The mTORC1 protein kinase complex integrates nutrient and growth stimuli to modulate signalling pathways that regulate cellular metabolism and physiology, but the molecular nature of the amino acid sensing mechanism at the lysosome is unknown; here, an orphan member of the human solute carrier group of proteins, SLC38A9, is shown to be an integral component of the lysosomal machinery that can directly sense amino acids and activate mTORC1. The mTORC1 protein kinase complex integrates nutrients and growth stimuli to modulate signalling pathways that regulate cellular metabolism and physiology. Although amino acids are essential for mTORC1 activity, the molecular nature of the amino-acid-sensing mechanism at the lysosome is unknown. Here, Giulio Superti-Furga and colleagues discover a member of the human solute carrier group of proteins, SLC38A9, to be an integral component of the lysosomal machinery that can directly sense amino acids and activate mTORC1. 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 responses1,2,3. 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+-ATPase4,5,6,7,8. 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.

Infection with influenza virus can result in bacterial superinfection, but the mechanisms underlying this process are unclear. Bergthaler and colleagues demonstrate that influenza virus upregulates the methyltransferase Setdb2, which attenuates select proinflammatory gene expression and heightens susceptibility to bacterial infection. Immune responses are tightly regulated to ensure efficient pathogen clearance while avoiding tissue damage. Here we report that Setdb2 was the only protein lysine methyltransferase induced during infection with influenza virus. Setdb2 expression depended on signaling via type I interferons, and Setdb2 repressed expression of the gene encoding the neutrophil attractant CXCL1 and other genes that are targets of the transcription factor NF-κB. This coincided with occupancy by Setdb2 at the Cxcl1 promoter, which in the absence of Setdb2 displayed diminished trimethylation of histone H3 Lys9 (H3K9me3). Mice with a hypomorphic gene-trap construct of Setdb2 exhibited increased infiltration of neutrophils during sterile lung inflammation and were less sensitive to bacterial superinfection after infection with influenza virus. This suggested that a Setdb2-mediated regulatory crosstalk between the type I interferons and NF-κB pathways represents an important mechanism for virus-induced susceptibility to bacterial superinfection.

Significance: Without a vaccine or cost-effective antivirals against hepatitis C virus (HCV) there is a need to understand better the molecular mechanisms underlying the establishment of productive HCV infection and chronic liver disease. Recently, the Crohn's disease and tuberculosis risk factor immunity-related GTPase M (IRGM) was found to promote HCV replication, but the mechanisms involved are unknown. Here we provide mechanistic insight into how IRGM stimulates the two membrane-remodeling pathways, Golgi fragmentation and autophagy, to facilitate HCV replication. Our findings offer insights into the replication strategies used by HCV that should be useful for antiviral approaches. Also, these findings might aid understanding how IRGM functions in infection and inflammation in the setting of other diseases, e.g., in Crohn’s disease.

Lipid metabolism and receptor-mediated signaling are highly intertwined processes that cooperate to fulfill cellular functions and safeguard cellular homeostasis. Activation of Toll-like receptors (TLRs) leads to a complex cellular response, orchestrating a diverse range of inflammatory events that need to be tightly controlled. Here, we identified the GPI-anchored Sphingomyelin Phosphodiesterase, Acid-Like 3B (SMPDL3B) in a mass spectrometry screening campaign for membrane proteins co-purifying with TLRs. Deficiency of Smpdl3b in macrophages enhanced responsiveness to TLR stimulation and profoundly changed the cellular lipid composition and membrane fluidity. Increased cellular responses could be reverted by re-introducing affected ceramides, functionally linking membrane lipid composition and innate immune signaling. Finally, Smpdl3b-deficient mice displayed an intensified inflammatory response in TLR-dependent peritonitis models, establishing its negative regulatory role in vivo. Taken together, our results identify the membrane-modulating enzyme SMPDL3B as a negative regulator of TLR signaling that functions at the interface of membrane biology and innate immunity.