The third cytoplasmic loop of a yeast G-protein-coupled receptor controls pathway activation, ligand discrimination, and receptor internalization

Variable Dependence of Signaling Output on Agonist Occupancy of Ste2p, a G Protein-coupled Receptor in Yeast

We report here on the relationship between ligand binding and signaling responses in the yeast pheromone response pathway, a well characterized G protein-coupled receptor system. Responses to agonist (α-factor) by cells expressing widely varying numbers of receptors depend primarily on fractional occupancy, not the absolute number of agonist-bound receptors. Furthermore, the concentration of competitive antagonist required to inhibit α-factor-dependent signaling is more than 10-fold higher than predicted based on the known ligand affinities. Thus, responses to a particular number of agonist-bound receptors can vary greatly, depending on whether there are unoccupied or antagonist-bound receptors present on the same cell surface. This behavior does not appear to be due to pre-coupling of receptors to G protein or to the Sst2p regulator of G protein signaling. The results are consistent with a signaling response that is determined by the integration of positive signals from agonist-occupied receptors and inhibitory signals from unoccupied receptors, where the inhibitory signals can be diminished by antagonist binding.

Evolution of a G protein-coupled receptor response by mutations in regulatory network interactions

All cellular functions depend on the concerted action of multiple proteins organized in complex networks. To understand how selection acts on protein networks, we used the yeast mating receptor Ste2, a pheromone-activated G protein-coupled receptor, as a model system. In Saccharomyces cerevisiae, Ste2 is a hub in a network of interactions controlling both signal transduction and signal suppression. Through laboratory evolution, we obtained 21 mutant receptors sensitive to the pheromone of a related yeast species and investigated the molecular mechanisms behind this newfound sensitivity. While some mutants show enhanced binding affinity to the foreign pheromone, others only display weakened interactions with the network's negative regulators. Importantly, the latter changes have a limited impact on overall pathway regulation, despite their considerable effect on sensitivity. Our results demonstrate that a new receptor–ligand pair can evolve through network-altering mutations independently of receptor–ligand binding, and suggest a potential role for such mutations in disease.

Co-evolution of a new receptor-ligand pair will affect the downstream signal transduction network. Here, the authors use experimental evolution of yeast mating receptor Ste2 to show the effect of enhanced binding affinity and weakened interactions with the network's negative regulators on protein evolution.

Analysis of random PCR-originated mutants of the yeast Ste2 and Ste3 receptors

The G protein-coupled receptors Ste2 and Ste3 bind α- and a-factor, respectively, in Saccharomyces cerevisiae. These receptors share a similar conformation, with seven transmembrane segments, three intracellular loops, a C-terminus tail, and three extracellular loops. However, the amino acid sequences of these two receptors bear no resemblance to each other. Coincidently the two ligands, α- and a-factor, have different sequences. Both receptors activate the same G protein. To identify amino acid residues that are important for signal transduction, the STE2 and STE3 genes were mutagenized by a random PCR-based method. Mutant receptors were analyzed in MATα cells mutated in the ITC1 gene, whose product represses transcription of a-specific genes in MATα. Expression of STE2 or STE3 in these cells results in autocrine activation of the mating pathway, since this strain produces the Ste2 receptor in addition to its specific ligand, α-factor. It also produces a-factor in addition to its specific receptor, Ste3. Therefore, this strain provides a convenient model to analyze mutants of both receptors in the same background. Many hyperactive mutations were found in STE3, whereas none was detected in STE2. This result is consistent with the different strategies that the two genes have adopted to be expressed.

Functional fusions of T4 lysozyme in the third intracellular loop of a G protein-coupled receptor identified by a random screening approach in yeast

The insertion of a stable soluble protein into loops of transmembrane proteins has proved to be a successful approach for enhancing their stabilities and crystallization, and may also be useful in contexts where the inserted proteins can modulate or report on the activities of membrane proteins. While the use of T4 lysozyme to replace portions of the third intracellular loops of G protein-coupled receptors (GPCRs) has allowed determination of the structures of members of this important class of receptors, the creation of such fusion proteins generally leads to loss of signaling function of the resulting fusion protein, since the third intracellular loops of GPCRs play critical roles in their interactions with G proteins. We describe here a random screening approach allowing insertion of T4 lysozyme into diverse positions in the third loop of the yeast α-pheromone receptor, a GPCR encoded by the yeast STE2 gene. Insertions were accompanied by varying extents of deletion or duplication of the loop. A set of phenotypic screens allow detection of potentially rare variant receptors that are expressed, bind to agonist and are capable of signal transduction via activation of the cognate G protein. A large fraction of screened full-length receptor variants containing at least partial duplications of the loop on either side of the inserted T4 lysozyme retain the ability to activate the downstream signaling pathway in response to binding of ligand. However, we were unable to identify any receptors with truncated C-termini that retain significant signaling function in the presence of inserted T4 lysozyme. Our results establish the feasibility of creating functional receptors containing insertions of T4 lysozyme in their third intracellular loops.

Defining pheromone-receptor signaling in Candida albicans and related asexual Candida species

The pheromone response in Candida albicans is mediated by the Ste2 receptor. Intracellular (IC) loop 3 and C-terminal tail regions of Ste2 are required for signaling, whereas the large IC1 region is dispensable. Heterologous expression of receptors from asexual species can also drive signaling in C. albicans, allowing functional pheromone-receptor couples to be analyzed.

Candida albicans is an important human fungal pathogen in which sexual reproduction is under the control of the novel white–opaque switch. Opaque cells are the mating-competent form, whereas white cells do not mate but can still respond to pheromones, resulting in biofilm formation. In this study, we first define the domains of the α-pheromone receptor Ste2 that are necessary for signaling in both white and opaque forms. Both cell states require the IC loop 3 (IC3) and the C-terminal tail of Ste2 for the cellular response, whereas the first IC loop (IC1) of Ste2 is dispensable for signaling. To also address pheromone-receptor interactions in related species, including apparently asexual Candida species, Ste2 orthologues were heterologously expressed in Candida albicans. Ste2 receptors from multiple Candida clade species were functional when expressed in C. albicans, whereas the Ste2 receptor of Candida lusitaniae was nonfunctional. Significantly, however, expression of a chimeric C. lusitaniae Ste2 receptor containing the C-terminal tail of Ste2 from C. albicans generated a productive response to C. lusitaniae pheromone. This system has allowed us to characterize pheromones from multiple Candida species and indicates that functional pheromone-receptor couples exist in fungal species that have yet to be shown to undergo sexual mating.

