Our laboratory uses the fruit fly, Drosophila melanogaster, as a model system to understand the mechanisms by which animal cells sense the requirements for (and regulate the production of) fatty acids. In mammals, a complicated assemblage of protein machinery, known as the SREBP pathway, is central to these processes.
The SREBPs (sterol regulatory element binding proteins) are membrane-bound transcription factors. When lipid supplies are sufficient, they remain in the membranes of the endoplasmic reticulum - when cellular demand for lipid rises, another protein, SCAP, escorts SREBP to the Golgi apparatus where two different proteases are required to release the transcription factor from the membrane and enable it to travel to the nucleus. There, SREBP binds to DNA sequences in the upstream region of target genes and mediates their increased transcription.
The best studied role of the SREBP pathway in mammals is the regulation of cholesterol uptake and production. The SREBPs are also involved in aspects of fatty acid metabolism as well. This role in fatty acid metabolism is easier to study in flies than in mammals because flies cannot make cholesterol from scratch and must get it as part of their diet. Vertebrates are capable of making their own cholesterol as well as fatty acids and have two separate genes that encode three different SREBP proteins.
By contrast, Drosophila has only a single SREBP gene. In addition, the considerable array of genetic and molecular tools available for Drosophila make it a productive experimental system in which to study fatty acid metabolism.
In earlier work, we demonstrated that the SREBP pathway in Drosophila cells responded to palmitate rather than sterols. We went on to demonstrate that phospholipids, rather than palmitate itself, are the principle regulatory lipids in flies. Our extensive characterization of the SREBP pathway in Drosophila cells enabled us to use that system to show that mammalian Insig proteins were both necessary and sufficient to confer sterol regulation on the mammalian SREBP pathway.
Continuing to use mammalian cell genetics, we and colleagues described two roles for cholesterol in mammalian cells: 1) a bulk requirement that could be satisfied by several different sterols, and which was not enantioselective, and 2) a specific requirement for very smal1 amounts of cholesterol itself.
We isolated mutant flies lacking the gene for SREBP and discovered that these animals die at the 2nd-to-3rd larval instar transition. We overcame this lethality by supplementing the larval diet with extra lipid. Free fatty acids were far more effective at promoting survival than were more complex lipids (e.g. triglycerides and phospholipids.) Our ability to recover adult flies lacking all SREBP function allowed us to show, in collaboration with Sara Cherry of Norbert Perrimon’s laboratory, that SREBP is essential for the replication of certain kinds of polio-like viruses.
Current projects in the lab include characterization of mutant flies lacking SCAP or Site-2 protease function. We are also conducting genome-wide screens for modifiers of SREBP activity in Drosophila and characterizing genes that are targets of SREBP transcriptional up-regulation.
RESEARCH INTERESTS
Genetics of the control of lipid metabolism in insects
Cherry, S., Kunte, A., Wang, H., Coyne, C., Rawson, R. B., and Perrimon, N., "COPI activity coupled with fatty acid biosynthesis is required for viral replication." PLOS Pathogens, 2(10):0900-0912, September 2006
Kunte, A. S., Matthews, K. M., and Rawson, R. B., "Fatty acid auxotrophy in Drosophila larvae lacking SREBP" Cell Metabolism, 3:439-448, June 2006
Xu F., Rychnovsky, S.D., Belani, J.D., Hobbs H.H., Cohen J.C., Rawson R.B., "Dual roles for cholesterol in mammalian cells" PNAS (USA), 102:14551-14556, October 2005
SIGNIFICANT PUBLICATIONS
Kunte, A. S., Matthews, K. M., and Rawson, R. B., "Fatty acid auxotrophy in Drosophila larvae lacking SREBP" Cell Metabolism, 3:439-448, June 2006
Dobrosotskaya, I. Y., Seegmiller, A. C., Brown, M. S., Goldstein, J. L., and Rawson, R. B., "Regulation of SREBP Processing and Membrane Lipid Production by Phospholipids in Drosophila." Science, 269:879-883, 2002
Seegmiller, A. C., Dobrosotskaya, I., Goldstein, J. L., Ho, Y. K., Brown, M. S., Rawson, R.B., "The SREBP Pathway in Drosophila: Regulation by Palmitate, Not Sterols." Developmental Cell, 2:229-238, 2002
Ye, J., Rawson, R. B., Komuro, R., Chen, X., Dave, U. P., Prywes, R., Brown, M.S., and Goldstein, J. L., "ER Stress Induces Cleavage of Membrane-Bound ATF6 by the Same Proteases that Process SREBPs." Molecular Cell, 6:1355-1364, 2000
Rawson, R. B., Zelenski, N. G., Nijhawan, D., Ye, J., Sakai, J., Hasan, M. T., Chang, T. Y., Brown, M. S., and Goldstein, J. L., "Complementation Cloning of S2P, a Gene Encoding a Putative Metalloprotease Required for Intramembrane Cleavage of SREBPs." Molecular Cell, 1:47-57, 1997
Kunte, A. S., Matthews, K. M., and Rawson, R. B., "Fatty acid auxotrophy in Drosophila larvae lacking SREBP" Cell Metabolism, 3:439-448
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