ERMES in Candida albicans: A key mediator of lipid dynamics

Organelles are specialized compartments within a cell and cross talk between them is compulsory to achieve cell homeostasis. Lipid transport is a necessary facet of this cross talk. The main organelle of lipid synthesis is the endoplasmic reticulum. However, synthesis of the phospholipid phosphatidylethanolamine occurs within mitochondria. The nature of transport processes which supply mitochondria with lipids, as well as, the transport of phosphatidylethanolamine to the endoplasmic reticulum, is largely unknown. Lipid exchange between these organelles likely occurs via membrane contact sites. Recently, a multi-protein integral membrane complex, ERMES (ER-mitochondria encounter structure), was shown to physically link these two organelles, coordinating lipid exchange. However, the precise biochemical role of ERMES in lipid trafficking is unclear and several other roles for this complex have been suggested, such as maintenance of Ca2+- and mitochondrial genome homeostasis. ERMES consists of four subunits and was first discovered in Saccharomyces cerevisiae, but its subunits are conserved across diverse fungal species. Preliminary data generated with Candida albicans showed that the phenotypic consequences of ERMES inactivation are significantly more severe, with at least two protein components of the ERMES complex, Mdm10p and Mmm1p, being essential. In contrast, none of the ERMES proteins are essential in Saccharomyces cerevisiae. These findings make Candida albicans an excellent genetic system to study ERMES function. This proposal focuses on elucidating the role of ERMES in Candida albicans cell physiology from various angles: shut-down experiments of the essential genes will be used to analyze phenotypic consequences of ERMES inactivation and will help clarify the relationship between distinct phenotypes of different ERMES mutants. The stoichiometry of the ERMES complex remains to be determined, therefore purification of ERMES will be performed and subunit ratios estimated on SDS- PAGES. As ERMES is proposed to function in many different cellular pathways, it will be addressed whether or not other proteins join ERMES, changing complex stoichiometry and modulating the different functions of the complex. Three of the ERMES complex proteins contain a lipid-binding domain. The ability of these proteins to bind phospholipids will be assayed by Quartz Crystal Microbalance. Furthermore, ERMES has been proposed to be a key player in ER-mitochondria cross talk that serves as a ``hub`` for the coordinated control of mitochondrial dynamics and function. To elucidate pathways closely linked with ERMES, RNAseq will be used to identify up- or down- regulated pathways upon ERMES mutations. This project will provide significant insight into the enigmatic role of ERMES with great benefits for basic and applied research. It will further establish Candida albicans as a eukaryotic model to study genetics and cell biology by developing new, state of the art technologies optimized for this organism.

Candida albicans is an opportunistic pathogenic yeast that exists as part of the human microflora on the skin, the oral cavity and the urogenital tract. If the microflora becomes imbalanced favouring an overgrowth of C. albicans, the fungus may penetrate the mucosal barrier and cause an infection. This infection remains mostly localized and superficial for healthy individuals, but it can also enter the bloodstream producing severe systemic infections especially in immunocompromised patient. These infections reach a mortality of up to 70%. A small assortment of drugs is available to treat fungal infections, but efficacy is declining due to the ability of Candida to quickly become resistant. My work focused on the characterisation of the ERMES complex which is of special interest as a new potential drug target for therapeutics. This complex is responsible for maintaining the correct morphology of cell organelles known as mitochondria. These organelles play a major role in the virulence of C. albicans and are therefore an important drug target. Furthermore, I discovered a molecule which manipulates the metabolism of C. albicans towards a non-virulent growth behaviour.