REGULATION OF EIF-4E BP1 PHOSPHORYLATION BY MTOR

The proteins eIF-4E BP1 and p70 S6 kinase each undergo an insulin/mito gen-stimulated phosphorylation in situ that is partially inhibited by rapamycin. Previous work has established that the protein known as mTO R/RAFT-1/FRAP is the target through which the rapamycin.FKBP12 complex acts to dephosphorylate/deactivate the p70 S6 kinase; thus, some mTOR mutants that have lost the ability to bind to the rapamycin.FKBP12 co mplex in vitro can protect the p70 S6 kinase against rapamycin-induced dephosphorylation/deactivation in situ, We show herein that such mTOR mutants also protect eIF-4E BP1 against rapamycin-induced dephosphory lation, and for both p70 S6 kinase and eIF-4E BP1, such protection req uires that the rapamycin-resistant mTOR variant retains an active cata lytic domain, In contrast, mutants of p70 S6 kinase rendered intrinsic ally resistant to inhibition by rapamycin in situ are not able to prot ect coexpressed eIF-4E BP1 from rapamycin-induced dephosphorylation. W e conclude that mTOR is an upstream regulator of eIF-4E BP1 as well as the p70 S6 kinase; moreover, these two mTOR targets are regulated in a parallel rather than sequential manner.