Global arrest of translation during invertebrate quiescence

Comparing the translational capacities of cell-free systems from aerobically developing embryos of the brine shrimp Artemia franciscana vs. quiescent embryos has revealed a global arrest of protein synthesis. Incorporation rates of [3H]leucine by lysates from 4-h anoxic embryos were 8% of those from aerobic (control) embryos, when assayed at the respective pH values measured for each treatment in vivo. Exposure of embryos to 4 h of aerobic acidosis (elevated CO2 in the presence of oxygen) suppressed protein synthesis to 3% of control values. These latter two experimental treatments promote developmental arrest of Artemia embryos and, concomitantly, cause acute declines in intracellular pH. When lysates from each treatment were assayed over a range of physiologically relevant pH values (pH 6.4-8.0), amino acid incorporation rates in lysates from quiescent embryos were consistently lower than values for the aerobic controls. Acute reversal of pH to alkaline values during the 6-min assays was not sufficient to return the incorporation rates of quiescent lysates to control values. Thus, a stable alteration in translational capacity of quiescent lysates is indicated. Addition of exogenous mRNA did not rescue the suppressed protein synthesis in quiescent lysates, which suggests that the acute blockage of amino acid incorporation is apparently not due to limitation in message. Thus, the results support a role for intracellular pH as an initial signaling event in translational control during quiescence yet, at the same time, indicate that a direct proton effect on the translational machinery is not the sole proximal agent for biosynthetic arrest in this primitive crustacean.