Pseudohyphal and invasive growth in the yeast Saccharomyces cerevisiae is regulated by the kelch repeat-containing proteins Gpb1p and Gpb2p, which act downstream of the G protein α-subunit Gpa2p.
Here we show that deletion of GPB1 and GPB2 causes increased haploid invasive growth in cells containing any one of the three protein kinase A (PKA) catalytic subunits,
suggesting that Gpb1p and Gpb2p are able to inhibit each of these kinases. Cells containing gpb1Δ gpb2Δ mutations also display increased phosphorylation of the PKA substrates Sfl1p and Msn2p, indicating that Gpb1p and Gpb2p
are negative regulators of PKA substrate phosphorylation. Stimulation of PKA-dependent signaling by gpb1Δ gpb2Δ mutations occurs in cells that lack both adenylyl cyclase and the high-affinity cyclic AMP (cAMP) phosphodiesterase. This
effect is also seen in cells that lack the low-affinity cAMP phosphodiesterase. Given that these three enzymes control the
synthesis and degradation of cAMP, these results indicate that the effect of Gpb1p and Gpb2p on PKA substrate phosphorylation
does not occur by regulating the intracellular cAMP concentration. These findings suggest that Gpb1p and Gpb2p mediate their
effects on the cAMP/PKA signaling pathway either by inhibiting the activity of PKA in a cAMP-independent manner or by activating
phosphatases that act on PKA substrates.
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"However, recently the existence of a cAMP-independent PKA pathway has begun to emerge in multiple organisms (Blackstone and Chang, 2011; Brown et al., 2013; Graef and Nunnari, 2011; McInnis et al., 2010). In S. cerevisiae the existence of a glucose-sensing cAMP-independent signaling pathway for activation of PKA has been described previously (Budhwar et al., 2010; Lu and Hirsch, 2005 ). Starvation in mammalian cells triggers autophagy, yet mitochondria enlarge in size, which has been shown to sustain ATP production and prevent mitophagy (Blackstone and Chang, 2011). "
[Show abstract][Hide abstract]ABSTRACT: The utilisation of lignocellulosic plant biomass as an abundant, renewable feedstock for green chemistries and biofuel production is inhibited by its recalcitrant nature. In the environment, lignocellulolytic fungi are naturally capable of breaking down plant biomass into utilisable saccharides. Nonetheless, within the industrial context, inefficiencies in the production of lignocellulolytic enzymes impede the implementation of green technologies. One of the primary causes of such inefficiencies is the tight transcriptional control of lignocellulolytic enzymes via carbon catabolite repression. Fungi coordinate metabolism, protein biosynthesis and secretion with cellular energetic status through the detection of intra- and extra-cellular nutritional signals. An enhanced understanding of the signals and signalling pathways involved in regulating the transcription, translation and secretion of lignocellulolytic enzymes is therefore of great biotechnological interest. This comparative review describes how nutrient sensing pathways regulate carbon catabolite repression, metabolism and the utilisation of alternative carbon sources in Saccharomyces cerevisiae and ascomycete fungi.
"proteins, Krh1 and Krh2, which were also called Gpb2 and Gpb1, referring to a possible role as Gβ subunit for Gpa2 (Harashima & Heitman, 2002). Later work, however, showed that these proteins function in an adenylate cyclase bypass pathway, allowing direct activation of PKA by activated Gpa2 (Lu & Hirsch, 2005, Peeters, et al., 2006). The kelch repeat proteins directly bind to the catalytic subunits of PKA and thereby stimulate association of the catalytic and regulatory subunits of PKA, lowering PKA activity. "
[Show abstract][Hide abstract]ABSTRACT: The yeast Saccharomyces cerevisiae has been a favorite organism for pioneering studies on nutrient-sensing and signaling mechanisms. Many specific nutrient responses have been elucidated in great detail. This has led to important new concepts and insight into nutrient-controlled cellular regulation. Major highlights include the central role of the Snf1 protein kinase in the glucose repression pathway, galactose induction, the discovery of a G-protein coupled receptor system and role of Ras in glucose-induced cAMP signaling, the role of the protein synthesis initiation machinery in general control of nitrogen metabolism, the cyclin-controlled protein kinase Pho85 in phosphate regulation, nitrogen catabolite repression and the nitrogen-sensing TOR pathway, and the discovery of transporter-like proteins acting as nutrient sensors. In addition, a number of cellular targets, like carbohydrate stores, stress tolerance and ribosomal gene expression, are controlled by the presence of multiple nutrients. The PKA signaling pathway plays a major role in this general nutrient response. It has led to the discovery of nutrient transceptors (transporter-receptors) as nutrient sensors. Major shortcomings in our knowledge are the relationship between rapid and steady-state nutrient signaling, the role of metabolic intermediates in intracellular nutrient sensing and the identity of the nutrient sensors controlling cellular growth. This article is protected by copyright. All rights reserved.
Full-text · Article · Feb 2014 · FEMS microbiology reviews
"However, substantial evidence has accumulated discounting Gpb1/Gpb2 as b subunits (Peeters et al. 2007), including the fact that the site on Gpa2 at which the proteins bind does not correspond to the classic Gb-binding domain (Niranjan et al. 2007 ). Nonetheless , Gpb1 and Gpb2 play redundant roles in negatively regulating the activity of the Ras/PKA pathway, either by interference with the Gpr1/Gpa2 interaction (Harashima and Heitman 2005), or through stabilization of the Ras– GAP proteins, Ira1 and Ira2 (Harashima et al. 2006), or by stabilization of the interaction between the regulatory subunit, Bcy1, and the catalytic subunits, Tpk1–3, of protein kinase A (Lu and Hirsch 2005; Peeters et al. 2006; Budhwar et al. 2010), or by some combination of all three mechanisms. One should note that the studies on Gpr1, Gpa2, and Gpb1/2 have not examined the dynamic nature of these components in the context of signal transduction. "
[Show abstract][Hide abstract]ABSTRACT: Availability of key nutrients, such as sugars, amino acids, and nitrogen compounds, dictates the developmental programs and the growth rates of yeast cells. A number of overlapping signaling networks-those centered on Ras/protein kinase A, AMP-activated kinase, and target of rapamycin complex I, for instance-inform cells on nutrient availability and influence the cells' transcriptional, translational, posttranslational, and metabolic profiles as well as their developmental decisions. Here I review our current understanding of the structures of the networks responsible for assessing the quantity and quality of carbon and nitrogen sources. I review how these signaling pathways impinge on transcriptional, metabolic, and developmental programs to optimize survival of cells under different environmental conditions. I highlight the profound knowledge we have gained on the structure of these signaling networks but also emphasize the limits of our current understanding of the dynamics of these signaling networks. Moreover, the conservation of these pathways has allowed us to extrapolate our finding with yeast to address issues of lifespan, cancer metabolism, and growth control in more complex organisms.