Carbon Catabolite Control in Candida albicans: New Wrinkles in Metabolism

Most microorganisms maintain strict control of nutrient assimilation pathways to ensure that they preferentially use compounds that generate the most energy or are most efficiently catabolized. In doing so, they avoid potentially inefficient conflicts between parallel catabolic and metabolic pathways. The regulation of carbon source utilization in a wide array of bacterial and fungal species involves both transcriptional and posttranscriptional mechanisms, and while the details can vary significantly, carbon catabolite control is widely conserved. In many fungi, the posttranslational aspect (carbon catabolite inactivation [CCI]) involves the ubiquitin-mediated degradation of catabolic enzymes for poor carbon sources when a preferred one (glucose) becomes available. A recent article presents evidence for a surprising exception to CCI in the fungal pathogen Candida albicans, an organism that makes use of gluconeogenic carbon sources during infection (D. Sandai, Z. Yin, L. Selway, D. Stead, J. Walker, M. D. Leach, I. Bohovych, I. V. Ene, S. Kastora, S. Budge, C. A. Munro, F. C. Odds, N. A. Gow, and A. J. Brown, mBio 3[6]:e00495-12). In vitro, addition of glucose to cells grown in a poor carbon source rapidly represses transcripts encoding gluconeogenic and glyoxylate cycle enzymes, such as phosphoenolpyruvate carboxykinase (Pck1p) and isocitrate lyase (Icl1p), in both C. albicans and Saccharomyces cerevisiae. Yet, uniquely, the C. albicans proteins persist, permitting parallel assimilation of multiple carbon sources, likely because they lack consensus ubiquitination sites found in the yeast homologs. Indeed, the yeast proteins are rapidly degraded when expressed in C. albicans, indicating a conservation of the machinery needed for CCI. How this surprising metabolic twist contributes to fungal commensalism or pathogenesis remains an open question.