Quasimodo mediates daily and acute light effects on Drosophila clock neuron excitability

Significance Most organisms developed a circadian clock to adapt their behavior to daily changes of light and temperature. The molecular clock is remarkably conserved across species with much of our current understanding coming from Drosophila studies. To generate circadian behavior, appropriate levels of neuronal electrical activity are crucial, but the regulators of this activity have remained largely elusive. Here we identify three membrane proteins that interact to set the clock neurons to “day” or “night,” forming a light-input pathway to the circadian clock. The membrane-anchored extracellular protein Quasimodo affects both the daily changes in physiological properties and light responses of brain clock neurons, possibly acting upstream of the potassium channel Shaw and the Na + , K + , Cl − ion transporter NKCC.