRs (17)(18)(19)(20)(21)(22).
Rab GTPases are the largest branch of the Ras-related GTPase superfamily, consisting of more then 60 members in mammals, and regulate almost every step of vesicle-mediated protein transport, particularly the targeting, tethering, and fusion of transport vesicles with the appropriate acceptor membrane. Some Rab GTPases are expressed in specialized cells and modulate specific types of membrane trafficking. For example, Rab26 was found on the parotid acinar cell secretory granules to regulate amylase release (23,24). Rab26 was also shown to interact with Rim1, a Rab3-interacting protein (25). Interestingly, a recent study demonstrated that the expression of Rab26 was regulated by the transcriptional factor MIST1 and was associated with the maturation of secretory granules (26). In a continuing effort to search for small GTPases in controlling export trafficking of GPCRs, we determined the role of Rab26 by using ␣ 2 -ARs as representatives. We found that Rab26 is able to regulate the cell surface transport, specifically from the Golgi, of ␣ 2 -ARs, and this function is likely mediated by its direct and activation-dependent interaction with the receptors.
Plasmid Constructions-Human ␣ 2B -AR tagged with GFP at its C terminus (CT) (␣ 2B -AR-GFP) in the pEGFP-N1 vector was generated as described previously (12). The ORF of Rab26 was obtained by RT-PCR using total RNA extracted from HEK293 cells and the forward primer 5Ј-GAATTCTCCAGGAAGAA-GACCCCCAAG-3Ј and the reverse primer 5Ј-GGATCCT-CAAGGGCGGCAGCAGGAGG-3Ј. The ORF was then digested with the restriction enzymes EcoRI and BamHI and subcloned in frame into the pEGFP-C1 vector. Rab26 was also cloned into the pDsRed-monomer-C1 vector using the same restriction sites. To generate GST fusion proteins encoding the first (ICL1, 10 residues from Thr-39 to Leu-48) and second (ICL2, 15 residues from Trp-112 to Pro-126) intracellular loops and the CT (24 residues from Thr-427 to Trp-450) of ␣ 2B -AR, oligonucleotides containing the BamHI and XhoI restriction sites and encoding the ICL1, the ICL2 or the CT were synthesized, annealed, and ligated into the pGEX-4T-1 vector. To generate GST fusion proteins encoding the third intracellular loop (ICL3, 169 residues from Arg-194 to Val-372) of ␣ 2B -AR, full-length ␣ 2B -AR was amplified by PCR using the forward primer 5Ј-GCGGATCCCGCATCTACCTGATCGCCAAACG-3Ј and the reverse primer 5Ј-GCCTCGAGTCACACGAAGGT-GAAGCGCTTCTCCC-3Ј. The PCR product was digested with BamHI and XhoI and ligated into the pGEX-4T-1 vector. To generate the Rab26-Rluc8 construct, Rab6 was amplified by PCR using the forward primer 5Ј-GAATTCATGTCCAG-GAAGAAGACCCCCAAG-3Ј and the reverse primer 5Ј-GGA-TCCCGAGGGCGGCAGCAGGAGGCCCC-3Ј. The PCR product was digested with EcoRI and BamHI and then subcloned into the Rluc8-N1 vector. To construct ␣ 2B -AR-Venus, ␣ 2B -AR in the pEGFP-N1 vector was released by digestion using BamHI and Not1, and the fragment was subcloned into the Venus-1 vector. For in vitro translation, Rab26 was cloned into the pcDNA-3.1(Ϫ) vector at the EcoRI and BamHI restriction sites by PCR using the forward primer 5Ј-GAATTC-ATGTCCAGGAAGAAGACCCCCAAG-3Ј and the reverse primer 5Ј-GGATCCTCAAGGGCGGCAGCAGGAGG. Rab mutants were generated using QuikChange site-directed mutagenesis (Agilent Technologies, La Jolla, CA). The structure of each construct used in the present study was verified by restriction mapping and nucleotide sequence analysis.
