| Description |
Plasma membrane integrity is paramount to cell viability. The separation between the extra- and intracellular environment is established by the plasma membrane and the plethora of proteins embedded within it. Nutrients that are unable to freely diffuse across the plasma membrane must be transported. Transportation is a highly regulated process. The proteins that facilitate nutrient transport, plasma membrane nutrient transporters, are multispanning integral membrane proteins, which utilize the energy of ion gradients to transport nutrients into the cell. Metabolic demands of the cell regulate the abundance of plasma membrane nutrient transporters by influencing new protein synthesis or protein degradation. Appropriate downregulation and vacuole degradation of plasma membrane nutrient transporters is imperative to maintain cellular homeostasis. Downregulation of nutrient transporters has been observed both on a global, cellwide scale, targeting many different transporters congruently, and on a proteinspecific basis, resulting in a single transporter's downregulation. In Saccharomyces cerevisiae, downregulation is facilitated by the E3 ubiquitin ligase Rsp5. For specific downregulation of a nutrient transporter to occur, Rsp5 must recognize the correct substrate before ubiquitin conjugation. How this is achieved is an open question in the field. Identification and subsequent downregulation of damaged cell surface nutrient transporters require Rsp5 to properly distinguish between a damaged and nondamaged protein. It has been observed that Fur4, the high affinity uracil transporter, is efficiently downregulated in response to both peroxide and heat stress, but the underlying mechanism was unknown. Utilizing the crystal structure of Mhp1, a bacterial homolog of Fur4, an intrinsic protein-fold sensing domain was identified and termed the Loop Interaction Domain (LID). Through extensive mutational analysis, it was discovered that the LID of Fur4 functions as a built-in chaperone: The LID directly relays the folded status of Fur4 to the ubiquitin machinery of the cell by exposure or sequestration of a degron. The data presented here resulted with the discovery of the LID-degron mode of degradation, which is a conformational model explaining both quality control and substrate-dependent downregulation and how Rsp5 is able to identify specific substrates. |
