Defining the biochemical functions of the TSC complex and its regulation of Rheb-mTORC1 signaling

Abstract

Nutrients and growth factors are sensed by a ubiquitous signaling network that converges on the protein kinase complex mechanistic target of rapamycin (mTOR) complex 1 (mTORC1). mTORC1 functions to integrate a diverse set of growth cues to coordinate the important anabolic processes fundamental to cell growth and proliferation. Upstream of mTORC1, growth factor signaling pathways impinge on the tuberous sclerosis complex (TSC) complex, comprised of the tumor suppressors TSC1 and TSC2, and a third component, TBC1D7, to inhibit its ability to regulate the direct mTORC1 activator, Rheb. The pathways that impinge on the TSC complex are comprised of some of the most common oncogenes and tumor suppressors genetically altered in human cancers. Moreover, direct mutations on TSC1 and TSC2 lead to the genetic tumor syndrome tuberous sclerosis complex (TSC) and have also been identified in sporadic cancers. In all these settings, mTORC1 is aberrantly activated and results in uncontrolled cell growth. The TSC complex acts as a GTPase Activating Protein (GAP) to stimulate GTP hydrolysis on Rheb, converting it from its active to inactive form, and this regulatory function is mapped exclusively to a small c-terminal domain on TSC2. Although significant progress has been made towards understanding the canonical GAP domain of TSC2, many longstanding questions underlying the mechanistic function of the TSC complex and its regulation of Rheb remain unanswered. Through mechanistic studies, I have identified a novel binding interaction between the TSC complex and Rheb that occurs outside of the canonical GAP domain of TSC2. I find that the TSC complex preferentially engages Rheb in is inactive form and that this interaction occurs in a highly phosphorylated region of TSC2 containing the most essential growth factorstimulated phosphorylation sites. Finally, I show that this interaction appears to be important for complete mTORC1 suppression upon growth factor withdrawal from the cell. Collectively, these studies have revealed previously unrecognized regulatory functions of the TSC complex and provide updated molecular details towards our understanding of the rheb-mTORC1 signaling axis in both normal settings and disease.