Abstract Neuronal systems must adapt to fast and slow changes in the environment. A classic example is the visual system that can adjust to changes of several orders of magnitude in light levels within just seconds. Adaptation on a much longer time scale has been observed as well upon the shift from a regular light:dark cycle to continuous illumination. In Drosophila, this shift triggers a reduced sensitivity to light and a down regulation of synaptic active zones in photoreceptors. We recently discovered that the unfolded protein response (UPR) plays a major role in regulating this adaptation. After just one night with continuous light exposure, both the Ire1 and the PERK arm of the UPR exhibit strong activation. Interference with the normal regulation of the UPR results in the loss of visual neurotransmission and severe structural deterioration of rhabdomeres, the microvillar arrays that house the key elements of the phototransduction cascade. This phenotype was observed for fic and BiP mutants that interfere with the regulation of the activity of BiP, a major regulator of the UPR. Screening for additional elements of this pathway, we identified an unconventional kinase, called Allnighter (Aln), as a candidate. Its sequence predicts this kinase to be acting in the secretory pathway. Preliminary data indicate that, similar to fic and BiP mutants, continued illumination causes aln mutants to lose visual neurotransmission and structural integrity of rhabdomeres. This proposal aims to characterize the role of aln in long-term visual adaption (i) by defining its subcellular and cell- type specific side of action, (ii) by testing its role in regulating the Ire1 or PERK arms of the UPR, and (iii) by identifying Aln substrates necessary for long-term visual adaption. Completion of these experiments will significantly enhance our understanding of the mechanism by which the UPR aids in the maintenance of visual acuity during long-term adaption.