Generation of Cell Lines Stably Expressing ␣ 2B -AR-HEK293 cells were cultured on 6-well dishes and transfected with 2 g of HA-␣ 2B -AR in the pcDNA3.1(ϩ) vector for 24 h using FuGENE HD transfection reagent according to the manufacturer's protocol. The cells were split into four 100-mm dishes and cultured for additional 24 h. The cells were then incubated with G418 at a concentration of 600 g/ml for 2 weeks. Stable transfectants were isolated and grown in DMEM containing 300 g/ml of G418. The cells stably expressing HA-␣ 2B -AR were confirmed by immunoblotting using HA antibodies and intact cell ligand binding using [ 3 H]RX821002 to quantify ␣ 2B -AR expression at the cell surface as described below. Eight stable cell lines expressing different amounts of ␣ 2B -AR were generated, and the cell line with the binding ability of 1.14 ϫ 10 Ϫ18 mol of [ 3 H]RX821002/cell was used in the current study.
Cell Culture and Transient Transfection-HEK293 and MCF-7 cells were cultured in DMEM with 10% fetal bovine serum, 100 units/ml of penicillin, and 100 g/ml of streptomycin. Transient transfection of the cells was carried out using Lipofectamine 2000 as described previously (27). For transfection of HEK293 cells stably expressing HA-␣ 2B -AR, the cells were cultured on 6-well dishes and transfected with 1.8 g of empty vectors or Rab constructs. For co-transfection, HEK293 cells were cultured on 6-well dishes and transfected with 0.2 g of ␣ 2A -AR plasmids together with 1.8 g of empty vectors or Rab constructs. The transfection efficiency was estimated to be greater than 75% based on the GFP fluorescence.
For transient transfection of MCF-7 cells, the cells were cultured in 12-well dishes to 70 -80% confluence and transfected with 1.0 g of the pEGFP-C1 vector or Rab26N177I in the pEGFP-C1 vector. After 24 h, the cells were transferred to 6-well dishes and cultured for another 24 h. The cells were then transfected for the second time with 1.8 g of the pEGFP-C1 vector or Rab26T177I. The experiments were performed at 60 h after the first transfection.
siRNA-mediated Knockdown of Rab26-Two Stealth TM RNAi duplexes (siRNA) targeting to human Rab26 (accession number NM_014353; number 1, 5Ј-GCUUCCGGCUGCAU-GAUUACHUUAA-3Ј; number 2, 5Ј-UGGUCAGGAGCG-GUUCCGCAGUGUU-3Ј) and an negative control med GC duplex were purchased from Invitrogen. HEK293 cells were cultured in 12-well dishes at a density of 1.5 ϫ 10 5 cells/well for 24 h before transfection. Two l of Lipofectamine 2000 and 1 l of 20 M siRNA were added separately into 50 l of Opti-MEM medium. After incubation for 5 min, both solutions were mixed and incubated for 20 min. The transfection mixture was added to culture dishes containing 0.9 ml of fresh DMEM without antibiotics. After incubation at 37°C for 6 -8 h, the cells were split into 6-well dishes and cultured for additional 24 h. The cells were then transfected for the second time with 2.5 l of 20 M siRNA together with 400 ng of ␣ 2B -AR. All of the experiments were performed at 72 h after the first siRNA transfection.
Confocal Fluorescence Microscopy-To visualize the subcellular distribution and co-localization of Rab26, giantin, and ␣ 2B -AR, HEK293 cells grown on coverslips precoated with poly-L-lysine on 6-well dishes were transfected and fixed with 4% paraformaldehyde in PBS for 15 min. The cells were then permeabilized with 0.1% saponin for 20 min and blocked with PBS containing 5% BSA, 0.1% gelatin, and 0.1% saponin for 30 min. The cells were sequentially stained with primary antibodies and fluorophore-conjugated secondary antibodies. The coverslips were mounted with prolong antifade reagent (Invitrogen), and images were captured using a LSM510 Zessis confocal microscope equipped with a 63ϫ objective (NA ϭ 1.3).
For visualization of stably expressed HA-␣ 2B -AR, HEK293 cells were transfected and then fixed as described above. To permeabilize the cells, the cells were incubated with PBS containing 0.2% Triton X-100 for 5 min. The cells were then incubated with PBS containing 5% BSA and 0.1% gelatin for 30 min followed by staining with HA antibodies (1:1500 dilution) and Alexa 594-conjugated secondary antibodies (1:300 dilution).
