The Basolateral Localization Signal of the Follicle-stimulating Hormone Receptor*

The follicle-stimulating hormone receptor (FSHR) is physiologically localized in the basolateral compartment of the membrane of Sertoli cells. This localization is also observed when the receptor is experimentally expressed in Madin-Darby canine kidney cells. We thus used in vitro mutagenesis and transfection into these polarized cells to delineate the basolateral localization signal of the receptor. The signal was localized in the C-terminal tail of the intracellular domain (amino acids 678–691) at a marked distance of the membrane. Mutation of individual amino acids highlighted the importance of Tyr684 and Leu689. The 14-amino acid sequence was grafted onto the p75 neurotrophin receptor and redirected this apical protein to the basolateral cell membrane compartment. Deletion of amino acids 677–695 did not modify the internalization of the FSHR, showing that the basolateral localization signal of the FSHR is not colinear with its internalization signal.

The FSH 1 receptor (FSHR), along with the LH and TSH receptors, belongs to a subgroup of G protein-coupled receptors (reviewed in Refs. [1][2][3][4][5]. These highly homologous proteins are unusual among G protein-coupled receptors in that they contain a very large extracellular hormone binding domain. FSH transduction pathways involve mainly G s proteins coupling and the ensuing activation of adenylate cyclase (5).
The FSH receptor has a central role in reproduction through the control of gonadal development and gamete production (5). In males it is expressed in testicular Sertoli cells. These cells form an epithelial blood barrier and control the development of spermatogenesis (6). In females, the FSHR is expressed in the granulosa cells of the ovaries. It controls follicular growth and, in cooperation with the LH receptor, ovarian steroidogenesis (2).
Little is known of the intracellular trafficking of G proteincoupled receptors and, more specifically, of this subgroup of receptors. The pathways of internalization of the LH and TSH receptors have been studied at the ultrastructural level (7), 2 but the molecular signals involved are still unknown. The LH receptor is also present in the vascular endothelium of the testes (8). This endothelial LH receptor is involved in receptormediated hormone transcytosis, leading to the accumulation of the hormone in close proximity to the target cells. FSHR has been detected in the vascular endothelial cells of the testes (9), but its role in hormone transcytosis has not been studied to date.
Another particularity of gonadotrophin and thyrostimulin receptors is their polarized cellular expression in several target tissues. Immunocytochemical studies with monoclonal antibodies have shown that the FSH receptor in Sertoli cells and the TSH receptor in thyroid follicular cells have a polarized basolateral expression (9,10). By contrast, the LH receptor is expressed circumferentially in the ovarian granulosa and theca cells and in the testicular Leydig cells (11,12). This difference in receptor distribution could result from either a difference in the structure of the receptors or from differential expression in polarized or nonpolarized cells. To distinguish between these possibilities, we have transfected the three receptors into polarized MDCK cells (13). We observed that all three receptors were directly delivered to the basolateral membrane of these cells, suggesting that they all contain a basolateral targeting signal.
The molecular signals involved in the polarized targeting of G protein-coupled receptors and more generally of hormone receptors in epithelial cells have still not been identified. We have thus undertaken to delineate this signal in the FSH receptor. In vitro mutagenesis was followed by establishment of permanent MDCK cell lines expressing mutated receptors. We report here the localization of the basolateral targeting signal, its structure, the functional importance of the individual amino acids, and the noncolinearity of this signal with the signal for receptor endocytosis.
In the case of the FSHR⌬657-695, a 821-base pair fragment (positions 1244 -2065 of the cDNA sequence, ϩ1 being the first base of the initiation codon) containing a stop codon at residue 657 was constructed. After digestion with PflMI, the purified fragment was subcloned into the pSG5-FSHR vector previously digested with the same enzyme.
* 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  In the case of the FSHR⌬677-695, the same strategy was used. A 486-base pair fragment (positions 1793-2279 of the cDNA sequence) containing a stop codon at residue 677 was digested with SapI and EcoRI. It was cloned into the pSG5-FSHR vector previously digested with SapI and EcoRI.
The FSHR⌬692-695 mutant was prepared by generating a KpnI site at position 3098 of pSG5-FSHR using the same strategy as described above for the FSHR⌬677-695 mutant. This mutation yields a conservative substitution of threonine 682 for a serine. This substitution does not alter the polarity or the functional properties of the receptor. A synthetic double strand mutated oligonucleotide was then inserted between the KpnI and the EcoRI sites.