Fungal mating pheromones: choreographing the dating game

Pheromones are ubiquitous from bacteria to mammals - a testament to their importance in regulating inter-cellular communication. In fungal species, they play a critical role in choreographing interactions between mating partners during the program of sexual reproduction. Here, we describe how fungal pheromones are synthesized, their interactions with G protein-coupled receptors, and the signals propagated by this interaction, using Saccharomyces cerevisiae as a reference point. Divergence from this model system is compared amongst the ascomycetes and basidiomycetes, which reveals the wealth of information that has been gleaned from studying pheromone-driven processes across a wide spectrum of the fungal kingdom.

A Candida albicans-specific region of the alpha-pheromone receptor plays a selective role in the white cell pheromone response

Candida albicans strains homozygous at the mating type locus can switch from white to opaque, and must do so to mate. Opaque cells then secrete mating pheromones that stimulate opaque cells of opposite mating type to undergo mating. These same pheromones stimulate mating-incompetent white cells to become cohesive and adhesive, and enhance white cell biofilm development, a pathogenic trait. Stimulation is mediated through the same receptor, G protein complex and mitogen-activated protein kinase pathway. Here we present evidence that a C. albicans-specific 55-amino-acid region of the first intracellular loop, IC1, of the alpha-pheromone receptor Ste2p, is required for the alpha-pheromone response of white cells, but not that of opaque cells. This represents a unique regulatory configuration in which activation of a common pathway by the same ligand, the same receptor and the same signal transduction pathway is dependent on a unique region of an intracellular loop of the common receptor in one of the two responding phenotypes.

Effects of N- and C-terminal addition of oligolysines or native loop residues on the biophysical properties of transmembrane domain peptides from a G-protein coupled receptor

Transmembrane domains (TMDs) of G-protein coupled receptors (GPCRs) have very low water solubility and often aggregate during purification and biophysical investigations. To circumvent this problem many laboratories add oligolysines to the N- and C-termini of peptides that correspond to a TMD. To systematically evaluate the effect of the oligolysines on the biophysical properties of a TMD we synthesized 21 peptides corresponding to either the second (TPIFIINQVSLFLIILHSALYFKY) or sixth (SFHILLIMSSQSLLVPSIIFILAYSLK) TMD of Ste2p, a GPCR from Saccharomyces cerevisiae. Added to the termini of these peptides were either Lys(n) (n = 1,2,3) or the corresponding native loop residues. The biophysical properties of the peptides were investigated by circular dichroism (CD) spectroscopy in trifluoroethanol-water mixtures, sodium dodecyl sulfate (SDS) micelles and dimyristoylphosphocholine (DMPC)-dimyristoylphosphoglycerol (DMPG) vesicles, and by attenuated total reflection Fourier transform infrared (ATR-FTIR) in DMPC/DMPG multilayers. The results show that the conformation assumed depends on the number of lysine residues and the sequence of the TMD. Identical peptides with native or an equal number of lysine residues exhibited different biophysical properties and structural tendencies.

Rapid identification of functionally critical amino acids in a G protein-coupled receptor

G protein-coupled receptors (GPCRs) comprise one of the largest protein families found in nature. Here we describe a new experimental strategy that allows rapid identification of functionally critical amino acids in the rat M(3) muscarinic acetylcholine receptor (M3R), a prototypic class I GPCR. This approach involves low-frequency random mutagenesis of the entire M3R coding sequence, followed by the application of a new yeast genetic screen that allows the recovery of inactivating M3R single point mutations. The vast majority of recovered mutant M3Rs also showed substantial functional impairments in transfected mammalian (COS-7) cells. A subset of mutant receptors, however, behaved differently in yeast and mammalian cells, probably because of the specific features of the yeast expression system used. The screening strategy described here should be applicable to all GPCRs that can be expressed functionally in yeast.

Investigating RGS proteins in yeast

Regulator of G protein signalling (RGS) proteins are vital in the adaptation of cells to stimulation via G protein-coupled receptors. Yeast have been integral in elucidating the important role that RGS proteins play within cellular processes. In addition to extensive characterisation of the endogenous RGS proteins, these organisms have enabled the identification and analysis of numerous mammalian homologues. The simplicity and plasticity of the yeast pheromone-response pathway has facilitated studies which would have been impossible in mammalian systems and it is certain that yeast will continue to have a great impact on this field of research in the future.

Interacting Residues in an Activated State of a G Protein-coupled Receptor

Ste2p, the G protein-coupled receptor (GPCR) for the tridecapeptide pheromone alpha-factor of Saccharomyces cerevisiae, was used as a model GPCR to investigate the role of specific residues in the resting and activated states of the receptor. Using a series of biological and biochemical analyses of wild-type and site-directed mutant receptors, we identified Asn(205) as a potential interacting partner with the Tyr(266) residue. An N205H/Y266H double mutant showed pH-dependent functional activity, whereas the N205H receptor was non-functional and the Y266H receptor was partially active indicating that the histidine 205 and 266 residues interact in an activated state of the receptor. The introduction of N205K or Y266D mutations into the P258L/S259L constitutively active receptor suppressed the constitutive activity; in contrast, the N205K/Y266D/P258L/S259L quadruple mutant was fully constitutively active, again indicating an interaction between residues at the 205 and 206 positions in the receptor-active state. To further test this interaction, we introduced the N205C/Y266C, F204C/Y266C, and N205C/A265C double mutations into wild-type and P258L/S259L constitutively active receptors. After trypsin digestion, we found that a disulfide-cross-linked product, with the molecular weight expected for a receptor fragment with a cross-link between N205C and Y266C, formed only in the N205C/Y266C constitutively activated receptor. This study represents the first experimental demonstration of an interaction between specific residues in an active state, but not the resting state, of Ste2p. The information gained from this study should contribute to an understanding of the conformational differences between resting and active states in GPCRs.

