Substrate-induced differential degradation and partitioning of the two tryptophan permeases Tat1 and Tat2 into eisosomes in Saccharomyces cerevisiae

Tryptophan is a relatively rare amino acid whose influx is strictly controlled to meet cellular demands. The yeast Saccharomyces cerevisiae has two tryptophan permeases, namely Tat1 (low-affinity type) and Tat2 (high-affinity type). These permeases are differentially regulated through ubiquitination based on inducible conditions and dependence on arrestin-related trafficking adaptors, although the physiological significance of their degradation remain unclear. Here, we demonstrated that Tat2 was rapidly degraded in an Rsp5-Bul1-dependent manner upon the addition of tryptophan, phenylalanine, or tyrosine, whereas Tat1 was unaffected. The expression of the ubiquitination-deficient variant Tat2^5K>R led to a reduction in cell yield at 4 μg/mL tryptophan, suggesting the occurrence of an uncontrolled, excessive consumption of tryptophan at low tryptophan concentrations. Eisosomes are membrane furrows that are thought to be storage compartments for some nutrient permeases. Tryptophan addition caused rapid Tat2 dissociation from eisosomes, whereas Tat1 distribution was unaffected. The 5 K > R mutation had no marked effect on Tat2 dissociation, suggesting that dissociation is independent of ubiquitination. Interestingly, the D74R mutation, which was created within the N-terminal acidic patch, stabilized Tat2 while reducing the degree of partitioning into eisosomes. Moreover, the hyperactive I285V mutation in Tat2, which increases V_max/K_m for tryptophan import by 2-fold, reduced the degree of segregation into eisosomes. Our findings illustrate the coordinated activity of Tat1 and Tat2 in the regulation of tryptophan transport at various tryptophan concentrations and suggest the positive role of substrates in inducing a conformational transition in Tat2, resulting in its dissociation from eisosomes and subsequent ubiquitination-dependent degradation.

Effect of parathyroid hormone on release of interleukin 1 and interleukin 6 from cultured mouse osteoblastic cells

This study was undertaken to examine the possibility that parathyroid hormone promotes the production of interleukin 1 and 6 in MC3T3-E1 osteoblastic cells derived from mouse calvaria. The cells were incubated in serum-free medium with or without synthetic human parathyroid hormone(1-34). The concentrations of interleukin 1 and 6 in the culture medium were determined by using specific bioassay. The cells cultured without parathyroid hormone for 24 hr released both of interleukins, and parathyroid hormone stimulated the release in a dose-dependent manner. When the cells were cultured with 10(-6) M parathyroid hormone, the release of both interleukins from the cells remained higher than control up to 144 hr. These results suggest that interleukin 1 and 6 release stimulated by parathyroid hormone may be involved in the bone resorbing activity of the hormone.

Vectorial redox reactions of physiological quinones. II. A study of transient semiquinone formation

Transient absorption changes during reduction of quinone in liposomes by external dithionite, in the absence and presence of initially trapped ferricyanide, were matched with absorption spectra of semiquinone and quinone in the blue region. Plastoquinone, ubiquinone-9 and phylloquinone, each having an isoprenoid side chain were compared with trimethyl-p-benzoquinone, ubiquinone-9 and menadione, which lack a long side chain. Semiquinone transients could only be observed by our spectroscopic technique during reduction of quinones lacking the chain. If Triton X-100 was added to the liposomes preparation semiquinone transients were also observed with the isoprenoid quinones. This result is consistent with the view that isoprenoid quinones build domains in the membranes, in which the life time of the semiquinone might be decreased by fast disproportionation, and to which dithionite has limited access.

Vectorial redox reactions of physiological quinones. I. Requirement of a minimum length of the isoprenoid side chain

Physiological quinones carrying isoprenoid side chains have been compared with homologues lacking the side chain, for their ability to carry electrons and protons from dithionite to ferricyanide, trapped in liposomes. Six differential observations were made: (1) Plastoquinone and ubiquinones, with a side chain of more than two isoprene units, are by far better mediators than their short-chain homologues. Also other benzoquinones lacking a long side chain are poor catalysts, except dimethyl-methylenedioxy-p-benzoquinone, a highly autooxidizable compound. Tocopherol is a good catalyst. (2) Vitamin K-1 and K-2 are poor mediators compared to vitamin K-3. (3) The reaction catalyzed by quinones carrying long isoprenoid side chains has an about three-fold higher activation energy, irrespective of the catalytic efficiency. (4) The reaction catalyzed by quinones lacking a long side chain follows pseudo first-order kinetics, while the reaction with quinones carrying a long side chain is of apparently higher order. (5) The rate with ubiquinone-1 is increasing pH, while with ubiquinone-9 it is decreasing. (6) The reaction mediated by short-chain quinones seems to be satuarated at lower dithionite concentration. We conclude that isoprenoid quinones are able to translocate electrons and protons in lipid membranes, and that the side chain has a strong impact on the mechanism. This and the relevance of the model reaction for electron and proton transport in photosynthesis and respiration is discussed.