Characterization of Drosophila Scap: Analysis of Mutants and Evidence for a Retention Factor

The SREBP pathway is one of the major regulators of lipid homeostasis and it is highly conserved among metazoans. SREBP is a transcription factor whose precursor is an endoplasmic reticulum (ER) transmembrane protein. In order to be activated it must travel to the Golgi apparatus via interaction with an escort protein, Scap. Scap, in turn can interact with components of the coatamer protein complex II (COPII) when lipid levels fall. In the Golgi, SREBP is cleaved sequentially by two proteases, S1P and S2P. By contrast to mammalian cells, which cannot survive without S2P or Scap, flies lacking Scap or S2P can activate SREBP. These mutants survive owing to non-canonical mechanisms of SREBP activation. Scap has a intrinsic tendency to travel to Golgi. In vertebrates, the ER retention factor, Insig, anchors the Scap:SREBP complex to the ER membrane when de novo lipid synthesis is not required. In Drosophila dSREBP pathway there is no Insig orthologues. However, our data suggest that there should be an analogous component that retains dScap in the ER. In order to discover the putative retention factor and other modifiers of the dSREBP, I set up a high through-put genome-wide screen. Employing luciferase as reporter, knocking-down each gene in genome through RNA interference will reveal the genes that modulate the activity of dSREBP.