Inositol transport in Saccharomyces cerevisiae is regulated by transcriptional and degradative endocytic mechanisms during the growth cycle that are distinct from inositol-induced regulation.

Abstract

Regulation of inositol uptake activity in Saccharomyces cerevisiae during the growth cycle was examined. Activity increased as the cell population transited from lag phase to exponential growth, and continued to increase until late exponential phase. The increase in activity was due to increased transcription of the ITR1 gene and synthesis of the Itr1 permease. When the culture reached stationary phase, uptake activity decreased and dropped to a minimum within 4 h. The decrease was due to repression of ITR1 transcription, independent of the negative regulator Opi1p, and degradation of the existing permease. Degradation depended on delivery of the permease to the vacuole through the END3/END4 endocytic pathway. During exponential growth in inositol-containing medium the permease is also rapidly degraded, whereas in inositol-free medium the permease is highly stable. Rapid degradation of the permease at stationary phase occurred in inositol-free medium, indicating that there are two distinct mechanisms that trigger endocytosis and degradation in response to different physiological stimuli. In addition, the level of the enzyme required for inositol biosynthesis, inositol-1-phosphate synthase, encoded by INO1, is not reduced in stationary-phase cells, and this contrast in the regulation of inositol supply is discussed.

Arabidopsis INOSITOL TRANSPORTER4 Mediates High-Affinity H+ Symport of Myoinositol across the Plasma Membrane

“…AF280431) and MITR2 (accession no. AF280432) from ice plant (Chauhan et al, 2000;52.2% to 72.7% sequence identity on the protein level) and with ScITR1 from yeast (Robinson et al, 1996; 35.8% to 37.1% sequence identity on the protein level). Because the function of the plant transporters had been predicted from growth analyses of transgenic yeast (Chauhan et al, 2000), we decided to perform detailed analyses for one of the Arabidopsis proteins in different expression systems.…”

Two Major Inositol Transporters and Their Role in Cryptococcal Virulence

“…Inositol can also be imported from the extracellular environment via inositol transporters. The inositol transporter (ITR) gene family is part of the sugar transporter superfamily and plays an important role in inositol sensing in fungi, including Saccharomyces cerevisiae (30,31,(42)(43)(44)51), Candida albicans (12,28,50), C. neoformans (64), and Schizosaccharomyces pombe (40). Studying ITR genes in fungi has clinical significance as ITR genes identified in lower eukaryotes, such as fungi (28) and protozoa (15,39,52), are proton coupled and differ both kinetically and pharmacologically from the sodium-dependent inositol transporter system (SMIT) in humans.…”

Hxt-Carrier-Mediated Glucose Efflux upon Exposure of Saccharomyces cerevisiae to Excess Maltose

“…Which signal triggers this catabolite inactivation is still a matter of debate. Some authors have proposed that hexose transport via Hxt transporters is required for this pathway (24,25), whereas other authors have stated that galactose and even maltose can also elicit catabolite inactivation (40,46). In addition, trehalose and/or trehalose 6-phosphate have recently been mentioned as possible signals for catabolite inactivation (4).…”