UAS regulatory sequences drive expression of 'OptoSos', a two component system that can be used to optogenetically control Erk signaling in vivo. The two components are expressed as a single mRNA which gives rise to two distinct proteins due to the presence of a P2A peptide between the two coding sequence. The second open reading frame encodes a light-switchable membrane anchor (Tag:iLID fused to Tag:CAAX(hKRAS), represented in FlyBase by Ecol\ssrA^{iLID.UAS.Tag:CAAX(hKRAS)}) and the first open reading frame encodes a fluorescent Ras activator (the catalytic segment of Hsap\SOS2, tagged at the N-terminal end with TagRFP-T and Tag:SSPB, represented in FlyBase by Hsap\SOS2^{cat.UAS.TagRFP-T,Tag:SSPB}). Upon blue-light illumination, the Tag:iLID and Tag:SSPB sequences in the two components can bind each other, resulting in the fluorescent Ras activator being relocalized to the membrane.