Phototransduction in the photoreceptor outer segment (OS), a sensory cilium, is among the best- understood neural signaling systems, but the molecular details and regulation of assembly of the OS remain unclear. During the past grant period we focused on intraflagellar transport (IFT), a conserved pathway mediated by two kinesin 2 family motors, kinesin II and KIF17. Photoreceptors coordinate renewal and turnover of photosensitive discs according to light-dark cycles, and diurnal changes in intracellular calcium regulate the activity of kinases like CaMKII. This proposal is based on our recent finding that that phosphorylation of KIF17 at a conserved serine in its tail domain facilitates its association with IFT machinery. Furthermore, mutation of this serine in the zebrafish sequence to an aspartate results in entry into the OS and increased OS turnover through disc shedding, and KIF17 abundance and phosphorylation state are regulated in a diurnal pattern. These findings lead to the hypothesis that KIF17 mediated IFT is regulated in relationship to OS turnover and influences turnover through regulation of the dynamics of the distal ciliary axoneme at the OS tip. Aim 1 will analyze the KIF17 loss of function mutations in both zebrafish and mice to assess the overall importance of this motor in photoreceptors compared to other cilium types. Aim 2 will use coordinated analysis of OS turnover, axoneme organization and KIF17 dynamics in zebrafish using transgenic models to test the hypothesis a KIF17 regulates the distal axoneme dynamics, which in turn is coupled to OS turnover. Aim 3 will utilize real time imaging in isolated zebrafish photoreceptors to study KIF17 movement in the distal OS segment and its relationship to IFT motility. Completion of these aims will provide insight into fundamental regulatory functions of OS segment assembly that are directly relevant photoreceptor diseases.