Phosphorylation of the Bruchpilot N-terminus in Drosophila unlocks axonal transport of active zone building blocks

Protein scaffolds at presynaptic active zone membranes control information transfer at synapses. For scaffold biogenesis and maintenance, scaffold components must be safely transported along axons. A spectrum of kinases was suggested to control transport of scaffold components, but direct kinase/substrate relationships and operational principles steering phosphorylation-dependent active zone protein transport are presently unknown. Here we show that extensive phosphorylation of a 150-residue unstructured region at the N-terminus of the highly elongated BRP/ELKS active zone proteins is crucial for ordered active zone precursor transport. Point mutations that block SRPK79D-kinase-mediated phosphorylation of the BRP/ELKS N-terminus interfered with axonal transport, leading to BRP/ELKS-positive axonal aggregates that also contain additional active zone scaffold proteins. Axonal aggregates formed only in the presence of non-phosphorylatable BRP/ELKS isoforms containing the SRPK79D-targeted N-terminal stretch. We assume that specific active zone proteins are pre-assembled in transport packages and thus co-transported as functional scaffold building blocks. Our results suggest that transient post-translational modification of a discrete unstructured domain of the master scaffold component Bruchpilot blocks oligomerization of such building blocks during their long-range transport.