RAPAMYCIN BLOCKS THE PHOSPHORYLATION OF 4E-BP1 AND INHIBITS CAP-DEPENDENT INITIATION OF TRANSLATION

The immunosuppressant drug rapamycin blocks progression of the cell cy cle at the G(1) phase in mammalian cells and yeast. Here we show that rapamycin inhibits cap-dependent, but not cap-independent, translation in NIH 3T3 cells. Cap-dependent translation is also specifically redu ced in extracts from rapamycin-treated cells, as determined by in vitr o translation experiments. This inhibition is causally related to the dephosphorylation and consequent activation of 4E-BP1, a protein recen tly identified as a repressor of the cap-binding protein, eIF-4E, func tion. These effects of rapamycin are specific as FK506, a structural a nalogue of rapamycin, had no effect on either cap-dependent translatio n or 4E-BP1 phosphorylation. The rapamycin-FK506 binding protein compl ex is the effector of the inhibition of 4E-BP1 phosphorylation as exce ss of FK506 over rapamycin reversed the rapamycin-mediated inhibition of 4E-BP1 phosphorylation. Thus, inactivation of eIF-4E is, at least i n part, responsible for inhibition of cap-dependent translation in rap amycin-treated cells. Furthermore, these results suggest that 4E-BP1 p hosphorylation is mediated by the FRAP/TOR signalling pathway.