Eisosomes are lipid domains of the yeast plasma membrane that share similarities to caveolae of higher eukaryotes. Eisosomes harbor APC-type nutrient transporters for reasons that are poorly understood. Our analyses support the model that eisosomes function as storage compartments, keeping APC transporters in a stable, inactive state. By regulating eisosomes, yeast is able to balance the number of proton-driven APC transporters with the proton-pumping activity of Pma1, thereby maintaining the plasma membrane proton gradient. Environmental or metabolic changes that disrupt the proton gradient cause the rapid restructuring of eisosomes and results in the removal of the APC transporters from the cell surface. Furthermore, we show evidence that eisosomes require the presence of APC transporters, suggesting that regulating activity of nutrient transporters is a major function of eisosomes.

CLN5 is a soluble lysosomal protein with unknown function. Mutations in CLN5 lead to neuronal ceroid lipofuscinosis, a group of inherited neurodegenerative disorders that mainly affect children. CLN5 has eight potential N-glycosylation sites based on the Asn-X-Thr/Ser consensus sequence. Through site-directed mutagenesis of individual asparagine residues to glutamine on each of the N-glycosylation consensus sites, we showed that all eight putative N-glycosylation sites are utilized in vivo. Additionally, localization studies showed that the lack of N-glycosylation on certain sites (N179, N252, N304, or N320) caused CLN5 retention in the endoplasmic reticulum, indicating that glycosylation is important for protein folding. Interestingly, one particular mutant, N401Q, is mislocalized to the Golgi, suggesting that N401 is not important for protein folding but essential for CLN5 trafficking to the lysosome. Finally, we analyzed several patient mutations in which N-glycosylation is affected. The N192S patient mutant is localized to the lysosome, indicating that this mutant has a functional defect in the lysosome. Our results suggest that there are functional differences in various N-glycosylation sites of CLN5 which affect folding, trafficking, and lysosomal function of CLN5.

Eisosomes are furrows of the yeast plasma membrane that are involved in the regulation of nutrient transporters and membrane stress pathways. Environmental changes affect plasma membrane tension and fluidity, which change both the eisosome structure and the localization of nutrient transporters and regulatory proteins to the eisosome.

Nutrient import by APC‐type transporters is predicted to have a high energy demand because it depends on the plasma membrane proton gradient established by the ATP‐driven proton pump Pma1. We show that Pma1 is indeed a major energy consumer and its activity is tightly linked to the cellular ATP levels. The low Pma1 activity caused by acute loss of respiration resulted in a dramatic drop in cytoplasmic pH, which triggered the downregulation of the major proton importers, the APC transporters. This regulatory system is likely the reason for the observed rapid endocytosis of APC transporters during many environmental stresses. Furthermore, we show the importance of respiration in providing ATP to maintain a strong proton gradient for efficient nutrient uptake.

Even though it is a well-accepted fact that the energy metabolism of yeast is likely to impact all cellular activities, surprising little is known about the ATP homeostasis of particular yeast strains that are commonly used in cell biological studies. Therefore, we determined key parameters such as oxygen consumption and fermentation rates of the lab strain SEY6210. Our data indicated that even at high glucose concentrations, SEY6210 produces 30-50% of cellular ATP from oxidative phosphorylation. Loss of respiration, either by disrupting ATP synthase function or by growth in anaerobic conditions, was not fully compensated by fermentation and as a result affected energy intensive processes such as the maintenance of the plasma membrane proton gradient and the associated import of nutrients.