Intact Cell Radioligand Binding-The cell surface expression of ␣ 2A -AR and ␣ 2B -AR was measured by ligand binding of intact live cells using the membrane-impermeable ligand [ 3 H]RX821002 as described previously (29,30). Briefly, HEK293 cells cultured on 6-well dishes were transfected and then split into 12-well dishes. The cells were incubated with 300 l of DMEM plus [ 3 H]RX821002 at a concentration of 20 nM for 90 min at room temperature. The cells were washed twice with 1 ml of ice-cold PBS, and the cell surface-bound ligands were extracted by 1 M NaOH treatment for 2 h. The nonspecific binding was determined in the presence of rauwolscine (10 M). The radioactivity was counted by liquid scintillation spectrometry in 3.5 ml of Ecoscint A scintillation solution.
Measurement of ERK1/2 Activation-HEK293 cells were cultured on 6-well dishes and transfected with ␣ 2B -AR together with individual Rab26 mutant or Rab26 siRNA as described above. The cells were starved for at least 3 h before stimulation with UK14304 at a concentration of 1 M for 5 min. Stimulation was terminated by the addition of 200 l of ice-cold cell lysis buffer containing 50 mM Tris-HCl, pH 7.4, 150 mM NaCl, 1% Nonidet P-40, 1 mM sodium orthovanadate, 1 mM ␤-glycerophosphate, and complete Mini protease inhibitor mixture. After solubilizing the cells on ice for 20 min, 15 l of total cell lysates was separated by 12% SDS-PAGE. ERK1/2 activation was determined by measuring the levels of phosphorylation of ERK1/2 with phospho-specific ERK1/2 antibodies by immunoblotting (27).
Co-immunoprecipitation of ␣ 2B -AR and Rab26-HEK293 cells stably expressing HA-␣ 2B -AR were cultured on 100-mm dishes and transfected with 8 g of the pEGFP-C1 vector or Rab26 in the pEGFP-C1 vector for 48 h. The cells were washed twice with PBS, harvested, and lysed with 500 l of lysis buffer containing 50 mM Tris-HCl, pH 7.4, 150 mM NaCl, 1% Nonidet P-40, and complete Mini protease inhibitor mixture. After gentle rotation for 1 h at 4°C, samples were centrifuged for 15 min at 14,000 ϫ g. The supernatants were diluted with equal volume of buffer containing 50 mM Tris-HCl, pH 7.4, 150 mM NaCl, and 0.5% gelatin and then incubated with 1 g of ␣ 2B -AR antibodies overnight at 4°C with gentle rotation followed by incubation with 50 l of protein G-Sepharose 4B beads for 5 h. Resin was collected by centrifugation and washed four times each with 1 ml of lysis buffer. Immunoprecipitated receptors were eluted with 30 l of 1ϫ SDS gel loading buffer, and 20 l from each sample was separated by SDS-PAGE to probe for GFP-Rab26 in the immunoprecipitates by immunoblotting using GFP antibodies (1:3000 dilution). In parallel, each sample was further diluted five times with 1ϫ SDS gel loading buffer, separated by SDS-PAGE, and probed with HA antibodies (1:3000 dilution) to determine the amount of HA-␣ 2B -AR in the immunoprecipitates.
Bioluminescence Resonance Energy Transfer (BRET) Assays-The live cell-based BRET assay to measure the possible interaction between ␣ 2B -AR and Rab26 was carried out as described previously (31). HEK293 cells were cultured on 6-well dishes and transfected with 0.1 g of Rab26-Rluc8 and 1.5 g of ␣ 2B -AR-Venus for 24 h. The cells were then harvested and transferred to black 96-well plates. Coelenterazine h (5 mM) was added to all wells immediately prior to making measurements. Luminescence measurements were made using a Mithras LB940 photon-counting plate reader (Berthold Technologies GmbH, Bad Wildbad, Germany). Raw BRET signals were calculated by dividing the emission intensity at 520 -545 nm by the emission intensity at 475-495 nm. Net BRET was this ratio minus the same ratio measured from cells expressing only the BRET donor (Rluc8).
