Activation of G protein coupled receptors (GPCRs) by binding of ligand is the initial event in diverse cellular signaling pathways. To examine the frequency and diversity of mutations that cause constitutive activation of one particular GPCR, the yeast alpha-factor receptor, we screened libraries of random mutations for constitutive alleles. In initial screens for mutant receptor alleles that exhibit signaling in the absence of added ligand, 14 different point mutations were isolated. All of these 14 mutants could be further activated by alpha-factor. Ten of the mutants also acquired the ability to signal in response to binding of desTrp(1)¿Ala(3)ălpha-factor, a peptide that acts as an antagonist toward normal alpha-factor receptors. Of these 10 mutants, at least eight alleles residing in the third, fifth, sixth, and seventh transmembrane segments exhibit bona fide constitutive signaling. The remaining alleles are hypersensitive to alpha-factor rather than constitutive. They can be activated by low concentrations of endogenous alpha-factor present in MATa cells. The strongest constitutively active receptor alleles were recovered multiple times from the mutational libraries, and extensive mutagenesis of certain regions of the alpha-factor receptor did not lead to recovery of any additional constitutive alleles. Thus, only a limited number of mutations is capable of causing constitutive activation of this receptor. Constitutive and hypersensitive signaling by the mutant receptors is partially suppressed by coexpression of normal receptors, consistent with preferential association of the G protein with unactivated receptors.

The alpha-factor receptor of the yeast Saccharomyces cerevisiae is a member of the superfamily of G protein-coupled receptors that mediate signal transduction in response to sensory and chemical stimuli. All members of this superfamily contain seven predicted transmembrane segments. We have created a series of genes encoding alpha-factor receptors with amino- or carboxyl-terminal truncations at each of the loop regions connecting transmembrane segments. Split receptors containing a discontinuity in the peptide backbone were synthesized by coexpressing pairs of truncated receptor fragments in yeast. Complementary pairs of fragments split at sites within each of the cytoplasmic and extracellular loops were capable of assembling and transducing a signal in response to alpha-factor binding. One pair of noncomplementary fragments containing a deletion in the second intracellular loop of the receptor also yielded a functional receptor. Coexpression of certain combinations of overlapping fragments containing supernumerary transmembrane segments also led to formation of functional receptors, apparently because of proteolytic trimming of overlapping regions. Coexpression of truncated receptor fragments with full-length receptors had no effect on signaling by the full-length receptors. These results demonstrate the following: (1) Correct folding of the alpha-factor receptor does not require a covalent connection between any pair of transmembrane segments that are adjacent in the sequence. (2) Most of the second intracellular loop of the receptor is not required for function. (3) The structure of the receptor cannot, in most cases, tolerate the presence of extra transmembrane segments. (4) None of the truncated fragments of the alpha-factor receptor can efficiently oligomerize with normal receptors in such a way as to inhibit receptor function.

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