Active zone protein SYD-2/Liprin-α acts downstream of LRK-1/LRRK2 to regulate polarized trafficking of synaptic vesicle precursors through clathrin adaptor protein complexes

Synaptic vesicle proteins (SVps) are thought to travel in heterogeneous carriers dependent on the motor UNC-104/KIF1A. In C. elegans neurons, we found that some SVps are transported along with lysosomal proteins by the motor UNC-104/KIF1A. LRK-1/LRRK2 and the clathrin adaptor protein complex AP-3 are critical for the separation of lysosomal proteins from SVp transport carriers. In lrk-1 mutants, both SVp carriers and SVp carriers containing lysosomal proteins are independent of UNC-104, suggesting that LRK-1 plays a key role in ensuring UNC-104-dependent transport of SVps. Additionally, LRK-1 likely acts upstream of the AP-3 complex and regulates the membrane localization of AP-3. The action of AP-3 is necessary for the active zone protein SYD-2/Liprin-α to facilitate the transport of SVp carriers. In the absence of the AP-3 complex, SYD-2/Liprin-α acts with UNC-104 to instead facilitate the transport of SVp carriers containing lysosomal proteins. We further show that the mistrafficking of SVps into the dendrite in lrk-1 and apb-3 mutants depends on SYD-2, likely by regulating the recruitment of the AP-1/UNC-101. We propose that SYD-2 acts in concert with both the AP-1 and AP-3 complexes to ensure polarized trafficking of SVps.