The fundamental cellular roles of mechanistic (formerly mammalian) Target of Rapamycin Complex 1 (mTORC1) activity are evident from the wide range of major human diseases in which it is misregulated, including the majority of human cancers, neurodegenerative disorders, cardiovascular disease, diabetes, obesity and other age-related disorders. mTORC1 is a key signalling hub that controls cellular homeostasis by responding to a diverse range of physiological inputs, like endocrine insulin/insulin-like growth factors (IGFs), energy levels (ATP:AMP ratio), oxygen levels, and local amino acid (AAs) to modulate cellular metabolism and growth (reviewed in Malik et al., 2012 [epub]). There are, however, still significant gaps in our understanding of local AA regulation of mTORC1, which takes place at the surface of late endosomes and lysosomes (LELs; reviewed in Durán and Hall 2012). Intracellular AAs have been highlighted as playing a key role in activating mTORC1, via two very different, but not necessarily mutually exclusive mechanisms. The first involves sensing of cytosolic leucine and the second the sensing of AA within late endosomes and lysososomes (LELs). Charged Leucyl tRNA Synthetase (LRS) seems to act as a direct sensor of cytosolic leucine levels, leading to interaction with the Rag GTPases on LELs and subsequent activation of mTORC1. However, the proposed mechanism differs significantly between human cells and yeast, and a key residue involved in the human mechanism is not conserved in fly LRS (reviewed in Durán and Hall 2012). Unbiased screening in flies and subsequent analysis in human cells led us to identify the Proton-assisted Amino acid Transporter (PAT/SLC36) family located at the surface of LELs, as central players in AA-dependent mTORC1 activation (Heublein et al., 2010; Ögmundsdóttir et al., 2012). Our studies have focused on the two ubiquitous human PAT transporters, PAT1 and PAT4. While modest overexpression of PAT1 leads to mTORC1 activation and increased cell growth and proliferation (Ögmundsdóttir et al., 2012, higher levels reduce tissue growth and mTORC1 signalling (Goberdhan et al., 2005; Zoncu et al., 2011), most likely due to a dominant negative effect. The discovery that LEL-located PAT1 interacts with Rags and is required to activate mTORC1 and promotes cell proliferation, together with a substantial amount of work from the Sabatini lab, has highlighted a multi-protein complex that we have called the ‘nutrisome’, located on the LEL surface. The nutrisome appears to respond to the intraluminal pool of AA within LELs, reflects LEL extracellular AA levels (for details see Figure 1; reviewed in Malik et al., 2012 [epub]). Aggressive tumour cells are able to survive and grow even when poorly vascularised with limited access to growth factors and nutrients. These microenviromental stresses lead to the induction of autophagy, resulting in the build-up of AA within autolysosomes. The PAT1-containing nutrisome (Figure 1) is well placed to utilise these luminal nutrients to maintain a residual level of mTORC1 activity and maintain homeostasis within the cell until conditions improve (reviewed in Goberdhan, 2010). We have previously shown in vivo in flies that when PI3K signalling is increased, as is the case in the majority of human cancers, the growth-promoting properties of the PATs are enhanced. PATs, which are normally distributed between intracellular membranes and the cell surface, become more intracellular in these conditions, a trafficking process that is required for increased growth (Ögmundsdóttir et al., 2012). I will present data showing that PI3K is also involved in PAT regulation in colon cancer cell lines, particularly under stress conditions. I will also discuss the analysis we have performed using IPTG-inducible shRNA constructs targeting PAT4 in HCT116 tumours using in vitro and xenograft models. Combined with expression studies in human tumour samples, this work suggests a role for PAT4 in tumour progression.