SUMMARY PARAGRAPH for FUR4
Fur4p is a uracil permease that mediates the uptake of uracil, but does not transport other natural pyrimidines such as cytosine, thymine, or uridine (6, 7). It is localized to lipid microdomains of the plasma membrane that also contain Can1p and Sur7p (8) Fur4p functions as a symporter by actively translocating uracil with the co-transport of protons (9). Since the magnitude of the proton gradient across the plasma membrane influences the rate of uracil uptake by Fur4p, the rate fluctuates with varying ionic conditions (10). The prodrug 5-fluorouracil (5-FU) also enters the cell via Fur4p (6), and the immunosuppressant leflunomide inhibits Fur4p-mediated uracil uptake (11).
Fur4p abundance is an important determinant of intracellular uracil levels, and is therefore under several types of regulation. FUR4 transcription is induced in response to galactose, in a GAL4-dependent manner (12). Fur4p expression is downregulated in the presence of uracil (of either exogenous or catabolic origin) in order to prevent the accumulation of excess intracellular uracil-derived nucleotides (13, 3). FUR4 mRNA is of low abundance and has a high turnover rate, with a half-life of approximately 2 minutes (14, 7). Its abundance is further decreased in response to uracil, probably due to increased degradation (3). Newly synthesized Fur4p is normally delivered to the cell surface via the secretory pathway. However in the presence of excess uracil, newly synthesized Fur4p can be directed to the degradative vacuolar pathway without ever passing through the plasma membrane (13).
Normal degradation of Fur4p occurs through phosphorylation of Fur4p at the plasma membrane, followed by ubiquitination, endocytosis, and finally degradation in the vacuoles. Fur4p is phosphorylated at several serine residues within a well characterized N-terminal PEST sequence (15, 16, 14, 17,), and phosphorylation is dependent on Yck1p and Yck2p, which are serine/threonine kinases (15). Phosphorylation in turn facilitates ubiquitination of Fur4p, which occurs on lysine residues 38 and 41 and is dependent on the Rsp5p ubiquitin-protein ligase (15, 18, 19). Fur4p is then internalized and following endocytosis, is targeted to the vacuole for proteolysis (15, 20, 18). Endocytosis of Fur4p involves End3p and Sla2p functions, and degradation of Fur4p in vacuoles requires the function of Pep4p (21). Cells lacking functional ESCRT complexes (stp22, srn2, vps28, snf8, vps25, vps36, vps20, and vps24 null mutants) accumulate Fur4p at the plasma membrane (22). Uracil also promotes the normal degradation of Fur4p located in the plasma membrane (13, 3, 21). Fur4p is constitutively degraded in exponentially growing cells, and degradation increases in response to adverse conditions such as nutrient starvation, inhibition of protein synthesis, heat shock, or stationary phase (16, 21, 17).
FUR4 is not essential for normal growth of uracil-proficient S. cerevisiae strains (23). fur4 mutants cannot take up uracil and are resistant to 5-fluorouracil (7, 9). Conversely, in cells overexpressing Fur4p, uracil is toxic (24, 13). fur4-[R294A] mutant protein is stabilized compared to wild-type protein during nitrogen starvation or inhibition of protein synthesis (17). FUR4 has similarity to DAL4, FUI1 and THI7, YOR071C, and Schizosaccharomyces pombe fur4 (25, 26, 16, 27, 28). Fur4p can complement the defects of an S. pombe mutant lacking uracil transport activity (29).
Last updated: 2005-12-16