UASt regulatory sequences drive expression of a sensor that is composed of Syt1 fused at its N-terminal end to a mCherry-pHluorinSE fusion. The presence of Syt1 targets the protein to the synaptic vesicle membrane (the dual fluorophore faces the vesicle lumen). The mCherry-pHluorinSE dual fluorophore can act as an acidification sensor in vivo as pHluorinSE is pH sensitive but mCherry is pH-resistant; red fluorescence indicates an acidic environment (pH < 6), while yellow fluorescence indicates a pH greater or equal to 7. In addition, the C-terminal pHluorinSE moiety is expected to be more exposed to proteases than mCherry, such that the dual fluorophore can also be used as a degradation sensor by observing fluorescence in fixed samples; yellow fluorescence indicates both fluorophores are intact, red fluorescence indicates degradation of the C-terminal pHluorinSE moiety.