Random mutagenesis of the M3 muscarinic acetylcholine receptor expressed in yeast: identification of second-site mutations that restore function to a coupling-deficient mutant M3 receptor

The M(3) muscarinic receptor is a prototypical member of the class A family of G protein-coupled receptors (GPCRs). To gain insight into the structural mechanisms governing agonist-mediated M(3) receptor activation, we recently developed a genetically modified yeast strain (Saccharomyces cerevisiae) which allows the efficient screening of large libraries of mutant M(3) receptors to identify mutant receptors with altered/novel functional properties. Class A GPCRs contain a highly conserved Asp residue located in transmembrane domain II (TM II; corresponding to Asp-113 in the rat M(3) muscarinic receptor) which is of fundamental importance for receptor activation. As observed previously with other GPCRs analyzed in mammalian expression systems, the D113N point mutation abolished agonist-induced receptor/G protein coupling in yeast. We then subjected the D113N mutant M(3) receptor to PCR-based random mutagenesis followed by a yeast genetic screen to recover point mutations that can restore G protein coupling to the D113N mutant receptor. A large scale screening effort led to the identification of three such second-site suppressor mutations, R165W, R165M, and Y250D. When expressed in the wild-type receptor background, these three point mutations did not lead to an increase in basal activity and reduced the efficiency of receptor/G protein coupling. Similar results were obtained when the various mutant receptors were expressed and analyzed in transfected mammalian cells (COS-7 cells). Interestingly, like Asp-113, Arg-165 and Tyr-250, which are located at the cytoplasmic ends of TM III and TM V, respectively, are also highly conserved among class A GPCRs. Our data suggest a conformational link between the highly conserved Asp-113, Arg-165, and Tyr-250 residues which is critical for receptor activation.

Expression of brain-specific angiogenesis inhibitor 3 (BAI3) in normal brain and implications for BAI3 in ischemia-induced brain angiogenesis and malignant glioma

Murine brain-specific angiogenesis inhibitor 1 and 2 (mBAI1, mBAI2) are involved in angiogenesis after cerebral ischemia. In this study, mBAI3 was cloned and characterized. Northern and Western blot analyses demonstrated a unique developmental expression pattern in the brain. The level of mBAI3 in brain peaked 1 day after birth, unlike mBAI1 and mBAI2, which peaked 10 days after birth. In situ hybridization analyses of the brain showed the same localization of BAI3 as BAI1 and BAI2, which includes most neurons of cerebral cortex and hippocampus. In the in vivo focal cerebral ischemia model, BAI3 expression decreased from 0.5 h after hypoxia until 8 h, but returned to control level after 24 h. The expression of vascular endothelial growth factor following ischemia showed an inverse pattern. The decreased expressions of BAIs in high-grade gliomas were observed, but BAI3 expression was generally lower in malignant gliomas than in normal brain. Our results indicate that the expression and distribution of BAI3 in normal brain, but not its developmental expression, are very similar to those of BAI1 and BAI2, and that BAI3 may participate in the early phases of ischemia-induced brain angiogenesis and in brain tumor progression.

Expression analysis of PCSTE3, a putative pheromone receptor from the lung pathogenic fungus Pneumocystis carinii

The fungal pathogen Pneumocystis carinii remains the most prevalent opportunistic infection in patients infected with HIV. Fungal pheromone receptors are seven transmembrane domain G-protein-coupled receptors which are expressed on specific mating types, and have ligand-binding extracellular domains for specific pheromones from cells of the opposite mating type. We have cloned and characterized PCSTE3 from P. carinii, which encodes a seven transmembrane domain protein orthologous to the Saccharomyces cerevisiae pheromone receptor Ste3. We detect PCSTE3 by indirect immunofluorescence using antibodies designed to extracellular domains of the receptor in yeast expressing the protein. Using a downstream Fus1-lacZ reporter gene, we determined that PCSTE3 does not recognize a- or alpha-factor pheromones as ligands for the receptor. We isolated P. carinii life cycle stages and examined PCSTE3 expression by immunofluorescence microscopy and flow cytometry, and found PCSTE3 expression exclusively on a population of trophic forms. PCSTE3 receptor expression was not found on cysts.

Subunits of a Yeast Oligomeric G Protein-coupled Receptor Are Activated Independently by Agonist but Function in Concert to Activate G Protein Heterotrimers

G protein-coupled receptors (GPCRs) form dimeric or oligomeric complexes in vivo. However, the function of oligomerization in receptor-mediated G protein activation is unclear. Previous studies of the yeast alpha-factor receptor (STE2 gene product) have indicated that oligomerization promotes signaling. Here we have addressed the mechanism by which oligomerization facilitates G protein signaling by examining the ability of ligand binding- and G protein coupling-defective alpha-factor receptors to form complexes in vivo and to correct their signaling defects when co-expressed (trans complementation). Newly and previously identified receptor mutants indicated that ligand binding involves the exofacial end of transmembrane domain (TM) 4, whereas G protein coupling involves ic1, ic3, the C-terminal tail, and the intracellular ends of TM2 and TM3. Mutant receptors bearing substitutions in these domains formed homo-oligomeric or hetero-oligomeric complexes in vivo, as indicated by results of fluorescence resonance energy transfer experiments. Co-expression of ligand binding- and G protein coupling-defective mutant receptors did not significantly improve signaling. In contrast, co-expression of ic1 and ic3 mutations in trans but not in cis significantly increased signaling efficiency. Therefore, we suggest that subunits of the alpha-factor receptor: 1) are activated independently rather than cooperatively by agonist, and 2) function in a concerted fashion to promote G protein activation, possibly by contacting different subunits or regions of the G protein heterotrimer.

Extracellular fragment of brain-specific angiogenesis inhibitor 1 suppresses endothelial cell proliferation by blocking αvβ5 integrin

Brain-specific angiogenesis inhibitor 1 (BAI1) is a transmembrane protein with anti-angiogenic activity. The mechanisms underlying BAI1 activity are unknown. In this study, we found that overexpression of BAI1 increased cell death in human umbilical vein endothelial cells (HUVECs) and, to a lesser degree, in SHSY5Y and U343 cells. Conditioned medium from BAI1-transfected U343 cells inhibited proliferation of HUVECs, and this effect was neutralized by addition of anti-BAI1 serum. The conditioned medium contained four cleavage products of the BAI1 extracellular domain. BAI1's middle extracellular region containing five thrombospondin type 1 repeats (BAI1-TSR) was sufficient for BAI1's antiproliferative effect on HUVECs. BAI1's action on HUVECs was blocked by anti-alpha(v) integrin, but not by anti-CD36 antibody treatment. Introduction of alpha(v)beta(5) integrin into HEK293 cells rendered them susceptible to cell death by BAI1, and BAI1-TSR bound with alpha(v)beta(5) integrin, but not to alpha(v)beta(3) integrin in brain tissue. Fluorescent BAI1-TSR colocalized with alpha(v)beta(5) integrin in HUVECs. Together, our results indicate that BAI1 has antiproliferative action on surrounding endothelial cells by blocking alpha(v)beta(5) integrin, and its active region is BAI1-TSR. BAI1-TSR could be valuable for regulating brain angiogenesis.