All of the constructs were verified by double-stranded DNA sequencing.
Site-directed Mutagenesis of Receptor Basolateral Localization Signal-Alanine scanning mutagenesis of the FSH receptor basolateral signal was performed using a series of synthetic oligonucleotides inserted between two restriction sites flanking the signal. The oligonucleotides were introduced either between the SapI (position 3075 of the pSG5 FSHR vector) and the KpnI sites or between the KpnI and the EcoRI (position 3145) sites.
The mutants Gly 681 3 Ala and Ser 682 3 Ala were constructed using polymerase chain reaction-mediated oligonucleotide mutagenesis. A 486-base pair fragment (positions 1793-2279 of the cDNA sequence) containing the Gly 681 3 Ala or the Ser 682 3 Ala substitution was digested with SapI and EcoRI, purified, and cloned into the SapI-EcoRI-digested pSG5-FSHR.
Generation of the Truncated Neurotrophin Receptor (NTRt)/FSHR-(678 -691) Hybrid Receptor-The neurotrophin receptor (NTR) cDNA, cloned in the pCB6 vector, which lacks its intracellular domain (six residues downstream the transmembrane domain), has previously been described (14). The basolateral localization signal of the FSHR was introduced into the XbaI site. The stop codon TAG, located just before the basolateral signal, was then mutated to a GAG codon encoding a glutamine using the same strategy as described above, yielding the NTRt/FSHR-(678 -691) hybrid receptor.
The control vector (NTRt) that was used contained a stop codon just before the first residue of the signal.
Cell Culture and Expression of FSHR Mutants-MDCK cells (type II) were seeded and grown on coverslips (Nunc) or filters (0.4-m polycarbonate Transwell Costar Corp., Cambridge, MA) as described previously (13). The cells were cotransfected with expression vectors encoding either the wild-type or the mutated FSHR and with the plasmid pSV-neo, which confers resistance to the antibiotic G418. The clones were screened for receptor expression by immunocytochemistry (13) using the monoclonal anti-FSH receptor, FSHR323 (9). Sodium butyrate (10 mM) (Sigma) was added to increase the expression of the transfected cDNAs as described (15).
Control of Cell Polarity-The transepithelial resistance of the clones was ϳ300 ohms/cm 2 . Indirect immunofluorescence studies were performed as described (13) using antibodies (a gift of A. Le Bivic) directed against a basolateral (BC11) or an apical (BB18) endogenous protein of MDCK cells. Polarized apical secretion of the endogenous glycoprotein complex gp80 (16) was verified as described previously (17).
Immunofluorescence and Confocal Microscopy-MDCK cells were grown to confluence on coverslips as described (13). The cells were washed with PBS ϩ (phosphate-buffered saline (Dulbecco's formulation) with 0.1 mM CaCl 2 and 1 mM MgCl 2 ) and fixed for 15 min in 3% paraformaldehyde in PBS ϩ . The cells were washed, and the aldehyde groups were quenched with 50 mM NH 4 Cl in PBS ϩ for 30 min. After 1 h of saturation with PBS ϩ , 1% BSA (albumin, fraction V, Boehringer Mannheim), the cells were incubated for 2 h at room temperature in a humid chamber with the monoclonal anti-FSHR antibody FSHR323, (5 g/ml in PBS ϩ , 1% BSA). The cells where then washed with PBS ϩ , 1% BSA, 0.1% Tween 20 (Sigma) and incubated for 1 h with a Cy3-labeled rabbit anti-mouse IgG (Sigma). After washing, the cells were mounted with fluorescent Montaing medium (DAKO) and observed with a Zeiss microscope (Axiovert 135M) in conjunction with a confocal laser scanning unit (Zeiss LSM 410). To open the tight junctions, MDCK cells were incubated for 1 h with Ca 2ϩ -free Dulbecco's modified Eagle's medium, washed twice with calcium and magnesium-free PBS, and then incubated with 10 mM EGTA in the same buffer (13). The cells were then fixed in 3% paraformaldehyde for 15 min and further processed as described above.
cAMP Assay-cAMP was measured as described (9). The cells were incubated in the absence or in the presence of 50 nM FSH (Metrodine, Serono).
