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 allow it to travel to the nucleus.
There, SREBP binds to 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 twoseparate
genes that encode three different SREBP proteins. By contrast, Drosophila
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.