Pathogenic KIF1A R350 Variants Disrupt A Conserved Kinesin-Tubulin Salt Bridge.

Pathogenic variants in the motor domain of the kinesin-3 motor protein KIF1A cause a range of neurodevelopmental and neurodegenerative conditions collectively termed KIF1A-associated neurological disorder (KAND). Among these, mutations at residue R350 are linked to hereditary spastic paraplegia and altered motor function. Yet, the structural basis for their pathogeny remains unclear. Here, we present high-resolution cryo-electron microscopy (cryo-EM) structures of KIF1A R350G and R350W bound to microtubules in both the apo and AMP-PNP-bound states. We identify a previously unrecognized salt bridge between KIF1A residue R350 and α-tubulin E415 that is disrupted in both mutants. This loss of electrostatic interaction correlates with increased velocity and reduced processivity, as demonstrated by single-molecule assays. Our results reveal a conserved electrostatic interaction at the motor-microtubule interface that regulates KIF1A's motility behavior.