Scaffold protein of the presynaptic cytomatrix at the active zone (CAZ) which is the place in the synapse where neurotransmitter is released (PubMed:22875941). After synthesis, participates in the formation of Golgi-derived membranous organelles termed Piccolo-Bassoon transport vesicles (PTVs) that are transported along axons to sites of nascent synaptic contacts (PubMed:22875941). At the presynaptic active zone, regulates the spatial organization of synaptic vesicle cluster, the protein complexes that execute membrane fusion and compensatory endocytosis (PubMed:27907191). Functions also in processes other than assembly such as the regulation of specific presynaptic protein ubiquitination by interacting with SIAH1 or the regulation of presynaptic autophagy by associating with ATG5 (PubMed:23403927, PubMed:27907191). Mediates also synapse to nucleus communication leading to reconfiguration of gene expression by associating with the transcriptional corepressor CTBP1 and by subsequently reducing the size of its pool available for nuclear import (PubMed:25652077). Interacts with PCLO, ERC2/CAST1, RIMS1 and UNC13A (PubMed:12163476, PubMed:14734538). Interacts with TPRG1L (By similarity). Interacts with DYNLL1 and DYNLL2; these interactions potentially link PTVs to dynein and myosin V motor complexes (By similarity). Interacts with ATG5; this interaction is important for the regulation of presynaptic autophagy (By similarity). Interacts (via C-terminus) with TRIO (via N-terminus) (PubMed:27907191). Interacts with CTBP1 (PubMed:25652077). Interacts with SIAH1; this interaction negatively regulates SIAH1 E3 ligase activity (PubMed:23403927). In retina, is localized in the outer plexiform layer at ribbon synapses formed by rods and cones but was absent from basal synaptic contacts formed by cones. In the retinal inner plexiform layer localized to conventional inhibitory GABAergic synapses, made by amacrine cells, but absent from the bipolar cell ribbon synapses (PubMed:10564375). Detected at synapses in the stratum lucidum in the hippocampus CA3 region (at protein level). Detected at embryonic day E18 and at later stages. The expression does not significantly change during the developmental stages tested. Myristoylated. The N-terminal myristoylation is not sufficient for presynaptic localization. Loss of both Bassoon/BSN and Piccolo/PCLO leads to the aberrant degradation of multiple presynaptic proteins, culminating in synapse degeneration.