Surface Immunoprecipitation of Wild-type and Mutated FSH Recep-tors-Polarized monolayers of MDCK cells were obtained after 2 days of culture on 24-mm filters. Receptors present on the cell surface were immunoprecipitated as described previously (13). Briefly, cells were pulse-labeled for 1 h with 1 mCi/ml Express 35 S (NEN Life Science Products) and chased for 3 h in the same medium containing unlabeled amino acids and 1% BSA. Monoclonal antibody FSHR323 was added to the apical or to the basolateral compartment during the last hour of the chase period. Extraction and purification of receptor-antibody complex were performed as described (13). All experiments were performed at least twice with triplicate samples. Surface Immunoprecipitation of the NTRt/FSHR-(678 -691) Chimera-MDCK cells grown on filters were incubated for 30 min in DMEM without cysteine and pulsed for 30 min in the same medium containing 1 mCi/ml [ 35 S]cysteine (NEN Life Science Products) as described (18). After washing with DMEM, the cells were chased for 3 h in DMEM containing unlabeled amino acids.
Cell surface immunoprecipitation was performed as described (19). Briefly, the monoclonal antibody ME20.4 directed against the ectodomain of the human neurotrophin receptor was added either in the apical or in the basolateral compartment (5 l of ascites fluid/ml) for 1 h at 4°C. After the incubation, the filters were washed five times for 10 min with DMEM containing 0.5% BSA. Receptor-antibody complexes were extracted and purified as described (18).
Electrophoresis and Autoradiography-SDS-polyacrylamide gel electrophoresis was performed as described (13). The gels were fixed and processed for fluorography. Densitometric scanning was used for quantification.
Receptor-mediated FSH Internalization-COS-7 cells were transfected with wild-type FSHR or the truncated FSHR⌬677-695 using Superfect (Qiagen, Chatsworth, CA). Internalization from the cell surface of wild-type FSHR or of the truncated FSHR⌬677-695 was determined by using 125 I-labeled FSH (NEN Life Science Products, 130 Ci/g, 26 Ci/ml). COS-7 cells plated on six-well plates were quickly cooled to 4°C using two washes with ice-cold PBS, 1% BSA. The cells were incubated with 125 I-FSH (7000 cpm/l in PBS, 1% BSA) for 10 min at 37°C. The cells were then extensively washed at 4°C to remove unbound ligand and were allowed to internalize the ligand for various time periods at 37°C. One set of plates that were kept at 4°C was used to measure the initial FSH binding. The internalization was stopped by rapid cooling to 4°C. The ligand remaining on the cell surface was stripped with 50 mM glycine in PBS containing 1% BSA, pH 2, for 10 min at 4°C. Finally, the cells were treated with 0.2 M NaCl, and the total radioactivity was counted. Nonspecific binding was determined in the presence of an excess of unlabeled FSH (Serono) and with nontransfected COS-7 cells. Internalized ligand was expressed as the percentage of the total radioactivity (corrected for nonspecific binding) initially bound to the cells.
Internalization was also studied in MDCK cells permanently expressing the wild-type FSHR or the truncated FSHR⌬677-695. The cells were seeded on filters and incubated 48 h later with 125 I-FSH added to the basolateral compartment (10 min at 37°C). The MDCK cells were then processed as described above for the COS-7 cells.
Secondary Structure Predictions-Secondary structure predictions were performed using computer modeling. Two methods were used: the protein sequence analysis method (20) and neural network prediction (21).

RESULTS
The C-terminal Region of the Intracellular Domain of FHSR Is Involved in Basolateral Sorting-To establish whether the intracellular domain of the FSH receptor contains basolateral sorting information, we constructed three mutants deleted for various parts of the C-terminal tail of the receptor (Fig. 2A). The truncated receptors were stably expressed in MDCK cell lines. For each mutant, several (at least five) independent clones exhibiting different levels of expression were analyzed with the anti-FSH receptor monoclonal antibodies by confocal microscopy (Fig. 1). Monoclonal antibodies BC11 (Fig. 1a) and BB28 (Fig. 1b) that recognize the ectodomain of endogenous antigens restricted to the basolateral and the apical compartments of MDCK cell membrane, respectively (13), were used as controls.