Random mutagenesis of the M3 muscarinic acetylcholine receptor expressed in yeast. Identification of point mutations that "silence" a constitutively active mutant M3 receptor and greatly impair receptor/G protein coupling

The M3 muscarinic receptor is a prototypical member of the class I family of G protein-coupled receptors (GPCRs). To facilitate studies on the structural mechanisms governing M3 receptor activation, we generated an M3 receptor-expressing yeast strain (Saccharomyces cerevisiae) that requires agonist-dependent M3 receptor activation for cell growth. By using receptor random mutagenesis followed by a genetic screen in yeast, we initially identified a point mutation at the cytoplasmic end of transmembrane domain (TM) VI (Q490L) that led to robust agonist-independent M3 receptor signaling in both yeast and mammalian cells. To explore further the molecular mechanisms by which point mutations can render GPCRs constitutively active, we subjected a region of the Q490L mutant M3 receptor that included TM V-VII to random mutagenesis. We then applied a yeast genetic screen to identify second-site mutations that could suppress the activating effects of the Q490L mutation and restore wild-type receptor-like function to the Q490L mutant receptor. This analysis led to the identification of 12 point mutations that allowed the Q490L mutant receptor to function in a fashion similar to the wild-type receptor. These amino acid substitutions mapped to two distinct regions of the M3 receptor, the exofacial segments of TM V and VI and the cytoplasmic ends of TM V-VII. Strikingly, in the absence of the activating Q490L mutation, all recovered point mutations severely reduced the efficiency of receptor/G protein coupling, indicating that the targeted residues play important roles in receptor activation and/or receptor/G protein coupling. This strategy should be generally applicable to identify sites in GPCRs that are critically involved in receptor function.

The C-terminal tail preceding the CAAX box of a yeast G protein gamma subunit is dispensable for receptor-mediated G protein activation in vivo

The gamma subunits of heterotrimeric G proteins are required for receptor-G protein coupling. The C-terminal domains of Ggamma subunits can contact receptors and influence the efficiency of receptor-G protein coupling in vitro. However, it is unknown whether receptor interaction with the C terminus of Ggamma is required for signaling in vivo. To address this question, we cloned Ggamma homologs with diverged C-terminal sequences from five species of budding yeast. Each Ggamma homolog functionally replaced the Ggamma subunit of the yeast Saccharomyces cerevisiae (STE18 gene product). Mutagenesis of S. cerevisiae Ste18 likewise indicated that specific C-terminal sequence motifs are not required for signaling. Strikingly, an internal in-frame deletion removing sequences preceding the C-terminal CAAX box of Ste18 did not impair signaling by either of its cognate G protein-coupled pheromone receptors. Therefore, receptor interaction with the C-terminal domain of yeast Ggamma is not required for receptor-mediated G protein activation in vivo. Because the mechanism of G protein activation by receptors is conserved from yeast to humans, mammalian receptors may not require interaction with the tail of Ggamma for G protein activation in vivo. However, receptor-Ggamma interaction may modulate the efficiency of receptor-G protein coupling or promote activation of Gbetagamma effectors that co-cluster with receptors.

The extracellular N-terminal domain and transmembrane domains 1 and 2 mediate oligomerization of a yeast G protein-coupled receptor

G protein-coupled receptors (GPCRs) can form homodimers/oligomers and/or heterodimers/oligomers. The mechanisms used to form specific GPCR oligomers are poorly understood because the domains that mediate such interactions and the step(s) in the secretory pathway where oligomerization occurs have not been well characterized. Here we have used subcellular fractionation and fluorescence resonance energy transfer (FRET) experiments to show that oligomerization of a GPCR (alpha-factor receptor; STE2 gene product) of the yeast Saccharomyces cerevisiae occurs in the endoplasmic reticulum. To identify domains of this receptor that mediate oligomerization, we used FRET and endocytosis assays of oligomerization in vivo to analyze receptor deletion mutants. A mutant lacking the N-terminal extracellular domain and transmembrane (TM) domain 1 was expressed at the cell surface but did not self-associate. In contrast, a receptor fragment containing only the N-terminal extracellular domain and TM1 could self-associate and heterodimerize with wild type receptors. Analysis of other mutants suggested that oligomerization is facilitated by the N-terminal extracellular domain and TM2. Therefore, the N-terminal extracellular domain, TM1, and TM2 appear to stabilize alpha-factor receptor oligomers. These domains may form an interface in contact or domain-swapped oligomers. Similar domains may mediate dimerization of certain mammalian GPCRs.

Expression of brain-specific angiogenesis inhibitor 2 (BAI2) in normal and ischemic brain: involvement of BAI2 in the ischemia-induced brain angiogenesis

Previously, the authors cloned and characterized murine brain-specific angiogenesis inhibitor 1 (mBAI1). In this study, the authors cloned mBAI2 and analyzed its functional characteristics. Northern and Western blot analyses demonstrated a unique developmental expression pattern of mBAI2 in the brain. The expression level of mBAI2 appeared to increase as the development of the brain progressed. Reverse transcription-polymerase chain reaction (RT-PCR) analyses demonstrated the existence of alternative splice variants of mBAI2, which were defective in parts of type I repeat of thrombospondin or the third cytoplasmic loop of the seven-span transmembrane domain that were considered essential to the functions of mBAI2. The expressions of spliced variants in the brain were differently regulated compared with wild-type mBAI2 during development and ischemic conditions. In situ hybridization analyses of the brain showed the same localization of BAI2 as BAI1, such as in most neurons of cerebral cortex. In the in vivo focal cerebral ischemia model and the in vitro hypoxic cell culture model with cobalt, BAI2 expression decreased after hypoxia and preceded the increased expression of vascular endothelial growth factor (VEGF). RT-PCR analysis of antisense BAI2 cDNA-transfected SHSY5Y cells showed an increased VEGF expression as well as a decreased BAI2 expression. Immunohistochemical study of focal ischemic cortex showed that the regional localization of decreased BAI2 was related to the formation of new vessels. These results suggest that the brain-specific developmental expression pattern of angiostatic BAI2 is correlated with the decreased neovascularization in the adult brain, and that angiostatic BAI2 participates in the ischemia-induced brain angiogenesis in concert with angiogenic VEGF.

