RESEARCH HIGHLIGHTS Nature Reviews Cancer | AOP, published online 14 November 2013; doi:10.1038/nrc3632 SIGNALLING that FLCN−FNIP activity is required for mTORC1 activation. Moreover, the authors found that the interac- Finding the GAPs in mTORC1 signalling tion between FLCN−FNIP and the RAGs in cell culture was strength- ened in the absence of amino acids. This suggests that FLCN−FNIP is part of the mTORC1 nutrient- sensing machinery. Furthermore, when FLCN is knocked down by NPG short hairpin RNAs, mTORC1 fails to properly localize and does not Tsun and colleagues have identi- Tsun and colleagues first re- become active in response to amino fied folliculin (FLCN), a tumour examined the nucleotide (GTP- or acid stimulation, which suggests suppressor that is implicated in GDP-bound) status of each of the that FLCN−FNIP is required for Birt−Hogg−Dubé hereditary cancer RAGs. GTP-bound RAGA/B was nutrient-sensitive activation of syndrome, as a GTPase-activating previously reported to be the major mTORC1. protein (GAP) that regulates the determinant of mTORC1 binding, This study raises several interest- nucleotide status of the RAG pro- but this was inferred by examining ing questions, which include why teins. The RAG proteins are crucial RAGA/B-mutant proteins, which FLCN (a tumour suppressor that is for the nutrient-sensing response preferentially had an affinity for lost in several tumour types) activates and activation of mTOR complex 1 either GTP or GDP. Tsun and the growth-promoting mTORC1 (mTORC1) kinase. colleagues sought to conclusively kinase. The authors speculate that The RAG GTPases directly ascertain whether RAGA/B is the in tumours with FLCN loss other interact with mTORC1 to form a major determinant by individually growth-promoting pathways are acti- heterodimer that consists of RAGA or examining the nucleotide state of vated, and they observe that there are RAGB (RAGA/B) bound to RAGC each RAG, including RAGC and reports of pathways such as MAPK or RAGD (RAGC/D); the interaction RAGD. Surprisingly, they found that being hyperactivated in FLCN- of this heterodimer with mTORC1 the nucleotide state of RAGC was the deleted tumours. Alternatively, there is required for mTORC1 activation. most important; specifically, it had to may be other, undescribed proteins In addition, the nucleotide status be GDP-bound for mTORC1 binding that compensate for FLCN loss. (that is, whether the RAGs are in a and activation. Following this, the This study provides further insight GTP-bound or a GDP-bound form) authors attempted to identify regula- into the molecular components of is also important in mTORC1 activa- tors of the nucleotide state of RAGC. mTORC1 signalling, in particular tion. Thus, the GAPs, which promote They applied an immunoprecipita- those relevant to amino acid sens- conversion of GTP to GDP, and FLCN−FNIP tion−mass-spectrometry approach ing, and it describes one of the first guanine nucleotide exchange factors activity is using epitope-tagged RAG proteins, molecular roles for FLCN. (GEFs), which promote conversion of required for which consistently identified FLCN Isabel Lokody GDP to GTP, are crucial to mTORC1 as a RAG interactor. They further mTORC1 ORIGINAL RESEARCH PAPER Tsun, Z.-Y. et al. regulation. Although RAGA/B have showed that FLCN, and its binding The folliculin tumor suppressor is a GAP for the been reasonably well studied, the role activation. partner folliculin-interacting protein RagC/D GTPases that signal amino acid levels to of RAGC/D in mTORC1 signalling (FNIP), is a GAP that is specific for mTORC1. Mol. Cell http://dx.doi.org/10.1016/j. molcel.2013.09.016 (2013) has been less clear. RAGC/D and not for RAGA/B, and NATURE REVIEWS | CANCER VOLUME 13 | DECEMBER 2013 © 2013 Macmillan Publishers Limited. All rights reserved.