In the first mutant (FSHR⌬657-695), the major part of the intracellular domain of the receptor was deleted. The cell lines expressing mutant or wild-type receptors were incubated with the anti-FSHR ectodomain antibody in the presence of EGTA (Fig. 1). This compound opens the tight junctions (13) and thus allows antibodies to reach the totality of the cell surface. In cells expressing the wild-type receptor, a strong labeling of the basolateral membrane was observed with a reticular pattern characteristic of basolateral antigens (see in Fig. 1c a confocal horizontal section close to the basal pole of the cell). In cells expressing the truncated FSHR⌬657-695 receptor, there was a strong apical labeling (see in Fig. 1d a confocal horizontal section close to the apical pole of the cell). Confocal vertical sections confirmed that the wild-type FSHR was predominantly expressed in the basolateral domain (Fig. 1c) and the FSHR⌬657-695 in the apical domain of cell membranes (Fig. 1d).
To narrow down the sequences of the cytoplasmic tail of the FSHR necessary for basolateral targeting, we next analyzed by confocal microscopy two additional deletion mutants: FSHR⌬677-695 and FSHR⌬692-695. While the first mutant exhibited mainly an apical localization very similar to that of the FSHR⌬657-695 mutant (Fig. 1e), the second mutant, FSHR⌬692-695, had a predominant basolateral polarized expression indistinguishable from the wild-type receptor (Fig. 1f). Thus, the sequence located between residues 678 and 691 is very likely to contain basolateral sorting information.
To quantify the relative proportion of receptor targeted to each membrane domain at steady state, we performed surface immunoprecipitation experiments. MDCK cells expressing wild-type or mutated receptors were pulse-labeled with [ 35 S]methionine and cysteine for 1 h and chased for 3 h with a medium containing unlabeled amino acids (see "Materials and Methods"). Antireceptor antibodies were added either to the basal or to the apical compartment of MDCK cells during the last hour of the chase period. The receptor-antibody complexes were purified and analyzed by gel electrophoresis. For the wild-type FSHR, ϳ85% of the receptor molecules were immunoprecipitated from the basolateral surface (Fig. 2B). The same result was obtained for the FSHR⌬692-695 truncated mutant, while for clones expressing the ⌬677-695 or ⌬657-695 truncated mutants only ϳ30% of the receptor molecules were found on the basolateral surface of MDCK cells, the majority of receptors being detected on the apical domain.
These results thus confirm the immunocytochemical studies localizing the signal between residues 678 and 691 of the FSHR.
The Truncated FSH Receptor ⌬677-695 Is Directly Targeted to the Apical Membrane of MDCK Cells-The truncation of FSH receptor at amino acid 677 might have provoked a change in receptor polarization by several mechanisms: loss of a basolateral sorting signal, increased transcytosis, or deletion of a basolateral anchoring sequence (reviewed in Refs. 22 and 23).
Analysis of the kinetics of appearance of the newly synthesized protein on the basolateral and apical cell surfaces allows us to distinguish between these mechanisms (15, 19, 24 -26). Such an analysis was performed by metabolic labeling of the cells followed by selective surface immunoprecipitation at various time intervals. As shown in Fig. 3, after 30 min of chase, During the chase, an antibody specific for the extracellular domain of the FSHR was added either in the apical (Ap) or in the basolateral (Bl) compartment. The antibody-receptor complexes were purified (see "Materials and Methods"), and the radiolabeled receptor was analyzed by SDS-polyacrylamide gel electrophoresis and fluorography. Molecular mass markers (kDa) are indicated on the left. Note that a variable proportion of the precursor high mannose receptor localized in the endoplasmic reticulum coprecipitates with the mature receptor present on the cell surface. This observation has previously been reported and ascribed to the tendency of these receptors to aggregate during the purification process (13). the newly synthesized FSH receptor ⌬677-695 was already detected on the apical surface of MDCK cells. A minority of receptor molecules (ϳ25%) were also delivered to the basolateral surface of MDCK cells at this time. This was in marked contrast to what has been previously observed for the wild-type receptor, which was mainly detected on the basolateral membrane of MDCK cells in the same conditions (13). Throughout the chase period, delivery of the mutant receptor to each surface remained constant at each time period. Approximately 60 -70% of the labeled receptor molecules were found on the apical surface. There was no indication that the truncated receptor presented an increased transcytosis or decreased anchorage at the basolateral surface of the cells. This experiment thus suggested that a basolateral localization signal has been inactivated in the truncated receptor. Furthermore, the fact that the majority of receptor molecules were delivered to the apical cell surface indicates that apical sorting signal(s) may be present elsewhere in the FSHR protein and be revealed by the deletion of basolateral targeting sequences.