GST Fusion Protein Pulldown Assays-The interaction of the intracellular domains of ␣ 2B -AR with Rab26 was measured by using the MagneGST pulldown system as described essentially (18,28). Briefly, GST fusion proteins were expressed in BL21 bacteria and purified using MagneGST glutathione particles. Immobilized fusion proteins were either used immediately or stored at 4°C for no longer than 3 days. Each batch of fusion proteins used in experiments was first analyzed by Coomassie Brilliant Blue staining following SDS-PAGE. HEK239 cells were transiently transfected with GFP-tagged Rab26 or its individual mutant, and the total HEK293 cell homogenates were incubated with GST fusion proteins tethered to the glutathione particles in 500 l of binding buffer containing 20 mM Tris-HCl, pH 7.5, 140 mM NaCl, 1% Nonidet P-40, and 0.5% BSA for 4 h at 4°C. The particles were washed four times with 0.5 ml of binding buffer, and the retained proteins were solubilized in 1ϫ SDS gel loading buffer. GFP-Rab26 bound to the GST fusion proteins was detected by immunoblotting using GFP antibodies.
Translation of Rab26 in Vitro-Rab26 in the pcDNA3.1(Ϫ) vector was translated using the TNT T7 quick in vitro translation system (Promega) according to the manufacturer's protocol. Briefly, the reaction was carried out in a total volume of 50 l containing 40 l of TNT T7 Quick Master Mix, 1 l of 1 mM methionine, and 1 g of the Rab26 plasmid at 30°C for 90 min. After translation, 20 l of the reaction mixtures was diluted into 200 l of binding buffer containing 20 mM Tris-HCl, pH 7.5, 140 mM NaCl, 1% Nonidet P-40, and 0.5% BSA. Rab26 was then incubated in binding buffer containing 100 M GTP␥S or GDP for 1 h at room temperature followed by incubation with GST fusion proteins tethered to MagneGST particles as described above.
Statistical Analysis-Differences were evaluated using Student's t test, and p Ͻ 0.05 was considered as statistically significant. The data are expressed as the means Ϯ S.E.

Effects of Rab26 and Its Mutants on the Cell Surface
Transport of ␣ 2A -AR and ␣ 2B -AR-We first studied the subcellular distribution of endogenous Rab26 and exogenously expressed GFP-tagged Rab26 and its constitutively active and dominant-negative mutants in HEK293 cells. As revealed by confocal microscopy, endogenous Rab26 was strongly co-localized with giantin, a Golgi marker, in HEK293 cells (Fig. 1A). The subcellular distribution of GFP-tagged Rab26 was very much similar to endogenous Rab26 (Fig. 1B). Consistent with the subcellular distribution of many other Rab mutants, the GTP-bound mutant Rab26Q123L was largely localized to the perinuclear region, whereas the GDP-bound mutant Rab26T77N and the guanine nucleotide binding-deficient Rab26N177I were predominantly expressed in the cytoplasm (Fig. 1B). Giantin Merge We then determined the effect of transient expression of Rab26 and all three Rab26 mutants on the cell surface expression of both ␣ 2A -AR and ␣ 2B -AR. The expression of Rab26 and its mutants was measured by immunoblotting using both GFP and Rab26 antibodies (Fig. 1C), and the cell surface expression of ␣ 2A -AR and ␣ 2B -AR was determined by ligand binding in intact live cells using the radioligand [ 3 H]RX821002. Transient expression of all three Rab26 mutants significantly inhibited the cell surface expression of ␣ 2 -ARs with the order of N177I Ͼ T77N Ͼ Q123I, and the inhibitory effects were very similar between ␣ 2A -AR and ␣ 2B -AR (Fig. 1D). In contrast, expression of wild-type Rab26 only moderately enhanced the cell surface expression of both receptors and expression of Rab26 mutants did not significantly alter the total expression of ␣ 2B -AR (Fig.  1D).