The cytoplasmic end of transmembrane domain 3 regulates the activity of the Saccharomyces cerevisiae G-protein-coupled alpha-factor receptor

The binding of alpha-factor to its receptor (Ste2p) activates a G-protein-signaling pathway leading to conjugation of MATa cells of the budding yeast S. cerevisiae. We conducted a genetic screen to identify constitutively activating mutations in the N-terminal region of the alpha-factor receptor that includes transmembrane domains 1-5. This approach identified 12 unique constitutively activating mutations, the strongest of which affected polar residues at the cytoplasmic ends of transmembrane domains 2 and 3 (Asn84 and Gln149, respectively) that are conserved in the alpha-factor receptors of divergent yeast species. Targeted mutagenesis, in combination with molecular modeling studies, suggested that Gln149 is oriented toward the core of the transmembrane helix bundle where it may be involved in mediating an interaction with Asn84. These residues appear to play specific roles in maintaining the inactive conformation of the protein since a variety of mutations at either position cause constitutive receptor signaling. Interestingly, the activity of many mammalian G-protein-coupled receptors is also regulated by conserved polar residues (the E/DRY motif) at the cytoplasmic end of transmembrane domain 3. Altogether, the results of this study suggest a conserved role for the cytoplasmic end of transmembrane domain 3 in regulating the activity of divergent G-protein-coupled receptors.

Targeted transgenic expression of beta(2)-adrenergic receptors to type II cells increases alveolar fluid clearance

Clearance of edema fluid from the alveolar space can be enhanced by endogenous and exogenous beta-agonists. To selectively delineate the effects of alveolar type II (ATII) cell beta(2)-adrenergic receptors (beta(2)-ARs) on alveolar fluid clearance (AFC), we generated transgenic (TG) mice that overexpressed the human beta(2)-AR under control of the rat surfactant protein C promoter. In situ hybridization showed that transgene expression was consistent with the distribution of ATII cells. TG mice expressed 4.8-fold greater beta(2)-ARs than nontransgenic (NTG) mice (939 +/- 113 vs. 194 +/- 18 fmol/mg protein; P < 0.001). Basal AFC in TG mice was approximately 40% greater than that in untreated NTG mice (15 +/- 1.4 vs. 10.9 +/- 0.6%; P < 0.005) and approached that of NTG mice treated with the beta-agonist formoterol (19.8 +/- 2.2%; P = not significant). Adrenalectomy decreased basal AFC in TG mice to 9.7 +/- 0.5% but had no effect on NTG mice (11.5 +/- 1.0%). Na(+)-K(+)-ATPase alpha(1)-isoform expression was unchanged, whereas alpha(2)-isoform expression was approximately 80% greater in the TG mice. These findings show that beta(2)-AR overexpression can be an effective means to increase AFC in the absence of exogenous agonists and that AFC can be stimulated by activation of beta(2)-ARs specifically expressed on ATII cells.

Interaction between transmembrane domains five and six of the alpha -factor receptor

The alpha-factor pheromone receptor (STE2) activates a G protein signal pathway that induces conjugation of the yeast Saccharomyces cerevisiae. Previous studies implicated the third intracellular loop of this receptor in G protein activation. Therefore, the roles of transmembrane domains five and six (TMD5 and -6) that bracket the third intracellular loop were analyzed by scanning mutagenesis in which each residue was substituted with cysteine. Out of 42 mutants examined, four constitutive mutants and two strong loss-of-function mutants were identified. Double mutants combining Cys substitutions in TMD5 and TMD6 gave a broader range of phenotypes. Interestingly, a V223C mutation in TMD5 caused constitutive activity when combined with the L247C, L248C, or S251C mutations in TMD6. Also, the L226C mutation in TMD5 caused constitutive activity when combined with either the M250C or S251C mutations in TMD6. The residues affected by these mutations are predicted to fall on one side of their respective helices, suggesting that they may interact. In support of this, cysteines substituted at position 223 in TMD5 and position 247 in TMD6 formed a disulfide bond, providing the first direct evidence of an interaction between these transmembrane domains in the alpha-factor receptor. Altogether, these results identify an important region of interaction between conserved hydrophobic regions at the base of TMD5 and TMD6 that is required for the proper regulation of receptor signaling.

Position 13 analogs of the tridecapeptide mating pheromone from Saccharomyces cerevisiae: design of an iodinatable ligand for receptor binding

Analogs of the alpha-factor tridecapeptide mating pheromone (WHWLQLKPGQPMY) from Saccharomyces cerevisiae in which Tyr13 was replaced with Phe, p-F-Phe, m-F-Phe, p-NO2-Phe, p-NH2-Phe or Ser were synthesized and purified to >99% homogeneity. These analogs were bioassayed using a growth arrest assay and a gene induction assay and evaluated for their ability to compete with binding of tritiated alpha-factor to its receptor Ste2p. The results showed that the phenolic OH of Tyr13 is not required for either biological activity or receptor recognition. Analogs containing fluorine, amino, nitro or a hydrogen in place of OH had 80-120% of the biological activity of the parent pheromone in the gene induction assay and had receptor affinities from nearly equal to 6-fold lower than that of alpha-factor. In contrast, substitution of Ser or Ala at position 13 resulted in a >100-fold decrease in receptor affinity suggesting that the aromatic ring is involved in binding to the receptor. The lack of a strict requirement for Tyr13 allowed the design of several multiple replacement analogs in which Phe or p-F-Phe were substituted at position 13 and Tyr was placed in other positions of the peptide. These analogs could then be iodinated and used in the development of a highly sensitive receptor-binding assay. One potential receptor ligand [Tyr(125I)1,Nle12, Phe13] alpha-factor exhibited saturable binding with a KD of 81 nM and was competed by alpha-factor for binding in a whole-cell assay. Thus a new family of radioactive ligands for the alpha-factor receptor has been revealed. These ligands should be extremely useful in defining active site residues during mutagenesis and cross-linking studies.

