Propping Up Our Knowledge of G Protein Signaling Pathways: Diverse Functions of Putative Noncanonical Gβ Subunits in Fungi

Janganan

et al. 2007

Molecular Microbiology

SummaryParacoccidioides brasiliensis is a human pathogenic fungus that switches from a saprobic mycelium to a pathogenic yeast. Consistent with the morphological transition being regulated by the cAMP-signalling pathway, there is an increase in cellular cAMP levels both transiently at the onset (< 24 h) and progressively in the later stages (> 120 h) of the transition to the yeast form, and this transition can be modulated by exogenous cAMP. We have cloned the cyr1 gene encoding adenylate cyclase (AC) and established that its transcript levels correlate with cAMP levels. In addition, we have cloned the genes encoding three Ga (Gpa1-3), Gb (Gpb1) and Gg (Gpg1) G proteins. Gpa1 and Gpb1 interact with one another and the N-terminus of AC, but neither Gpa2 nor Gpa3 interacted with Gpb1 or AC. The interaction of Gpa1 with Gpb1 was blocked by GTP, but its interaction with AC was independent of bound nucleotide. The transcript levels for gpa1, gpb1 and gpg1 were similar in mycelium, but there was a transient excess of gpb1 during the transition, and an excess of gpa1 in yeast. We have interpreted our findings in terms of a novel signalling mechanism in which the activity of AC is differentially modulated by Gpa1 and Gpb1 to maintain the signal over the 10 days needed for the morphological switch.

Section: Discussionmentioning

confidence: 99%

The cAMP pathway is important for controlling the morphological switch to the pathogenic yeast form of Paracoccidioides brasiliensis

Janganan

et al. 2007

Molecular Microbiology

Journal of Biological Chemistry

“…Asc1 joins a small but growing list of multifunctional or atypical G␤ proteins (77). Recently Gpa1 was shown to bind directly to Vps15 in a GDP-dependent manner.…”

Section: Discussionmentioning

confidence: 99%

The RACK1 Ortholog Asc1 Functions as a G-protein β Subunit Coupled to Glucose Responsiveness in Yeast

Zeller

Parnell

Dohlman

2007

136

According to the prevailing paradigm, G-proteins are composed of three subunits, an ␣ subunit with GTPase activity and a tightly associated ␤␥ subunit complex. In the yeast Saccharomyces cerevisiae there are two known G␣ proteins (Gpa1 and Gpa2) but only one G␤␥, which binds only to Gpa1. Here we show that the yeast ortholog of RACK1 (receptor for activated protein kinase C1) Asc1 functions as the G␤ for Gpa2. As with other known G␤ proteins, Asc1 has a 7-WD domain structure, interacts directly with the G␣ in a guanine nucleotide-dependent manner, and inhibits G␣ guanine nucleotide exchange activity. In addition, Asc1 binds to the effector enzyme adenylyl cyclase (Cyr1), and diminishes the production of cAMP in response to glucose stimulation. Thus, whereas Gpa2 promotes glucose signaling through elevated production of cAMP, Asc1 has opposing effects on these same processes. Our findings reveal the existence of an unusual G␤ subunit, one having multiple functions within the cell in addition to serving as a signal transducer for cell surface receptors and intracellular effectors.

“…Vps15 has a 7-bladed propeller domain structure, similar to that of other Gb proteins, as well as a protein kinase domain of unknown function. The discovery of atypical and multifunctional Gb proteins, including Asc1 and Vps15, suggests that the superfamily of Gb subunits may be far larger and more complex than previously recognized (53).…”

Section: Discussionmentioning

confidence: 99%

Multi-omics analysis of glucose-mediated signaling by a moonlighting Gβ protein Asc1/RACK1

Rushing

et al. 2021

Preprint

G proteins were originally discovered through efforts to understand the effects of hormones, such as glucagon and epinephrine, on glucose metabolism. On the other hand, many cellular metabolites, including glucose, serve as ligands for G protein-coupled receptors. Here we investigate the consequences of glucose-mediated receptor signaling, and in particular the role of a Gα subunit Gpa2 and a non-canonical Gβ subunit, known as Asc1 in yeast and RACK1 in animals. Asc1/RACK1 is of particular interest because it has multiple, seemingly unrelated, functions in the cell. The existence of such “moonlighting” operations has complicated the determination of phenotype from genotype. Through a comparative analysis of individual gene deletion mutants, and by integrating transcriptomics and metabolomics measurements, we have determined the relative contributions of the Gα and Gβ protein subunits to glucose-initiated processes in yeast. We determined that Gpa2 is primarily involved in regulating sugar metabolism while Asc1 is primarily involved in amino acid metabolism. Both proteins are involved in regulating purine metabolism. Of the two subunits, Gpa2 regulates a greater number of gene transcripts and was particularly important in determining the amplitude of response to glucose addition. We conclude that the two G protein subunits regulate distinct but complementary processes downstream of the glucose-sensing receptor, as well as processes that lead ultimately to changes in cell growth and metabolism.AUTHOR SUMMARYDespite the societal importance of glucose fermentation in yeast, the mechanisms by which these cells detect and respond to glucose have remained obscure. Glucose detection requires a cell surface receptor coupled to a G protein that is comprised of two subunits, rather than the more typical heterotrimer: an α subunit Gpa2 and the β subunit Asc1 (or RACK1 in humans). Asc1/RACK1 also serves as a subunit of the ribosome, where it regulates the synthesis of proteins involved in glucose fermentation. This manuscript uses global metabolomics and transcriptomics to demonstrate the distinct roles of each G protein subunit in transmitting the glucose signal. Whereas Gpa2 is primarily involved in the metabolism of sugars, Asc1/RACK1 contributes to production of amino acids necessary for protein synthesis and cell division. These findings reveal the initial steps of glucose signaling and several unique and complementary functions of the G protein subunits. More broadly, the integrated approach used here is likely to guide efforts to determine the topology of complex G protein and metabolic signaling networks in humans.