To confirm the inhibitory effect of the Rab26 mutants on the cell surface expression of ␣ 2 -ARs as measured by intact cell ligand binding, the subcellular distribution of HA-␣ 2B -AR was then visualized by confocal microscopy after staining with anti-HA antibodies. In nonpermeabilized cells, ␣ 2B -AR was robustly expressed at the cell surface in cells expressing GFP alone, whereas the cell surface expression of ␣ 2B -AR was markedly attenuated in cells expressing Rab26N177I (Fig. 1E). In Triton X-100-treated cells, expression of Rab26N177I induced an extensive accumulation of ␣ 2B -AR in a discrete compartment in the perinuclear region (Fig. 1F). Quantitative analysis showed that ϳ3 and 90% cells had intracellular accumulation of the receptor in cells expressing GFP and GFP-tagged Rab26N177I, respectively (Fig. 1G). These data demonstrate that Rab26 is required for the cell surface transport of ␣ 2 -ARs.
Inhibition of the Cell Surface Expression of ␣ 2A -AR and ␣ 2B -AR by siRNA-mediated Knockdown of Rab26-We next determined the effect of siRNA-mediated depletion of endogenous Rab26 on the cell surface targeting of both ␣ 2A -AR and ␣ 2B -AR. Introduction of each Rab26 siRNA markedly and specifically knocked down Rab26 by ϳ90% without affecting the expression of the closely related small GTPase Rab4 and ␤-actin when compared with cells transfected with control siRNA and cells without transfection (Fig. 2, A and B). siRNA-mediated knockdown of Rab26 strongly inhibited the cell surface expression of both ␣ 2A -AR and ␣ 2B -AR as measured by intact cell ligand binding (Fig. 2C). In contrast, siRNA-mediated knockdown of Rab26 did not significantly influence the overall expression of ␣ 2B -AR (Fig. 2C). Subcellular distribution analysis showed that expression of Rab26 siRNA arrested ␣ 2B -AR in the intracellular compartments, and the receptors were unable to transport to the cell surface (Fig.  2D). These data further suggest that the normal function of Rab26 is essential for the cell surface movement of ␣ 2A -AR and ␣ 2B -AR.
Effect of Rab26 Mutants and Rab26 siRNA on the Cell Surface Expression of Endogenous ␣ 2 -ARs-Similar to the results obtained in HEK293 cells transfected with exogenous ␣ 2A -AR and ␣ 2B -AR, transient expression of the dominant-negative mutant Rab26N177I (Fig. 3A) significantly inhibited the cell surface expression of endogenous ␣ 2 -ARs in MCF-7 cells by 45% as measured by intact cell ligand binding (Fig. 3B). Expression of each Rab26 siRNA to knockdown Rab26 (Fig. 3, C and D) also significantly reduced the cell surface expression of endogenous ␣ 2 -ARs by ϳ35% (Fig. 3E). These data demonstrate an important role of Rab26 in the cell surface transport of endogenous ␣ 2 -ARs.
Effect of Rab26 Mutants and Rab26 siRNA on ERK1/2 Activation by ␣ 2B -AR-To define whether the attenuated ␣ 2 -AR expression at the cell surface caused by expression of Rab26 mutants or Rab26 siRNA could result in a concomitant defective signaling, we measured the activation of the mitogen-activated protein kinase pathway, specifically ERK1/2, in response to stimulation with the ␣ 2 -AR agonist UK14304. ERK1/2 activation by UK14304 was significantly compromised in cells expressing ␣ 2B -AR together with individual Rab26 mutant, whereas expression of wild-type Rab26 only slightly enhanced ERK1/2 activation as compared with cells expressing ␣ 2B -AR alone (Fig. 4, A and B). Furthermore, ERK1/2 activation by UK14304 was markedly blocked by each Rab26 siRNA as compared with cells transfected with control siRNA (Fig. 4, C and  D). These data are consistent with the inhibitory effect of Rab26 mutants and Rab26 siRNA on the cell surface expression of ␣ 2B -AR and suggest that Rab26 is able to modulate not only the trafficking but also the signaling of ␣ 2 -ARs.
Inhibition of Exit from the Golgi of ␣ 2B -AR by Rab26 Mutants and Rab26 siRNA-To define the intracellular compartment in which Rab26 regulates ␣ 2 -AR transport, Rab26 and its mutants were tagged with DsRed. The subcellular distribution of DsRed-tagged Rab26 was almost identical to their GFP-tagged forms (Fig. 5A), suggesting that epitope tagging (i.e., GFP and DsRed) did not have significant impacts on the subcellular localization of Rab26.