Cellular and molecular mechanisms of sexual incompatibility in plants and fungi

Plants and fungi show an astonishing diversity of mechanisms to promote outbreeding, the most widespread of which is sexual incompatibility. Sexual incompatibility involves molecular recognition between mating partners. In fungi and algae, highly polymorphic mating-type loci mediate mating through complementary interactions between molecules encoded or regulated by different mating-type haplotypes, whereas in flowering plants polymorphic self-incompatibility loci regulate mate recognition through oppositional interactions between molecules encoded by the same self-incompatibility haplotypes. This subtle mechanistic difference is a consequence of the different life cycles of fungi, algae, and flowering plants. Recent molecular and biochemical studies have provided fascinating insights into the mechanisms of mate recognition and are beginning to shed light on evolution and population genetics of these extraordinarily polymorphic genetic systems of incompatibility.

Ligand recognition in multiallelic pheromone receptors from the basidiomycete Schizophyllum commune studied in yeast

The homobasidiomycete Schizophyllum commune encodes a multiallelic pheromone receptor system that distinguishes more than 20 nonself from at least 2 self pheromones. The well-investigated pheromone response system of the yeast Saccharomyces cerevisiae was used to link the FUS1::lacZ reporter system to the heterologous pheromone receptors from S. commune. To investigate yeast G-protein binding, the unchanged heterologous receptor was compared to constructs carrying an exchange of the 3rd cytoplasmatic loop for the Ste2 sequence. A better coupling could be achieved with the altered constructs. In order to examine activation by single pheromones, an artificial peptide based on the sequence of a new putative pheromone gene, bap2(1), in the Balpha2 mating-type locus encoding the shortest pheromone found so far in fungal mating types was used. Thus, we have reassembled the pheromone recognition of the basidiomycete S. commune and constructed a system ideal for specificity analysis in the yeast S. cerevisiae.

Extragenic suppressors of loss-of-function mutations in the aspergillus FlbA regulator of G-protein signaling domain protein

We showed previously that two genes, fl bA and fadA, have a major role in determining the balance between growth, sporulation, and mycotoxin (sterigmatocystin; ST) production by the filamentous fungus Aspergillus nidulans. fadA encodes the alpha subunit for a heterotrimeric G-protein, and continuous activation of FadA blocks sporulation and ST production while stimulating growth. fl bA encodes an A. nidulans regulator of G-protein signaling (RGS) domain protein that antagonizes FadA-mediated signaling to allow development. To better understand FlbA function and other aspects of FadA-mediated growth control, we have isolated and characterized mutations in four previously undefined genes designated as sfaA, sfaC, sfaD, and sfaE (suppressors of flbA), and a new allele of fadA (fadAR205H), all of which suppress a fl bA loss-of-function mutation ( fl bA98). These suppressors overcome fl bA losses of function in both sporulation and ST biosynthesis. fadAR205H, sfaC67, sfaD82, and sfaE83 mutations are dominant to wild type whereas sfaA1 is semidominant. sfaA1 also differs from other suppressor mutations in that it cannot suppress a fl bA deletion mutation (and is therefore allele specific) whereas all the dominant suppressors can bypass complete loss of fl bA. Only sfaE83 suppressed dominant activating mutations in fadA, indicating that sfaE may have a unique role in fadA- fl bA interactions. Finally, none of these suppressor mutations bypassed fl uG loss-of-function mutations in development-specific activation.

Mutations affecting ligand specificity of the G-protein-coupled receptor for the Saccharomyces cerevisiae tridecapeptide pheromone

Random mutations were generated in the G-protein-coupled receptor (Ste2p) for the tridecapeptide pheromone (alpha-factor) of Saccharomyces cerevisiae. These mutants were screened for variants that responded to antagonists. Because multiple mutations were detected in each mutant receptor recovered from the screen, site-directed mutagenesis was used to create single-site mutant receptors. Three receptors containing mutations F55V, S219P, and S259P were analyzed for their biological responses to various alpha-factor analogs and for their ligand binding profiles. Cells expressing each of the mutant receptors responded to alpha-factor as well as or better than wild-type cells in a growth arrest assay. In contrast, the binding of alpha-factor to the F55V and S219P mutant receptors was at least 10-fold reduced in comparison to wild-type receptor indicating a complex non-linear correlation between binding affinity and biological activity. Cells expressing mutant receptors responded to some normally inactive analogs in biological assays, despite the fact that these analogs had a low affinity for Ste2p. The analysis of these mutant receptors confirms previous findings that the first and sixth transmembrane regions of Ste2p are important for ligand interaction, ligand specificity, and/or receptor activation to initiate the signal transduction pathway. Changes in binding affinity of pheromone analogs to wild-type and mutant receptors indicate that residue 55 of Ste2p is involved with both ligand binding and signal transduction.

Structure of segments of a G protein-coupled receptor: CD and NMR analysis of the Saccharomyces cerevisiae tridecapeptide pheromone receptor

Peptides representing both loop and the sixth transmembrane regions of the alpha-factor receptor of Saccharomyces cerevisiae were synthesized by solid-phase procedures and purified to near homogeneity. CD, nmr, and modeling analysis indicated that in aqueous media the first extracellular loop peptide E1(107-125), the third intracellular loop peptide I3(231-243), and the carboxyl terminus peptide I4(350-372) were mostly disordered. In contrast, the second extracellular loop peptide E2(191-206) assumed a well-defined structure in aqueous medium and the sixth transmembrane domain peptide receptor M6(252-269, C252A) was highly helical in trifluoroethanol/water (4:1), exhibiting a kink at Pro258. A synthetic peptide containing a sequence similar to that of the sixth transmembrane domain of a constitutively active alpha-factor receptor M6(252-269, C252A, P258L) in which Leu replaces Pro258 exhibited significantly different biophysical properties than the wild-type sequence. In particular, this peptide had very low solubility and gave CD resembling that of a beta-sheet structure in hexafluoroacetone/water (1:1) whereas the wild-type peptide was partially helical under identical conditions. These results would be consistent with the hypothesis that the constitutive activity of the mutant receptor is linked to a conformational change in the sixth transmembrane domain. The study of the receptor segments also indicate that peptides corresponding to loops of the alpha-factor receptor do not appear to assume turn structures.

