Raptor recruits mTOR to non-phosphorylated 4E-BP1 bound to eIF4E and positively modulates phosphorylation of 4E-BP1 by mTOR. 4E-BP1 is further phosphorylated on multiple sites by other unknown kinases, also contributing to the dissociation of 4E-BP1 from eIF4E. Thus mTORC1 relieves the inhibitory effect of 4E-BP1 on eIF4E dependent translation initiation (Inoki et al. 2005, Gingras et al. 1999, 2001).
protein serine/threonine kinase activity of Active mTORC1 complex [lysosomal membrane]