Rab26 Regulation of ␣ 2 -AR Export
with different intracellular markers was visualized by confocal microscopy. ␣ 2B -AR was clearly expressed at the cell surface in cells transfected with the pEGFP-C1 vector and wild-type Rab26. ␣ 2B -AR was partially co-localized with the Golgi marker giantin in cells expressing Rab26Q123L. In contrast, ␣ 2B -AR was remarkably co-localized with giantin, but not ER markers (data not shown), in cells transfected with Rab26T77N and Rab26N177I (Fig. 5A). Furthermore, ␣ 2B -AR was also extensively co-localized with giantin but not ER markers (data not shown) in cells transfected with Rab26 siRNA (Fig. 5B). These data demonstrate that inhibition of Rab26 function by expressing Rab26 mutants and Rab26 siRNA induces defective ␣ 2B -AR transport at the level of the Golgi. Activation-dependent Interaction of Rab26 with Full Length ␣ 2B -AR-To elucidate the possible molecular mechanisms underlying the function of Rab26 in regulating ␣ 2 -AR transport, we sought to define whether Rab26 is able to interact with ␣ 2B -AR. In the first series of experiments, we determined the interaction of ␣ 2B -AR with Rab26 and its three mutants by co-immunoprecipitation. HEK293 cells stably expressing HA-␣ 2B -AR were transiently transfected with GFP-Rab26, and HA-␣ 2B -AR was immunoprecipitated by ␣ 2B -AR antibodies. The amounts of Rab26 in the immunoprecipitates detected by immunoblotting using GFP antibodies were much higher from cells expressing wild-type Rab26 and the active mutant Q123L than from cells expressing the dominant-negative mutants T77N and N177I (Fig. 6A).
In the second series of experiments, we used the BRET assay to measure whether Rab26 and ␣ 2B -AR are in close proximity in live cells. HEK293 cells were transfected with Venus-tagged ␣ 2B -AR and Rluc8-tagged Rab26 or individual Rab26 mutant. The net BRET between ␣ 2B -AR and wild-type Rab26 or the Rab26 siRNA#2  constitutively active mutant Rab26Q123L was significantly higher than those between ␣ 2B -AR and the dominant-negative mutants Rab26T77N and Rab26N177I (Fig. 6B). These data reveal that ␣ 2B -AR is able to form a complex with Rab26 GTPase, preferentially in its active GTP-bound form.
Direct and Activation-dependent Interaction of Rab26 with the Third Intracellular Loop of ␣ 2B -AR-To identify the intracellular domain mediating ␣ 2B -AR interaction with Rab26, the ICL1, the ICL2, the ICL3, and the CT of ␣ 2B -AR were generated as GST fusion proteins, and their abilities to interact with Rab26 were determined in a GST fusion protein pulldown assay. The GST fusion proteins containing the ICL3, but not the ICL1, the ICL2 and the CT, were capable of interacting with Rab26 (Fig. 7A). The interaction of the ICL3 with wild-type Rab26 and its GTP-bound mutant Rab26Q123L was stronger than with the mutants Rab26T77N and Rab26N177I (Fig. 7B).
We then determined whether the interaction between Rab26 and the ICL3 is direct or indirect. Rab26 was expressed using an in vitro translation system, and its interaction with the ICL3 was determined by the GST fusion protein pulldown assay. The interaction of Rab26 with the GST-ICL3 was more potent in the present of GTP␥S than in the presence of GDP (Fig. 7C), whereas the interaction between GST and Rab26 in the presence of GTP␥S or GDP was undetectable. These data indicate that Rab26 directly interacts with the ICL3 of ␣ 2B -AR, and the interaction largely depends on the activation of Rab26.