Structure-function analysis of the Saccharomyces cerevisiae tridecapeptide pheromone using alanine-scanned analogs

Twenty-six peptide analogs of the Saccharomyces cerevisiae alpha-factor, a tridecapeptide mating pheromone (W1H2W3L4Q5L6K7p8G9Ql0P11M12Y13) with either L- or D-alanine replacement of each amino acid residue (Ala-scanned) and with the isosteric replacement of methionine at position 12 by norleucine, were synthesized, purified to homogeneity and assayed for biological activity and receptor binding. Two new and effective antagonists, [D-Ala3,Nle12]alpha-factor and [D-Ala4,Nle12]alpha-factor, were found among the series, and the [D-Ala10,Nle12]alpha-factor demonstrated a marked ability to increase the biological activity of [Nle12]alpha-factor without having any effect by itself. One analog, the [L-Ala1 alpha-factor, showed a 3-fold increase in bioactivity over the [Nle12]alpha-factor, although its binding to the alpha-factor receptor was about 70-fold less than [Nle12]alpha-factor. Residues near the carboxyl terminus contributed more strongly to receptor binding than other residues, whereas those near the amine terminus of the alpha-factor played an important role in signal transduction. The effect of insertion of D-Ala residues at positions 7, 8, 9 and 10 on bioactivity and receptor binding of the peptide suggested a specific positioning role of the central loop in establishing optimal contacts between the receptor and the ends of the pheromone. We conclude that the alpha-factor may be divided into segments with dominant roles in forming the biologically active pheromone conformation, in receptor binding and in initiating signal transduction. The discovery of such relationships was made possible by the systematic variation of each residue in the peptide and by the testing of each analog in highly defined biological and binding assays.

Inhibition of signal transduction by a splice variant of the growth hormone-releasing hormone receptor expressed in human pituitary adenomas

We have previously shown that some of human growth hormone (GH)-producing pituitary adenomas preferentially express a larger transcript of GH-releasing hormone (GHRH) receptor (GHRH-R). This transcriptional variant is presumed to be produced by alternative messenger RNA splicing and contains premature stop codon in frame, predicted to yield a truncated GHRH-R. Functional expression study indicated that the variant receptor was unable to transduct GHRH signals. To determine the functional relationship between the splice-variant and the wild-type GHRH-R, the expression vector for the variant GHRH-R transcript was transfected into COS-7 cells together with or without that for the wild GHRH-R transcript. In cells transfected with both GHRH-R expression vectors, GHRH-dependent cyclic adenosine monophosphate (cAMP) induction was decreased to 39% of that in the cells transfected with the wild-type GHRH-R expression vector alone. This inhibition was found to be irrespective of the concentration (10(-8) to 10(-5) mol/L) of GHRH. These findings suggest that the splice variant form of GHRH-R functions as a dominant-negative modulator in GHRH-induced cellular signaling.

Mechanisms governing the activation and trafficking of yeast G protein-coupled receptors

We have addressed the mechanisms governing the activation and trafficking of G protein-coupled receptors (GPCRs) by analyzing constitutively active mating pheromone receptors (Ste2p and Ste3p) of the yeast Saccharomyces cerevisiae. Substitution of the highly conserved proline residue in transmembrane segment VI of these receptors causes constitutive signaling. This proline residue may facilitate folding of GPCRs into native, inactive conformations, and/or mediate agonist-induced structural changes leading to G protein activation. Constitutive signaling by mutant receptors is suppressed upon coexpression with wild-type, but not G protein coupling-defective, receptors. Wild-type receptors may therefore sequester a limiting pool of G proteins; this apparent "precoupling" of receptors and G proteins could facilitate signal production at sites where cell surface projections form during mating partner discrimination. Finally, rather than being expressed mainly at the cell surface, constitutively active pheromone receptors accumulate in post-endoplasmic reticulum compartments. This is in contrast to other defective membrane proteins, which apparently are targeted by default to the vacuole. We suggest that the quality-control mechanism that retains receptors in post-endoplasmic reticulum compartments may normally allow wild-type receptors to fold into their native, fully inactive conformations before reaching the cell surface. This may ensure that receptors do not trigger a response in the absence of agonist.

Pheromone signalling and polarized morphogenesis in yeast

Yeast cells respond to mating pheromones by activating a signal transduction pathway involving a seven transmembrane receptor/G protein complex linked to a mitogen-activated protein kinase module. Regulation of the G protein signal is controlled by the receptor and Sst2p; Sst2p may function as a GTPase-activating protein for the G protein alpha subunit. The Ste20 kinase acts in the linkage between the G protein and the MAP kinase module. Experiments suggest that binding of the Rho-like GTPase Cdc42p to Ste20p is not required for the mating response, yet is needed for the pseudohyphal growth response which involves many of the same kinases.

Random mutagenesis of the cAMP chemoattractant receptor, cAR1, of Dictyostelium. Evidence for multiple states of activation

cAMP receptor 1 (cAR1) of Dictyostelium couples to the G protein G2 to mediate activation of adenylyl and guanylyl cyclases, chemotaxis, and cell aggregation. Other cAR1-dependent events, including receptor phosphorylation and influx of extracellular Ca2+, do not require G proteins. To further characterize signal transduction through cAR1, we performed random mutagenesis of the third intracellular loop (24 amino acids), since the corresponding region of other seven helix receptors has been implicated in the coupling to G proteins. Mutant receptors were expressed in car1(-) cells and were characterized for G protein-dependent and -independent signal transduction. Our results demonstrate that cAR1 is remarkably tolerant to amino acid substitutions in the third intracellular loop. Of the 21 positions where amino acid substitutions were observed, one or more replacements were found that retained full biological function. However, certain alterations resulted in receptors with reduced ability to bind cAMP and/or transduce signals. There were specific signal transduction mutants that could undergo cAMP-dependent cAR1 phosphorylation but were impaired either in coupling to G proteins, in G protein-independent Ca2+ influx, or in both pathways. In addition, there were general activation mutants that failed to restore aggregation to car1(-) cells and displayed severe defects in all signal transduction events, including the most robust response, cAMP-dependent cAR1 phosphorylation. Certain of these mutant phenotypes were obtained in a complementary study, where the entire region of cAR1 from the third to the seventh transmembrane helices was randomly mutagenized. Considered together, these studies indicate that the activation cycle of cAR1 may involve a number of distinct receptor intermediates. A model of G protein-dependent and -independent signal transduction through cAR1 is discussed.

