Nutritional control of protein synthesis. Studies relating to tryptophan-induced stimulation of nucleocytoplasmic translocation of mRNA in rat liver.

Tryptophan has been demonstrated earlier to induce in the livers of rats and mice a rapid stimulation of polyribosomal aggregation and protein synthesis which has been attributed in part to stimulation of translocation of nuclear mRNA into the cytoplasm. This study was concerned with how tryptophan acts to affect the nuclei, particularly the nuclear membranes, in enhancing the nucleocytoplasmic translocation of mRNA of liver cells of rats fasted overnight. The results reveal that tryptophan rapidly becomes incorporated into proteins and also binds to proteins of hepatic cells, particularly proteins of the nuclear envelopes. In vitro binding of 3H-tryptophan to proteins (trichloroacetic-acid-precipitable) of nuclei and cytosols (when incubated together or separately) of livers of tryptophan-treated (10 minutes) rats is increased in comparison with binding to components of control rats. These findings correlate with the enhanced in vitro release of nuclear RNA and the increased activities of nuclear NTPase and protein phosphokinase of the livers of the experimental rats. Preincubation of hepatic nuclei with concanavalin A prevented the increases in in vitro binding of 3H-tryptophan to nuclear proteins, in prelabeled nuclear RNA release, and in nuclear NTPase activity of livers of the tryptophan-treated rats. The results suggest that tryptophan rapidly binds with hepatic proteins (possibly glycoproteins) associated with the nuclear membrane, leading to an increase in the activities of enzymes involved in phosphorylation and dephosphorylation and in release of nuclear mRNA into the cytoplasm.