RAPAMYCIN SELECTIVELY REPRESSES TRANSLATION OF THE POLYPYRIMIDINE TRACT MESSENGER-RNA FAMILY

The immunosuppressant rapamycin blocks p70(s6k)/p85(s6k) activation an d phosphorylation of 40S ribosomal protein S6 in Swiss 3T3 cells. The same net result is obtained when the macrolide is added 3 hr after ser um stimulation. In stimulated cells p70(s6k)/p85(s6k) inactivation is achieved within minutes, whereas S6 dephosphorylation requires 1-2 hr, supporting the concept that S6 dephosphorylation results from kinase inactivation. In parallel, rapamycin treatment causes a small, but sig nificant, reduction in the initiation rate of protein synthesis, as me asured both by [S-35]methionine incorporation into protein and by recr uitment of 80S ribosomes into polysomes. More striking, analysis of in dividual mRNA transcripts revealed that rapamycin selectively suppress es the translation of a family of mRNAs that is characterized by a pol ypyrimidine tract immediately after their N-7-methylguanosine cap, a m oth that can act as a translational modulator. This family includes tr anscripts for ribosomal proteins, elongation factors of protein synthe sis, and proteins of as-yet-unknown function. The results imply that ( i) 408 ribosomes containing phosphorytated S6 may selectively recogniz e this motif or proteins which bind to it and (ii) rapamycin may inhib it cell growth by blocking S6 phosphorylation and, thus, translation o f these mRNAs.