Acute blockage of the ubiquitin-mediated proteolytic pathway during invertebrate quiescence

Many organisms withstand adverse environmental conditions by entering a reversible state of quiescence that may last for months or years. In this report we provide evidence that the reduction in adenylate energy status and the associated intracellular acidosis occurring during anoxia-induced quiescence combine to inhibit, directly or indirectly, the initial step in the ubiquitin-mediated proteolytic pathway in embryos of the brine shrimp Artemia franciscana. The levels of ubiquitin-conjugated proteins drop to 37% of control (aerobic) values during the first hour of anoxia and reach 7% in 24 h. ATP falls to 5% of control values under anoxia, and AMP rises reciprocally. This energy limitation is accompanied by a simultaneous depression of intracellular pH (pHi). By comparison, when embryos are subjected to artificial acidosis under aerobic conditions (pHi drops sharply, but ATP does not change for hours), ubiquitin-conjugated proteins decline to 58% after 1 h. Thus, while the proximate mechanism for the suppression of ubiquitination has not been proven, alterations in the adenylate pool and the decrease in pHi both appear to contribute to the suppression of ubiquitination. Western blot analysis indicates that the decline in ubiquitin-conjugated protein is rapidly reversed on return of embryos to control conditions. We conclude that this arrest of ubiquitination likely serves to suppress ubiquitin-mediated degradation of protein, thereby preserving macromolecular integrity and potentially explaining the remarkable extension of protein half-life observed under anoxia in these embryos.