Cell growth and proliferation are tightly linked to nutrient availability. The 
mechanistic target of rapamycin complex 1 (mTORC1) integrates the presence of 
growth factors, energy levels, glucose and amino acids to modulate metabolic 
status and cellular responses. mTORC1 is activated at the surface of lysosomes 
by the RAG GTPases and the Ragulator complex through a not fully understood 
mechanism monitoring amino acid availability in the lysosomal lumen and 
involving the vacuolar H(+)-ATPase. Here we describe the uncharacterized human 
member 9 of the solute carrier family 38 (SLC38A9) as a lysosomal 
membrane-resident protein competent in amino acid transport. Extensive 
functional proteomic analysis established SLC38A9 as an integral part of the 
Ragulator-RAG GTPases machinery. Gain of SLC38A9 function rendered cells 
resistant to amino acid withdrawal, whereas loss of SLC38A9 expression impaired 
amino-acid-induced mTORC1 activation. Thus SLC38A9 is a physical and functional 
component of the amino acid sensing machinery that controls the activation of 
mTOR.