SUMMARY
Clathrin has established roles in endocytosis, with clathrin-cages enclosing membrane infoldings, followed by rapid disassembly and reuse of clathrin-monomers. In neurons, clathrin synthesized in cell-bodies is conveyed via slow axonal transport – enriching at presynapses – however, mechanisms underlying transport/targeting are unknown, and its role at synapses is debated. Combining live-imaging, mass-spectrometry (MS), Apex-labeled EM-tomography and super-resolution, we found that unlike dendrites where clathrin assembles and disassembles as expected, axons contain stable ‘clathrin transport-packets’ that move intermittently with an anterograde bias, conveying endocytosis-related proteins. The overall kinetics generate a slow biased flow of axonal clathrin, and we identify actin/myosin-VI as putative tethers. Inhibiting endocytosis does not affect clathrin transport-packets, suggesting specialized roles in trafficking and synaptic delivery. Presynapses have integer-numbers of clathrin assemblies resembling packets, circumferentially abutting the synaptic-vesicle cluster – an alignment that may have functional implications. The data offer a new nanoscale view of neuronal clathrin, and a mechanistic basis for its slow transport and presynaptic targeting.