Sequences in the intracellular loops of the yeast pheromone receptor Ste2p required for G protein activation.
The alpha-factor receptor of the yeast Saccharomyces cerevisiae encoded by the STE2 gene is a member of the large family of G protein-coupled receptors (GPCRs) that mediate multiple signal transduction pathways. The third intracellular loop of GPCRs has been identified as a likely site of interaction with G proteins. To determine the extent of allowed substitutions within this loop, we subjected a stretch of 21 amino acids (Leu228-Leu248) to intensive random mutagenesis and screened multiply substituted alleles for receptor function. The 91 partially functional mutant alleles that were recovered contained 96 unique amino acid substitutions. Every position in this region can be replaced with at least two other types of amino acids without a significant effect on function. The tolerance for nonconservative substitutions indicates that activation of the G protein by ligand-bound receptors involves multiple intramolecular interactions that do not strongly depend on particular sequence elements. Many of the functional mutant alleles exhibit greater than normal levels of signaling, consistent with an inhibitory role for the third intracellular loop. Removal of increasing numbers of positively charged residues from the loop by site-directed mutagenesis causes a progressive loss of signaling function, indicating that the overall net charge of the loop is important for receptor function. Introduction of negatively charged residues also leads to a reduced level of signaling. The defects in signaling caused by substitution of charged amino acids are not caused by changes in the abundance of receptors at the cell surface.
Celić
Andjelka
A
Department of Biochemistry and Biophysics, P.O. Box 712, University of Rochester School of Medicine and Dentistry, Rochester, New York 14642, USA.
Martin
Negin P
NP
Son
Cagdas D
CD
Becker
Jeffrey M
JM
Naider
Fred
F
Dumont
Mark E
ME
D000595:Amino Acid Sequence / Q000502:physiology*; D019943:Amino Acid Substitution / Q000235:genetics; D004252:DNA Mutational Analysis / Q000379:methods; D019204:GTP-Binding Proteins / Q000378:metabolism*; D007424:Intracellular Fluid / Q000737:chemistry / Q000502:physiology*; D008565:Membrane Proteins / Q000737:chemistry / Q000235:genetics / Q000502:physiology; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D010675:Pheromones / Q000737:chemistry / Q000235:genetics / Q000502:physiology*; D011487:Protein Conformation; D051964:Receptors, Mating Factor; D018000:Receptors, Peptide / Q000737:chemistry / Q000235:genetics / Q000502:physiology*; D029701:Saccharomyces cerevisiae Proteins / Q000737:chemistry / Q000235:genetics / Q000502:physiology*; D015398:Signal Transduction / Q000235:genetics / Q000502:physiology; D055672:Static Electricity; D014157:Transcription Factors
D008565:Membrane Proteins; D010675:Pheromones; D051964:Receptors, Mating Factor; D018000:Receptors, Peptide; D029701:Saccharomyces cerevisiae Proteins; D014157:Transcription Factors; D019204:GTP-Binding Proteins