The missing link: Piccolino is essential for tethering synaptic vesicles to rod photoreceptor ribbons.

Retinal photoreceptors transmit light signals to their postsynaptic neurons with high precision, speed and without fatigue. This high-throughput neurotransmission relies on a sophisticated molecular machinery centered on a presynaptic organelle, the synaptic ribbon (SR). A hallmark of SRs is the recruitment of synaptic vesicles (SVs) from the cytoplasmic SV pool via "tethering". However, the identity of the tether and the mechanism underlying SV tethering are unknown. Here, we show that cell-specific deletion of the SR-associated protein Piccolino from rod photoreceptors disrupts SR morphology and ablates SV tethering. Nanoscale epitope mapping suggests that Piccolino acts as an SV tether by extending its N terminus away from the SR into the SV-filled terminal cytoplasm. With in silico modeling and protein lipid-binding assays, we demonstrate that an amphipathic liquid packing sensor motif (ALPS) at the N terminus of Piccolino binds SV-like liposomes, implicating this interaction as the mechanism underlying SV tethering. Together, our findings identified Piccolino as the molecular link between the SR and SVs.