RalGAPs Regulate mTORC1 Signaling

The mTOR signaling pathway has emerged as an important regulator of cell growth and metabolism, and its deregulation has been linked to aging and cancer. The tuberous sclerosis complex (TSC), composed of TSC1 and TSC2, negatively regulates mTOR complex 1 (mTORC1) by acting as a GTPase activating protein (GAP) for the Rheb small GTPase, but additional signaling mechanisms may exist to modulate mTORC1. Martin and colleagues found that loss of RalGAP in Caenorhabditis. elegans, which lacks orthologs of TSC1/2, phenocopied CeTOR activation and reduced lifespan, suggesting that RalGAPs substitute for TSC1/2. Moreover, the phenotype of RalGAP mutants could be rescued by knockdown of Ral, but not Rheb, indicating that RalGAP acts through Ral to regulate aging in C. elegans. Consistent with these findings, mouse fibroblasts lacking the essential regulatory subunit RalGAPβ also exhibited increased mTORC1 activity that was dependent on RalB, but not Rheb. Furthermore, RalGAP overexpression or short hairpin RNA (shRNA) knockdown of RalB in TSC-null fibroblasts reduced mTORC1 activity. Loss of RalGAPβ correlated with a rapamycin-sensitive reduction in autophagy, indicating that RalGAPs act upstream of mTORC1 to regulate autophagy. In response to serum stimulation, active GTP-bound RalB interacted and colocalized with mTORC1 at the plasma membrane and was required for S6K phosphorylation. mTORC1 associated and colocalized with the known Ral effector SEC5, a member of the exocyst complex involved in vesicle sorting, and shRNA knockdown of SEC5 impaired mTORC1 activity and RalB association, implicating SEC5 as the key effector downstream of RalB involved in mTORC1 regulation. Intriguingly, shRNA knockdown of RalGAPβ caused a rapamycin-sensitive induction of pancreatic tumor cell invasion, emphasizing an mTORC1-dependent role for RalGAP–RalB signaling in promoting tumor invasion. These results identify cross-talk between Ral and mTOR signaling and suggest that RalB may be a potential therapeutic target for diseases linked to TSC1/2 loss and aberrant mTORC1 activation.

Martin TD, Chen XW, Kaplan RE, Saltiel AR, Walker CL, Reiner DJ, et al. Ral and Rheb GTPase activating proteins integrate mTOR and GTPase signaling in aging, autophagy, and tumor cell invasion. Mol Cell 2013 Jan 2 [Epub ahead of print].