DISCUSSION
The molecular mechanisms underlying the anterograde transport of nascent GPCRs, especially from the Golgi to the cell surface, remain poorly elucidated. We have previously demonstrated the roles of Ras-like small GTPases and found that Rab1, Rab2, and Rab6 modulate the transport of GPCRs between the ER and the Golgi, whereas Rab8 and ARF1 are likely involved in receptor transport between the Golgi and the plasma membrane (18,19,27,(32)(33)(34). As a continuing effort to search for small GTPases in controlling export trafficking of GPCRs, in this study we have defined the role of Rab26 in the cell surface expression of GPCRs by focusing on ␣ 2 -ARs. Compared with many other secretory GTPases, Rab26 is relatively less well characterized. Rab26 was found to highly express in secretory cells and to regulate vesicle-mediated secretion (23,24,26). However, whether or not Rab26 is able to regulate the transport of plasma membrane proteins has not been studied. The data presented in this paper have clearly demonstrated that Rab26 modulates the cell surface transport of both ␣ 2A -AR and ␣ 2B -AR. By contrast, expression of several other secretory Rab GTPases, including Rab3A, Rab3B, Rab3C, Rab3D, Rab27A, Rab27B, and Rab37 in the same functional group (23)(24)(25)(26)(35)(36)(37)(38)(39)(40) did not influence the cell surface expression of ␣ 2 -ARs (data not shown). These data suggest that Rab26 is a unique secretory GTPase in this group that may function as a crucial regulator for the Golgi to cell surface movement of ␣ 2 -ARs. These data also provide the first evidence indicating that, in addition to its secretory function, Rab26 may play an important role in the transport of plasma membrane proteins, such as GPCRs.
In this study, the gain of Rab26 function was achieved by overexpression of Rab26, and the loss of Rab26 function achieved by transient expression of Rab26 mutants and siRNA-mediated depletion of endogenous Rab26. We have shown that expression of Rab26 mutants and siRNA-mediated knockdown of Rab26 significantly inhibited the cell surface expression of exogenously transfected ␣ 2 -ARs in HEK293 cells as well as endogenous ␣ 2 -ARs in MCF-7 cells. It is well known that the function of Rab GTPases in coordinating vesicular transport is mediated through their GTP/GDP exchange cycles. The inactive, GDP-bound conformation of Rab GTPases is maintained in the cytosol and undergoes GDP exchange for GTP. GTP-bound Rab will be hydrolyzed to the GDP-bound form for recycling back to donor membrane. Similar to the effect of GDP-bound mutant Rab26T77N and the guanine nucleotide binding-deficient Rab26N177I, expression of GTPbound mutant Rab26Q123L inhibited the cell surface expression of ␣ 2 -ARs, suggesting that the recycling of Rab26 is an essential event in controlling the cell surface transport of ␣ 2 -ARs. A, interaction of the intracellular domains of ␣ 2B -AR with Rab26 as measured by GST fusion protein pulldown assays. ICL1, ICL2, ICL3, and CT of ␣ 2B -AR were generated as GST fusion proteins (FP) (lower panel). GFP-tagged Rab26 was expressed in HEK293 cells, and total cell homogenates were incubated with the purified GST fusion proteins as described under "Experimental Procedures." Rab26 bound to the GST fusion proteins was revealed by immunoblotting using GFP antibodies (upper panel). B, interaction of the ␣ 2B -AR ICL3 with Rab26 and its mutants. GFP-tagged Rab26 and its mutants were transiently expressed in HEK293 cells, and their interaction with the GST-ICL3 fusion proteins (FP) (lower panel) was determined by the GST pulldown assay. Bound Rab26 was detected by immunoblotting using GFP antibodies (upper panel). C, direct and activation-dependent interaction of Rab26 with the ␣ 2B -AR ICL3. Rab26 prepared by the in vitro translation system was incubated with 100 M GTP␥S or GDP for 1 h and then incubated with GST or the GST-ICL3 fusion proteins as described under "Experimental Procedures." In each panel, similar results were obtained in at least three different experiments. MW, molecular mass.
Expression of Rab26 mutants and Rab26 siRNA also attenuated ␣ 2 -AR-mediated signaling, specifically ERK1/2 activation. The reduction of ␣ 2 -AR-mediated ERK1/2 activation is presumably caused by the attenuation of receptor transport to the cell surface. However, we cannot exclude the possibility that inhibition of Rab26 function by expressing Rab26 mutants or Rab26 siRNA may alter ␣ 2 -AR coupling to other signaling molecules involved in receptor signaling systems, which may also contribute to the disruption of normal signal propagation of the receptors. By contrast, overexpression of Rab26 only moderately enhanced the cell surface numbers of ␣ 2 -ARs and receptor-mediated ERK1/2 activation, suggesting that the endogenous expression level of Rab26 may not be a rate-limiting factor for the cell surface transport of newly synthesized ␣ 2 -ARs. Nevertheless, these studies have strongly demonstrated for the first time that the small GTPase Rab26 plays a crucial role in the anterograde transport of ␣ 2 -ARs. However, whether or not Rab26 is involved in the cell surface transport of other GPCRs needs further investigation.