Individual Amino Acids Involved in the Basolateral Localization Signal-To define precisely the amino acids involved in receptor basolateral targeting, we mutated each one of the 14 residues of the 678 -691 sequence into an alanine ( Table I). The polarized expression of the receptor mutants was analyzed by confocal microscopy and quantified by cell surface immunoprecipitation. As shown in Figs. 4 and 5, the main effects were observed with mutations of Tyr 684 or Leu 689 . The corresponding receptor mutants exhibited a 55 and 45% delivery to the apical surface of MDCK cells, respectively (Fig. 5). More limited effects were observed when residues in the vicinity of Tyr 684 and Leu 689 were mutated; about 35% of Gly 681 , Ile 685 , and His 691 mutant receptors were detected at the apical surface of MDCK cells (Fig. 5 and Table I). Basolateral localization was not altered by point mutations of serine and threonine into alanine, indicating that phosphorylation of these amino acids is not involved in the vectorial sorting of the FSHR.
In conclusion, we have established that the basolateral signal of the FSH receptor comprises approximately 14 residues from positions 678 -691. The activity of this signal is mainly dependent on Tyr 684 and Leu 689 , while other residues play a less important role. Protein structure predictions (20,21) suggest the existence of a ␤-turn starting at Thr 680 and involving four residues (see Table I). It is followed by a ␤-sheet involving Tyr 684 -Val 688 . Tyrosine residues and ␤-turn have been shown -------------A 35 to have a role in several other basolateral localization signals (19, 24 -29).
The FSHR Contains a Basolateral Targeting Signal That Is Autonomous and Transferable to a Heterologous Protein-We have mapped within the C-terminal end of the intracellular tail of the FSHR a sequence corresponding to residues 678 -691 that was very likely to correspond to a basolateral sorting signal. Indeed, the deletion or mutation of this sequence altered the basolateral expression of the receptor. However, it remained possible that this sequence was only a part of the signal and thus necessary but not sufficient to target the receptor to the basolateral membrane. To establish unequivocally that amino acids 678 -691 corresponded to an autonomous basolateral signal sequence, it was necessary to demonstrate that this polypeptide could redirect a heterologous protein to the basolateral domain. We thus grafted this sequence onto an apically expressed protein: the p75 NTR truncated after the fifth amino acid of its cytoplasmic tail. This truncated receptor has been previously shown to be localized mainly (ϳ80%) in the apical compartment of the MDCK cell membrane (19).
A hybrid receptor NTRt/FSHR-(678 -691) was constructed, and the polarized expression of the hybrid receptor was studied by confocal microscopy and surface immunoprecipitation in parallel with that of the NTRt. Confocal microscopy confirmed the presence of NTRt at the apical surface of MDCK cells. In contrast, the chimera NTRt/FSHR-(678 -691) was detected at the basolateral surface of the cells (Fig. 6). Quantitative surface immunoprecipitation confirmed this result (Fig. 7). While 90% of the NTRt was detected at the apical surface of the cells, about 90% of the hybrid molecules were observed on the basolateral membrane.
We can thus conclude that residues 678 -691 of the FSHR encode an effective basolateral targeting signal. This signal can function independently and can be transferred to another membrane protein. Furthermore, this signal is a dominant sorting signal, since apical sorting information is known to be present in the extracellular domain of the neurotrophin receptor (30).
The Basolateral Localization Signal of the FSHR Is Different from Its Endocytosis Signal and Is Not Involved in G s Protein Coupling-In several proteins, basolateral localization signals have been shown to coincide or overlap with internalization signals (reviews in Refs. 23 and 31). We thus compared the internalization of the wild-type FSHR with that of the receptor devoid of its basolateral localization signal in COS-7 cells. As shown in Fig. 8, both receptors were internalized at the same rate.
Since there could have been differences between COS-7 and MDCK cells, we repeated this experiment in the latter cells. Again there was no difference in the internalization rate of 125 I-FSH mediated by wild-type receptor or FSHR⌬677-695.
