A novel role for yeast casein kinases in glucose sensing and signaling.

Yeasts have sophisticated signaling pathways for sensing glucose, their preferred carbon source, to regulate its uptake and metabolism. One of these is the sensor/receptor-repressor (SRR) pathway, which detects extracellular glucose and transmits an intracellular signal that induces expression of HXT genes. The yeast casein kinases (Ycks) are key players in this pathway. Our model of the SRR pathway had the Ycks functioning downstream of the glucose sensors, transmitting the signal from the sensors to the Mth1 and Std1 corepressors that are required for repression of HXT gene expression. However, we found that overexpression of Yck1 fails to restore glucose signaling in a glucose sensor mutant. Conversely, overexpression of a glucose sensor suppresses the signaling defect of a yck mutant. These results suggest that the Ycks act upstream or at the level of the glucose sensors. Indeed, we found that the glucose sensor Rgt2 is phosphorylated on Yck consensus sites in its C-terminal tail in a Yck-dependent manner and that this phosphorylation is required for corepressor binding and ultimately HXT expression. This leads to a revised model of the SRR pathway in which the Ycks prime a site on the cytoplasmic tails of the glucose sensors to promote binding of the corepressors.

Association between preoperative haemoglobin concentration and cardiopulmonary exercise variables: a multicentre study.

BACKGROUND: Preoperative anaemia and low exertional oxygen uptake are both associated with greater postoperative morbidity and mortality. This study reports the association among haemoglobin concentration ([Hb]), peak oxygen uptake (V˙O2 peak) and anaerobic threshold (AT) in elective surgical patients. METHODS: Between 1999 and 2011, preoperative [Hb] and cardiopulmonary exercise tests were recorded in 1,777 preoperative patients in four hospitals. The associations between [Hb], V˙O2 peak and AT were analysed by linear regression and covariance. RESULTS: In 436 (24.5%) patients, [Hb] was <12 g dl-1 and, in 83 of these, <10 g dl-1. Both AT and V˙O2 peak rose modestly with increasing [Hb] (r2 = 0.24, P <0.0001 and r2 = 0.30, P <0.0001, respectively). After covariate adjustment, an increase in [Hb] of one standard deviation was associated with a 6.7 to 9.7% increase in V˙O2 peak, and a rise of 4.4 to 6.0% in AT. Haemoglobin concentration accounted for 9% and 6% of the variation in V˙O2 peak and AT respectively. CONCLUSIONS: To a modest extent, lower haemoglobin concentrations are independently associated with lower oxygen uptake during preoperative cardiopulmonary exercise testing. It is unknown whether this association is causative.

Regulation of Hxt3 and Hxt7 turnover converges on the Vid30 complex and requires inactivation of the Ras/cAMP/PKA pathway in Saccharomyces cerevisiae.

Eukaryotic cells adjust their intracellular protein complement as a mechanism to adapt to changing environmental signals. In Saccharomyces cerevisiae the hexose transporters Hxt3 and Hxt7 are expressed and function on the plasma membrane in high and low glucose abundance, respectively. By contrast, Hxt3 is endocytosed and degraded in the vacuole when cells are starved of glucose and Hxt7 in response to rapamycin treatment or when nitrogen is limiting. Yeast uses several signaling pathways, including the TORC1 and Ras/cAMP/Protein Kinase A (PKA) pathways, to adapt to nutrient changes in the environment. The multi-protein Vid30 complex (Vid30c), an E3 ubiquitin ligase required for the degradation of FBPase, assists in this adaptation process in a mechanism that is poorly understood. Here we show the endocytosis and the subsequent degradation of both Hxt3 and Hxt7, in response to different nutrient signals, is dependent on components of the Vid30c. Additionally, we define the signaling events required for the turnover of Hxt3 and Hxt7 by showing that Hxt3 turnover requires Ras2 and PKA inactivation, whereas Hxt7 turnover requires TORC1 and Ras2 inactivation. Further investigation led us to identify Rim15, a kinase that is inhibited by both the TORC1 and Ras/cAMP/PKA pathways, as a key downstream effector in signaling both turnover events. Finally, we show that the turnover of both Hxt3 and Hxt7 is dependent on the essential E3 ubiquitin ligase, Rsp5, indicating that the role of the Vid30c might be indirect of Hxt ubiquitylation.

Components of the Vid30c are needed for the rapamycin-induced degradation of the high-affinity hexose transporter Hxt7p in Saccharomyces cerevisiae.

Saccharomyces cerevisiae adapts to changing nutrient conditions by regulating its genome-wide transcription profile and cell-wide protein complement in correlation with the reigning nutrient conditions. The target of rapamycin (TOR) signalling pathway is one of the major control mechanisms within the cell that facilitates these changes. The transcription, intracellular trafficking, and protein turnover of nutrient transporters, including the hexose transporter proteins (Hxts), are regulated in response to nutrient conditions. The Vid and Gid proteins facilitate the nutrient-dependent degradation of the gluconeogenic enzymes FBPase and Mdh2p when glucose-starved cells are replenished with glucose. Three members of the VID and GID gene families, VID30/GID1, GID2, and VID28/GID5 are needed for the rapamycin or nitrogen starvation-induced degradation of the high-affinity hexose transporter Hxt7p is shown here. In addition, evidence that the functions of several Vid and Gid proteins are in close relation to the TOR signalling pathway is provided.