Significance The mTORC1 (mammalian target of rapamycin complex 1) pathway plays a critical role in driving cancer growth. We have identified a phosphorylation-dependent mechanism that controls mTORC1 activity in which Pim and AKT kinases, 2 enzymes with increased activity in cancer phosphorylate DEPDC5, a member of the GATOR1 complex that senses cellular amino acid levels. The critical nature of this substrate to the activity of these protein kinases is demonstrated by the fact that deletion or mutation of DEPDC5 partially blocks the ability of Pim and Pim plus AKT inhibitors to suppress tumor cell growth. Thus, protein kinases regulate the amino acid sensing cascade to control mTORC1 activity and tumor cell growth.