We then examined the possible involvement of this region of FSHR in G s protein coupling. The truncated FSHR⌬677-695 was transfected into COS-7 cells and challenged with hormone. Stimulation of adenylate cyclase was similar to that observed with the wild-type receptor (not shown). DISCUSSION We have previously shown that the FSHR displays a polarized basolateral expression in Sertoli cells of the testes (9). We went on to demonstrate that this location can also be observed in the MDCK model system (13). We have now used in vitro mutagenesis and expression in these cells to define the basolateral sorting signal of the FSHR. Deletion mutants allowed us to map the sorting sequence to the distal part of the intracellular domain at a distance of 47 amino acids from the membrane. Point mutations have allowed us to determine that this sequence comprises approximately 14 residues and is mainly dependent for its activity on Tyr 684 and Leu 689 . This signal is autonomous and dominant; when transferred to an apically targeted heterologous membrane protein, the neurotrophin receptor, it redirects the chimeric construct to the basolateral domain of MDCK cells. Furthermore, we have shown that the FSHR basolateral targeting signal is independent from the endocytosis signal and is not linked to the G s protein transduction pathway.
In receptor mutants with complete deletion of the basolateral targeting signal, the majority of receptor molecules were detected at the apical surface of MDCK cells at steady state. This reversion of receptor polarity suggests the existence of apical sorting information in the remaining part of the receptor. Glycosylation of the extracellular domain of the receptor may be involved (32).
The insulin receptor has been shown to be localized in the basolateral compartment of rabbit kidney cells (33) and of transfected MDCK cells (34). Among G protein-coupled receptors, the parathyroid hormone (35), ␣ 2 -adrenergic (36,37), melatonin (38), serotonergic (39,40), calcium-sensing (41), and A1 adenosine (42) receptors also display a polarized apical or basolateral localization. However, in none of these cases has the targeting signal been identified. Recently, in vitro mutagenesis experiments have been performed on the ␣ 2A -adrenergic receptor but have failed to identify its targeting signal (43). The basolateral localization signal of the FSH receptor is thus the first signal identified for a G protein-coupled receptor and more generally for a hormone receptor.
Basolateral targeting signals have been described for a viral protein (vesicular stomatitis virus glycoprotein G) (27); for cell adhesion molecules (NCAM and CD44) (44,45); for receptors involved in the transport of ligands (transferrin (46), polymeric immunoglobulin (15), FcIgG (47), low density lipoprotein (26), and asialoglycoprotein (25)); and for a growth factor receptor (epidermal growth factor receptor (48)). All of these signals are located in the vicinity of the membrane. The basolateral tar-geting signal of the FSHR is unusually localized at a distance (47 residues) from the membrane. In many cases, basolateral localization signals are colinear with internalization signals (reviewed in Ref. 31). However, it has been shown that the two sorting processes are differentially sensitive to various point mutations (28,49,50,51). The basolateral localization signal of the epidermal growth factor (48), transferrin (46), and immunoglobulin (15) receptors; the distal signal of the low density lipoprotein receptor (26); and the signal of vesicular stomatitis virus glycoprotein G (27,52) share with the FSH receptor the property of being independent of the internalization signal.
Comparison of sequences and point mutations of basolateral localization signals has not allowed us to define a clear cut consensus sequence, although in most cases either a tyrosine or a hydrophobic amino acid doublet (Leu-Leu or Leu-Val) plays a major role in these signals (reviewed in Ref. 23). Structure predictions have indicated in several cases the existence of a ␤-turn (53,54). Only one basolateral localization signal has been studied by NMR (29); in the isolated polymeric immunoglobulin receptor signal, a ␤-turn was observed, followed by a nascent helix. Mutations that compromised this structure were shown to decrease the sorting fidelity of the protein.
In the case of the FSH receptor, both Tyr 684 and Leu 689 were shown to play a major role. This is thus different from the majority of known signals (see above). Only in the case of the basolateral targeting signal of the rat lysosomal glycoprotein 120 and of vesicular stomatitis virus glycoprotein G have a Tyr and an Ile been shown to be of special importance for function (49,52). Structure predictions using several algorithms (20,21) have indicated that the basolateral localization signal of FSHR contains a ␤-turn structure. Mutation of residues located within or at close proximity to this putative structure have been shown to alter the basolateral targeting of the receptor.
LH and TSH receptors also exhibit a basolateral localization in MDCK cells. These receptors are highly homologous to the FSH receptor. Mutagenesis experiments are now performed to compare their basolateral localization signals with that of the FSHR. These studies should shed light on the cellular trafficking mechanisms of G protein-coupled receptors. Furthermore, mutations of gonadotrophin and thyrostimulin receptors have been described in a variety of diseases (reviewed in Refs. 2 and 4). It is possible that in some cases such natural mutations may alter receptor cell trafficking.