Summary G proteins and their associated receptors process information from a variety of environmental stimuli to induce appropriate cellular responses. Generally speaking, each cell in a population responds within defined limits despite large variation in the expression of protein signaling components. Therefore we postulated that noise suppression is encoded within the signaling system. Using the yeast mating pathway as a model we evaluated the ability of a regulator of G protein signaling (RGS) protein to suppress noise. We found that the RGS protein Sst2 limits variability in transcription and morphogenesis in response to pheromone stimulation. While signal suppression is a result of both the GAP (GTPase accelerating) and receptor binding functions of Sst2, noise suppression requires only the GAP activity. Taken together our findings reveal a hitherto overlooked role of RGS proteins as noise suppressors, and demonstrate an ability to uncouple signal and noise in a prototypical stimulus-response pathway.

Summary Background Septins are well known to form a boundary between mother and daughter cells in mitosis, but their role in other morphogenic states is poorly understood. Results Using microfluidics and live cell microscopy, coupled with new computational methods for image analysis, we investigated septin function during pheromone-dependent chemotropic growth in yeast. We show that septins colocalize with the regulator of G-protein signaling (RGS) Sst2, a GTPase-activating protein that dampens pheromone receptor signaling. We show further that the septin structure surrounds the polar cap, ensuring that cell growth is directed toward the source of pheromone. When RGS activity is abrogated, septins are partially disorganized. Under these circumstances the polar cap travels toward septin structures and away from sites of exocytosis, resulting in a loss of gradient tracking. Conclusion Septin organization is dependent on RGS protein activity. When assembled correctly, septins promote turning of the polar cap and proper tracking of a pheromone gradient.

Extracellular signals, such as nutrients and hormones, cue intracellular pathways to produce adaptive responses. Often, cells must coordinate their responses to multiple signals to produce an appropriate outcome. We showed that components of a glucose-sensing pathway acted on components of a heterotrimeric guanine nucleotide–binding protein (G protein)–mediated pheromone signaling pathway in the yeast Saccharomyces cerevisiae. We demonstrated that the G protein α subunit Gpa1 was phosphorylated in response to conditions of reduced glucose availability and that this phosphorylation event contributed to reduced pheromone-dependent stimulation of mitogen-activated protein kinases, gene transcription, cell morphogenesis, and mating efficiency. We found that Elm1, Sak1, and Tos3, the kinases that phosphorylate Snf1, the yeast homolog of adenosine monophosphate–activated protein kinase (AMPK), in response to limited glucose availability, also phosphorylated Gpa1 and contributed to the diminished mating response. Reg1, the regulatory subunit of the phosphatase PP1 that acts on Snf1, was likewise required to reverse the phosphorylation of Gpa1 and to maintain the mating response. Thus, the same kinases and phosphatase that regulate Snf1 also regulate Gpa1. More broadly, these results indicate that the pheromone signaling and glucose-sensing pathways communicate directly to coordinate cell behavior.

MAPKs are activated by dual phosphorylation. Much of the MAPK Fus3 is monophosphorylated and acts to inhibit signaling in vivo. Computational models reveal how a kinase scaffold and phosphatase act together to dynamically regulate dual-phosphorylated and monophosphorylated MAPKs and the downstream signal.

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G protein–coupled receptor signaling is negatively regulated by both receptor internalization and regulator of G protein signaling (RGS) protein–stimulated inactivation of the G protein. The RGS protein can also positively regulate receptor signaling by binding to the receptor, thereby reducing receptor internalization.

Background:The yeast G␣ protein contains a unique domain that is monoubiquitinated, leading to vacuolar degradation. Results: A gene deletion screen reveals ubiquitin binding domain proteins necessary for G␣ trafficking. Loss of the ubiquitination domain impedes cellular morphogenesis and mating. Conclusion: Proper endocytosis of G␣ is required for sustained morphogenesis and efficient mating. Significance: G␣ endocytosis promotes signaling. Heterotrimeric G proteins are well known to transmit signals from cell surface receptors to intracellular effector proteins.There is growing appreciation that G proteins are also present at endomembrane compartments, where they can potentially interact with a distinct set of signaling proteins. Here, we examine the cellular trafficking function of the G protein ␣ subunit in yeast, Gpa1. Gpa1 contains a unique 109-amino acid insert within the ␣-helical domain that undergoes a variety of posttranslational modifications. Among these is monoubiquitination, catalyzed by the NEDD4 family ubiquitin ligase Rsp5. Using a newly optimized method for G protein purification together with biophysical measures of structure and function, we show that the ubiquitination domain does not influence enzyme activity. By screening a panel of 39 gene deletion mutants, each lacking a different ubiquitin binding domain protein, we identify seven that are necessary to deliver Gpa1 to the vacuole compartment including four proteins (Ede1, Bul1, Ddi1, and Rup1) previously not known to be involved in this process. Finally, we show that proper endocytosis of the G protein is needed for sustained cellular morphogenesis and mating in response to pheromone stimulation. We conclude that a cascade of ubiquitin-binding proteins serves to deliver the G protein to its final destination within the cell. In this instance and in contrast to the previously characterized visual system, endocytosis from the plasma membrane is needed for proper signal transduction rather than for signal desensitization.G␣ proteins are enzymatic switches that are part of a multicomponent signaling complex. The complex typically consists of a seven-transmembrane G protein-coupled receptor, a guanine nucleotide-binding protein (G␣), and an associated dimer consisting of ␤ and ␥ subunits (G␤␥) (1). Signaling is turned on and off based on receptor activation, which in turn dictates the nucleotide-bound state of the G␣ protein. When G␣ is GDPbound, G␤␥ is sequestered, and signaling pathways are off (1). When G␣ releases GDP and binds GTP, G␤␥ dissociates, and the signaling pathways are turned on. Subsequent GTP hydrolysis is accelerated by regulators of G protein signaling (RGS 3 proteins) (2, 3). Large G␣ proteins contain a Ras-like domain as well as an independently folded ␣-helical domain (1). Within this group of proteins there is a well established role for the Ras-like domain in specifying interactions with G␤␥, effectors, and RGS proteins (1). However, recent evidence has shown that the ␣-helical domain is also important for signa...

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