Many of the key physiological processes that take place within cells are regulated by nutrient and energy supplies, which are, for example, a prerequisite for cell growth and division. The major intracellular read-out of the nutrient environment is the highly conserved phosphoinositide 3-kinase (PI3K)/Akt/target of rapamycin complex 1 (TORC1) signalling network, which has a central role in regulating cellular metabolism and is now known to be misregulated in a wide range of major human diseases, including cancer, diabetes, neurodegenerative and other age-related disorders (Zoncu et al., 2011; Goberdhan, 2010). Both endocrine insulin/insulin-like growth factors and local amino acids are known to regulate this cellular pathway. However, there are still significant gaps in our understanding of local amino acid regulation of TORC1, despite its central importance in disease. We have investigated how local amino acids are sensed and can modulate TORC1 signalling by screening a wide spectrum of amino acid transporters under physiologically relevant conditions, using the fly as an in vivo model (Goberdhan et al., 2005). This study highlighted the Proton-assisted Amino acid Transporter (PAT/SLC36) family as having a particularly potent effect on TORC1-mediated signalling and cellular growth. Furthermore, our work suggested that PATs may act as so-called transceptors; that is molecules that look like transporters, but which actually signal independently of transport to downstream targets upon binding to substrate. There are several known examples of transceptors in yeast, but only the PATs and the related SNAT amino acid transporters (Hyde et al., 2007) have been proposed to function via this mechanism in higher eukaryotic organisms to date. We have now shown using both siRNA knockdown and overexpression approaches that the growth-promoting properties of the PATs are conserved in human cells (Heublein et al., 2010). siRNA knockdown in MCF-7 and HEK-293 cells results in typically >60% reduction in RNA levels and >60% decrease in cell proliferation, as well as >50% reduction in levels of activated downstream targets of TORC1 signalling. Intriguingly, these studies also highlighted an intracellular pool of PATs as being responsible for TORC1-mediated growth stimulation. Indeed PATs are located on late endosomes and lysosomes (LELs), the same location to which mTOR is recruited on amino acid stimulation and which requires the Ragulator-Rag GTPase complex (Sancak et al., 2008; Sanack et al., 2010). More detailed biochemical analysis suggests that PATs are part of a protein complex, including the Rag GTPases, that we have called the ‘nutrisome’, which acts as an intracellular amino acid sensor that interacts with TORC1 (see Figure 1). PATs would be predicted to export amino acids from LELs, but may also be able to detect amino acids on their cytosolic face, through which substrates would exit. This mechanism has been suggested for other amino acid transporters and transceptors (reviewed in Goberdhan, 2010), and it may help to explain how cytosolic amino acids like leucine, which are thought to be key regulators of TORC1, could interact with the sensing mechanism. More recently, we have also started to assess the role of PATs in the cell’s response to starvation, and recovery from amino acid depletion. Furthermore, our in vivo data highlight the differential localisation of PATs at the cell surface and LELs in cells with different growth rates, and suggests that the subcellular distribution of PATs might regulate TORC1-dependent growth. We conclude that PATs regulate a critical intracellular mechanism involved in amino acid sensing. Although protein structures of PATs suggest that they are transporters, it is unclear if transport is critical to key effects of the PATs on TORC1-mediated growth. Our data suggest that the regulation of PAT localisation is an important factor in the ability of this molecule to impact on TORC1-mediated growth and accumulation of intracellular PATs may help to isolate cells from detrimental changes in the extracellular nutrient environment. We propose that a PAT-dependent mechanism may be particularly important for the growth and proliferation of cancer cells, which are more able to survive and grow under such conditions than normal cells, which may be more dependent on cell surface transporters for growth regulation.