Characterizing human KIF1Bß motor activity by single-molecule motility assays and Caenorhabditis elegans genetics.

The axonal transport of synaptic vesicle precursors relies on KIF1A and UNC-104 ortholog motors. In mammals, KIF1Bß is also responsible for the axonal transport of synaptic vesicle precursors. Mutations in KIF1A and KIF1Bß lead to a wide range of neuropathies. Although previous studies have revealed the biochemical, biophysical and cell biological properties of KIF1A, and its defects in neurological disorders, the fundamental properties of KIF1Bß remain elusive. In this study, we determined the motile parameters of KIF1Bß through single-molecule motility assays. We found that the C-terminal region of KIF1Bß has an inhibitory role in motor activity. AlphaFold2 prediction suggests that the C-terminal region blocks the motor domain. Additionally, we established simple methods for testing the axonal transport activity of human KIF1Bß using Caenorhabditis elegans genetics. Taking advantage of these methods, we demonstrated that these assays enable the detection of reduced KIF1Bß activities, both in vitro and in vivo, caused by a Charcot-Marie-Tooth disease-associated Q98L mutation.

The FHA domain is essential for autoinhibition of KIF1A/UNC-104 proteins.

KIF1A/UNC-104 proteins, which are members of the kinesin superfamily of motor proteins, play a pivotal role in the axonal transport of synaptic vesicles and their precursors. Drosophila melanogaster UNC-104 (DmUNC-104) is a relatively recently discovered Drosophila kinesin. Although some point mutations that disrupt synapse formation have been identified, the biochemical properties of the DmUNC-104 protein have not been investigated. Here, we prepared recombinant full-length DmUNC-104 protein and determined its biochemical features. We analyzed the effect of a previously identified missense mutation in the forkhead-associated (FHA) domain, called bristly (bris). The bris mutation strongly promoted the dimerization of DmUNC-104 protein, whereas wild-type DmUNC-104 was a mixture of monomers and dimers. We further tested the G618R mutation near the FHA domain, which was previously shown to disrupt the autoinhibition of Caenorhabditis elegans UNC-104. The biochemical properties of the G618R mutant recapitulated those of the bris mutant. Finally, we found that disease-associated mutations also promote the dimerization of DmUNC-104. Collectively, our results suggest that the FHA domain is essential for autoinhibition of KIF1A/UNC-104 proteins, and that abnormal dimerization of KIF1A might be linked to human diseases.