F-box protein FBXB-65 regulates anterograde transport of a Kinesin-3 motor UNC-104 through a PTM near its cargo-binding PH domain

Axonal transport in neurons is essential for cargo movement between the cell body and synapses. UNC-104/KIF1A, a Kinesin-3 motor in C. elegans that anterogradely transports precursors of synaptic vesicles (pre-SVs), is degraded at synapses. Touch neurons-specific knockdown of the E1 ubiquitin-activating enzyme, uba-1, leads to UNC-104 accumulation at neuronal ends and synapses. An RNAi screen identified fbxb-65, an F-box protein, that leads to UNC-104 accumulation at neuronal distal ends, alters UNC-104 net anterograde movement and levels of UNC-104 on cargo without changing synaptic UNC-104 levels. Split fluorescence reconstitution shows that UNC-104 and FBXB-65 interact throughout the neuron. Our theoretical model suggests that UNC-104 may exhibit cooperative cargo binding that is regulated by FBXB-65. FBXB-65 regulates an unidentified post-translational modification (PTM) of UNC-104 in a region beside the cargo-binding PH domain. Both fbxb-65 and UNC-104 independent of FBXB-65 regulate axonal pre-SV distribution, transport of pre-SVs at branch points and organismal lifespan. FBXB-65 regulates a PTM of UNC-104 and the number of motors on the cargo surface, which can fine-tune cargo transport to the synapse.