Cell cycle arrest promotes trans-hammerhead ribozyme action in yeast

A hammerhead ribozyme designed to cleave the yeast ADE1 mRNA has been expressed in yeast under the control of a galactose-inducible promoter. RNA prepared from the galactose-induced yeast cultures possesses an activity that cleaves ADE1 mRNA in vitro. However, in spite of high expression levels of the ribozyme, no cleavage activity could be demonstrated in vivo. On the other hand, when the yeast cells expressing hammerhead RNA were treated with the alpha-factor mating pheromone, the level of ADE1 mRNA was reduced by 50%. Similar reductions were observed when this strain was cultured in the presence of lithium acetate or in nitrogen-free medium. Moreover, control experiments in which disabled hammerhead genes were expressed showed no such reductions. Extension of the length of the flanking recognition arms of the ribozyme from a total of 10 to 16 or 24 nucleotides diminished the inhibitory effect of the ribozyme. These data suggest that ribozymes are able to cleave a trans-RNA target in yeast.

Tetrapolar fungal mating types: sexes by the thousands

In order to achieve genetic rearrangement in a sexual cycle, eukaryotes go through the processes of meiosis and mating. Different mating types assure that mating is only possible between two genetically diverse individuals. Basidiomycetous fungi display thousands of different mating types that are determined by two genetically unlinked loci. One locus is multiallelic and contains genes for homeodomain transcription factors which are able to form heterodimers. The activation of target genes is dependent on heterodimers formed from the monomeric transcription factor proteins originating from different alleles of this genetic locus. The interactions between the two monomeric transcription factors and the activation of target genes by the heterodimeric proteins make this regulatory system both complex and interesting. The second locus contains a pheromone receptor system: the pheromone receptor is similar to the G protein-linked serpentine receptors in Saccharomyces cerevisiae that activate the pheromone response via a phosphorylation signal transduction cascade in S. cerevisiae. This pheromone perception is a trigger of sexual development and not, as with yeast, itself under control of mating type genes. Rather it directly senses diversity at the mating type loci. Whereas heterobasidiomycetes display a bi-allelic structure for this locus with recognition between one receptor and the opposite pheromone, homobasidiomycetes contain multiple specificities for pheromone receptors and pheromones.

Carboxyl-terminal domains determine internalization and recycling characteristics of bombesin receptor chimeras

To investigate the role of the carboxy terminus in the regulation of the bombesin (BN) receptor, we constructed two chimeric receptors with carboxyl termini transferred from either m3 muscarinic cholinergic (m3 ACh) (BMC) or cholecystokinin A (CCKA) (BCC) receptors and expressed them in Chinese hamster ovary cells. Previous studies showed that agonist treatment caused rapid internalization of CCKA but not m3 ACh receptors in these cells. In the current study we conducted separate analyses of ligand and receptor internalization and analyzed receptor recycling. Ligand internalization was assessed using acid washing. BN and CCKA receptors internalized ligand with 80 +/- 3 and 85 +/- 7% in an acid-resistant compartment at equilibrium. Ligand internalization of chimeric receptors generally assumed the properties of the donor receptors. Thus, BCC receptors internalized ligand to a similar extent as wild-type CCKA receptors (75 +/- 3%), whereas, BMC receptors showed reduced ligand internalization (38 +/- 1%). Receptor internalization was more directly assessed by determining agonist-induced loss of surface binding. BN and CCKA receptors were largely internalized (56 +/- 8 and 50 +/- 7%, respectively). BCC receptors were also extensively internalized (82 +/- 3%). In contrast, BMC receptors were minimally internalized (22 +/- 8%). Receptor recycling was assessed as recovery from agonist induced loss of binding. BN, CCKA, and BMC receptors showed rapid recycling. In contrast, BCC receptors did not recycle. These data indicate that carboxyl-terminal structures determine both internalization of ligand-receptor complexes and subsequent receptor recycling.

Influence of second and third cytoplasmic loops on binding, internalization, and coupling of chimeric bombesin/m3 muscarinic receptors

In order to investigate the molecular basis for differences in the characteristics of bombesin (Bn) and m3 muscarinic cholinergic (m3 ACh) receptors, chimeric Bn receptors possessing cytoplasmic domains from the m3 ACh receptor were produced. The receptors were expressed in CHO-K1 cells and binding, structural, and signal transduction characteristics were analyzed. Cell lines bearing chimeric Bn receptors possessing m3 ACh receptor domains in place of either the second cytoplasmic loop (BM2L), the third cytoplasmic loop (BM3L), or both loops (BM23L) each bound 125I-bombesin with a single affinity that was approximately the same as that of the Bn receptor (5-10 nM). However, Bn receptors possessing the m3 ACh third cytoplasmic loop were severely affected in other respects. Internalization of ligand in Bn and BM2L cells was rapid and extensive (> 80% of bound 125I-bombesin was acid-resistant). In contrast, internalization was dramatically reduced in BM3L and BM23L cells (approximately 20% of bound 125I-bombesin was acid-resistant). In Bn or BM2L cells 10 nM bombesin stimulated approximately 10-fold increases in phosphatidylinositol hydrolysis. Activation of Bn receptors also induced an increase in arachidonic acid release (478 +/- 32% of control, n = 3) and large increases in intracellular Ca2+. In contrast, in BM3L or BM23L cells, bombesin had no significant effect on phosphatidylinositol hydrolysis. Furthermore, BM3L receptor activation did not increase arachidonic acid release. However, BM3L and BM23L cells showed a small increase in intracellular Ca2+ at high concentrations of bombesin. These data indicate that the third cytoplasmic loop alone, or together with the second cytoplasmic loop, was not sufficient to transfer the characteristics of G protein interaction between m3 ACh and bombesin receptors. Furthermore, for the Bn receptor, ligand internalization does, whereas formation of the high affinity binding state does not, appear to require activation of G proteins.