Cell Biology

Coupling Lipid and Amino Acid Metabolism

Science Signaling  28 Feb 2012:
Vol. 5, Issue 213, pp. ec65
DOI: 10.1126/scisignal.2003000

Cells need to coordinate the lipid synthesis and trafficking systems with amino acid metabolism and protein translation in order to grow under the appropriate conditions. In yeast, two lipid-binding proteins regulate trafficking through the trans-Golgi network (TGN) and endosomal (TGN/endo) compartments. Sec14 [a phosphatidylinositol (PI) and phosphatidylcholine transfer protein] promotes traffic, and Kes1 (a sterol- and phosphatidylinositol 4-phosphate (PI4P)–binding protein] inhibits traffic. Mousley et al. found that Kes1 inhibited TGN/endo trafficking and altered sphingolipid metabolism to inhibit two master regulators of amino acid and protein metabolism, the kinase complex TORC1 and the transcriptional regulator Gcn4. Overexpression of Kes1 or mutants deficient in sterol binding caused growth arrest, exhibited enhanced association with the TGN/endo compartments, blocked access of other proteins to PI4P in the TGN/endo, impaired trafficking of proteins to the vacuole and plasma membrane, and impaired endocytosis. These effects were not seen with a Kes1 mutant deficient in PI4P binding, which failed to interact with the TGN/endo compartment. Furthermore, overexpression of Kes1 or a sterol-binding mutant caused autophagy, and blocking autophagy converted the growth arrest phenotype into a loss of viability. Nuclear magnetic resonance (NMR) analysis indicated that the Kes1-arrested cells had reduced pools of more than half of the amino acids detected in the NMR analysis, and amino acid supplementation of the Kes1-overexpressing cells restored growth without resolving the trafficking defects, suggesting that growth arrest was due to amino acid deficiencies. Overexpression of Kes1 or the sterol-binding mutant inhibited TORC1 activity, monitored as a reduction in the phosphorylation of the autophagy protein and TORC1 target Atg13, and increased phosphorylation of the translational inhibitor eIF2α. Normally, phosphorylated eIF2α inhibits overall protein translation but promotes translation of Gcn4 in a response called the general amino acid control (GAAC) pathway. Although phosphorylation of eIF2α was increased in the Kes1-overexpressing cells, the GAAC pathway leading to production of Gcn4 and transcriptional activation of Gcn4-dependent genes was not activated. Amino acid supplementation restored activation of the GAAC pathway. Kes1-arrested cells had increased amounts of ceramide and sphingolipids. Amino acid stress combined with exogenous addition of sphingolipid, which does not inhibit TGN/endo trafficking, caused growth arrest and reduced activation of the GAAC pathway, suggesting that Kes1 may act through a sphingolipid signaling mechanism to affect amino acid metabolism. The inhibition of the transcriptional activity of Gcn4 by either Kes1 overexpression or excess sphingolipids was due to a block after the formation of the preinitiation complex and required the cyclin-dependent kinase Cdk8. Thus, the authors propose that Kes1 integrates lipid metabolism with amino acid metabolism to establish growth or quiescence appropriate to the conditions.

C. J. Mousley, P. Yuan, N. A. Gaur, K. D. Trettin, A. H. Nile, S. J. Deminoff, B. J. Dewar, M. Wolpert, J. M. Macdonald, P. K. Herman, A. G. Hinnebusch, V. A. Bankaitis, A sterol-binding protein integrates endosomal lipid metabolism with TOR signaling and nitrogen sensing. Cell 148, 702–715 (2012). [PubMed]