Abstract. Monoclonal antibodies against the ll0-kD component of the yeast spindle pole body (SPB) were used to clone the corresponding gene SPC110. SPC110 is identical to NUF1 (Mirzayan, C., C. S. Copeland, and M. Snyder. 1992. J. Cell Biol. 116:1319-1332. SPC110/NUF1 has an MluI cell cycle box consensus sequence in its putative promoter region, and we found that the transcript was cell cycle regulated in a similar way to other MluI-regulated transcripts. Spcll0p/Nuflp has a long central region with a predicted coiled-coil structure. We expressed this region in Escherichia coli and showed by rotary shadowing that rods of the predicted length were present. The ll0-kD component is localized in the SPB to the gap between the central plaque and the sealed ends of the nuclear microtubules near the inner plaque (Rout, M., and J. V. Kilmartin. 1990. J. Cell Biol. 111:1913-1927. We found that rodlike structures bridge this gap. When truncations of SPC110 with deletions in the coiled-coil region of the protein replaced the wild-type gene, the gap between the central plaque and the ends of the microtubules decreased in proportion to the size of the deletion. This suggests that Spcll0p connects these two parts of the SPB together and that the coiled-coil domain acts as a spacer element.

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The mating pathway of Saccharomyces cerevisiae is widely used as a model system for G protein-coupled receptor-mediated signal transduction. Following receptor activation by the binding of mating pheromones, G protein βγ subunits transmit the signal to a MAP kinase cascade, which involves interaction of Gβ (Ste4p) with the MAP kinase scaffold protein Ste5p. Here, we identify residues in Ste4p required for the interaction with Ste5p. These residues define a new signaling interface close to the Ste20p binding site within the Gβγ coiled-coil. Ste4p mutants defective in the Ste5p interaction interact efficiently with Gpa1p (Gα) and Ste18p (Gγ) but cannot function in signal transduction because cells expressing these mutants are sterile. Ste4 L65S is temperature-sensitive for its interaction with Ste5p, and also for signaling. We have identified a Ste5p mutant (L196A) that displays a synthetic interaction defect with Ste4 L65S, providing strong evidence that Ste4p and Ste5p interact directly in vivo through an interface that involves hydrophobic residues. The correlation between disruption of the Ste4p-Ste5p interaction and sterility confirms the importance of this interaction in signal transduction. Identification of the Gβγ coiled-coil in Ste5p binding may set a precedent for Gβγ-effector interactions in more complex organisms.

A successful mating in the mushroom Coprinus cinereus brings together a compatible complement of pheromones and G-protein-coupled receptors encoded by multiallelic genes at the B mating-type locus. Rare B gene mutations lead to constitutive activation of B-regulated development without the need for mating. Here we characterize a mutation that arose in the B6 locus and show that it generates a mutant receptor with a single amino acid substitution (R96H) at the intracellular end of transmembrane domain III. Using a heterologous yeast assay and synthetic pheromones we show that the mutation does not make the receptor constitutively active but permits it to respond inappropriately to a normally incompatible pheromone encoded within the same B6 locus. Parallel experiments carried out in Coprinus showed that a F67W substitution in this same pheromone enabled it to activate the normally incompatible wild-type receptor. Together, our experiments show that a single amino acid replacement in either pheromone or receptor can deregulate the specificity of ligand-receptor recognition and confer a self-compatible B phenotype. In addition, we use the yeast assay to demonstrate that different receptors and pheromones found at a single B locus belong to discrete subfamilies within which receptor activation cannot normally occur.