Rab26 likely modulates the cell surface transport of ␣ 2 -ARs from the Golgi. This became evident as ␣ 2B -AR was extensively co-localized with the Golgi marker giantin in cells expressing the dominant-negative Rab26 mutants and Rab26 siRNA, indicating that attenuation of Rab26 function blocks the exit of newly synthesized ␣ 2B -AR out of the Golgi apparatus. This notion is also supported by the localization of Rab26 in the Golgi. Interestingly, ␣ 2B -AR was only partially accumulated in the Golgi in cells expressing the constitutively active GTPbound mutant Rab26Q123L, suggesting that expression of Rab26Q123L may also arrest ␣ 2B -AR in the post-Golgi compartments.
Another important finding presented in this manuscript is that, as determined by co-immunoprecipitation, BRET, and GST fusion protein pulldown assays, Rab26 directly and activation-dependently interacted with ␣ 2B -AR, providing a possible molecular mechanism responsible for Rab26 regulation of post-Golgi traffic of ␣ 2 -ARs. There are a number of interesting points regarding the interaction between ␣ 2B -AR and Rab26. First, the interaction of ␣ 2B -AR with Rab26 depends on the activation of Rab26, which is in contrast to the interaction of ␤ 2 -AR and thromboxane A2 receptor with Rab11 (41,42) and the interaction of ␣ 2B -AR and ␤ 2 -AR with Rab8, which prefer the inactive GDP-bound Rab mutants (18). These data suggest ␣ 2B -AR may be a downstream effector of Rab26 GTPase, because it has been well described that the downstream effectors strongly interact with the GTP-bound Rab GTPase mutants. Second, we have previously demonstrated that ␣ 2B -AR uses its CT to associate with Rab8 (18), suggesting that ␣ 2B -AR may utilize different intracellular domains to interact with distinct Rab GTPases (i.e., the CT with Rab8 versus the ICL3 with Rab26). Because ␣ 2B -AR preferentially interacts with the inactive form of Rab8 and the active form of Rab26, these interactions are likely to have different functions in modulating ␣ 2B -AR export. Third, Rab26 represents the first Rab GTPase interacting with the ICL3 of GPCRs. Several other Rab GTPases have been shown to interact with the CTs of the receptors. For example, Rab4 and Rab11 interact with the CTs of angiotensin II type 1 receptor, ␤ 2 -AR, and thromboxane A2 receptor to regulate their recycling from endosomes back to the plasma membrane (41)(42)(43). Fourth, we have demonstrated that Rab26 generated by the in vitro translation system was able to interact with purified GST-ICL3 fusion proteins, suggesting that the interaction between ␣ 2B -AR and Rab26 is direct. These data, together with other studies (18,28,(41)(42)(43), suggest that the cargo GPCRs may physically bind to components of transport machinery, such as Rab GTPases, to control their transport to the cell surface.
The cell surface targeting of nascent GPCRs is one of the most important factors determining the functionality of the receptors and is directly linked to the pathogenesis of human diseases (44 -47), including those essentially caused by naturally occurring mutations in the receptors resulting in the defective transport to the functional destination. Great progress has been achieved over the past several years in defining the role of small GTPases in the cell surface transport of GPCRs en route from the ER and the Golgi (17-19, 27, 32-34, 48 -50). These studies have demonstrated that the proper receptor expression at the cell surface is tightly controlled by a number of small GTPases, each of which may regulate a specific step in the biosynthesis and processing of GPCRs. Because the dysfunction of Rab GTPases has also been implicated in the development of human diseases (51)(52)(53)(54), to thoroughly explore the regulatory roles of small GTPases in the anterograde transport of nascent GPCRs, as well as possible underlying mechanisms, may reveal novel therapeutic targets for human diseases involving abnormal trafficking and signaling of GPCRs.