The rapamycin-sensitive mammalian target of rapamycin (mTOR) complex, mTORC1, regulates cell growth in response to mitogenic signals and amino acid availability. and in parallel to, the Rag pathway in regulating amino acid activation of mTORC1. Introduction The mammalian target of rapamycin (mTOR) is a Ser/Thr kinase critically involved in the regulation of many cellular and developmental processes including cell growth, differentiation, and metabolism. Two functionally distinct protein complexes containing mTOR have been characterized, namely mTORC1 and mTORC2, which mediate the rapamycin-sensitive and -insensitive signaling of mTOR, respectively (Sarbassov et al., 2005a). mTORC1 assembles a signaling network in the regulation of cell growth by mediating nutrient availability (amino acid sufficiency) and mitogenic signals. The two best-characterized 5725-89-3 manufacture immediate targets of mTORC1 are ribosomal S6 kinase 1 (S6K1) and eukaryotic initiation factorC4E-binding protein 1 (4E-BP1), both of which regulate protein synthesis at the translation initiation level (Hay and Sonenberg, 2004). The tumor suppressor tuberous sclerosis complex TSC1CTSC2 and the target of its GTPase-activating protein activity, Rheb, form a major hub that receives multiple upstream signals to activate mTORC1 (Manning and Rabbit polyclonal to PNO1 Cantley, 2003). The 5725-89-3 manufacture sensing and transduction of amino acid signals upstream of mTORC1 have been an issue of long-standing interest, as this mechanistically less well-understood aspect of mTOR regulation represents a fundamentally important signaling process and may be intimately linked to human diseases such as cancer and metabolic syndromes. To date, two major pathways have been reported to mediate amino acid signals to activate mTORC1, involving the class III phosphatidylinositol 3-kinase (PI-3-kinase) human vacuolar protein sorting 34 (hVps34) and the Rag family of small G proteins. hVps34 has been found to be activated by amino acids and required for mTORC1 activation in response to amino acid stimulation (Byfield et al., 2005; Nobukuni et al., 2005). In vivo validation of hVps34 as a key regulator of mTORC1 came from a recent study showing that hVps34-deficient embryos had drastically reduced levels of S6 phosphorylation and were defective in cell proliferation (Zhou et al., 2011). As upstream regulators, calcium and CaM have been shown to bind and activate hVps34 (Gulati et al., 2008), but others have questioned this mode of hVps34 regulation (Yan et al., 2009). Curiously, Vps34 does not regulate TOR signaling in (Juhasz et al., 2008), suggesting that the hVps34-mTOR regulatory branch may have evolved to accommodate the biological complexity in higher organisms. The Rag GTPase heterodimers, through the P18CP14CMP1 complex, recruit mTORC1 to the lysosomal surface upon amino acid stimulation, where Rheb presumably resides and mTORC1 activation occurs (Kim et al., 2008; Sancak et al., 2008, 2010). The Ste20 kinase MAP4K3 and its inhibitor PP2A/PR61- have also been reported to mediate amino acid signaling to mTORC1 in a Rag-dependent manner, although they may constitute a pathway parallel to Rag (Findlay et 5725-89-3 manufacture al., 2007; Yan et al., 2010). It is not known how the hVps34 and Rag pathways are connected or how hVps34 activates mTORC1. Mitogenic activation of mTORC1 also requires the lipid second messenger phosphatidic acid (PA), which binds to the FKBP12-rapamycinCbinding domain of mTOR (Fang et al., 2001; Foster, 2007; Sun and Chen, 2008). Phospholipase D (PLD), catalyzing the hydrolysis of phosphatidylcholine to PA, has been established as a key upstream component in the mitogenic mTORC1 pathway that regulates cell growth (Fang et 5725-89-3 manufacture al., 2003; Sun and Chen, 2008). Like hVps34, PLD does not regulate TOR in (Sun and Chen, 2008). Of the two mammalian isoforms of PLD, PLD2.