In the nervous system, nerves rely on microtubule-dependent transport along the axon to ensure that vesicles containing synaptic proteins are delivered to the synapse. Their vehicle — the microtubule-binding motor kinesin — is well characterized. However, the identity of the molecule that mediates the interaction between kinesin and its cargo in axons has remained elusive. Reporting in Cell and Neuron, Lawrence Goldstein and colleagues now show that there might be more than one chauffeur for the job.

To screen for proteins that might mediate this interaction, Bowman and colleagues used the genetically amenable Drosophila melanogaster. They mutagenized flies and screened larvae for behavioural phenotypes previously characterized in mutants lacking subunits of kinesin-1. Through this, they identified alleles of a candidate — Sunday driver ( SYD).

Their suspicions were confirmed when they found large accumulations of axonal membrane-bound cargos in the nerves of the larvae, indicating problems in axonal transport. Cloning of syd revealed that it is a membrane-associated protein containing two predicted protein–protein interaction domains. Through BLAST analysis they found that there are syd homologues in the worm, mouse and humans. So might SYD be the long-sought factor on vesicles that interacts with kinesin-1?

To test this, the authors first wondered whether syd is expressed in the right places. Expression of syd fused to green fluorescent protein in epithelial cells shows that it localizes to the Golgi, and to vesicles of the secretory pathway together with kinesin-1. Furthermore, expression analysis on Western blots of mouse brain and extracts from nervous tissue showed that SYD is probably present in axons.

Next, Bowman and colleagues tested whether SYD can bind to kinesin-1. They confirmed this in vitro and in vivo using the yeast two-hybrid system and co-immunoprecipitation, respectively. To show that this interaction is direct, they checked that the recombinant purified proteins bound together in a GST-pulldown assay. And they narrowed down the region responsible for this interaction to the tetratricopeptide (TPR) domain of the kinesin-1 light chain (KLC).

Does SYD mediate binding of all cargos with kinesin-1? Kamal and colleagues suggest not. They show that at least one cargo protein — the β-amyloid precursor protein ( APP) — interacts with KLC directly. And intriguingly, APP binds to the same region as SYD. So one possibility is that different cargos compete for interaction with KLC, thereby ensuring their safe delivery to the synapse.

But is it important for SYD to reach the synapse? Two other labs have also recently identified mouse syd as a neural-enriched protein that acts as a scaffold for the c-Jun amino-terminal kinase (JNK) mitogen-activated protein kinase (MAPK) cascade. Whether kinesin-1 is also important for this pathway remains unknown, but these results indicate that once SYD has reached the synapse, it might have other jobs beyond chauffeuring.