Pathogenic KIF1A R350 mutations disrupt a conserved and conformation-dependent 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 pathogenicity 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 salt bridge between KIF1A residue R350 and α-tubulin E415 that forms only in the open conformation of the motor domain and is disrupted in both mutants. The loss of this electrostatic interaction correlates with increased velocity, reduced processivity, and decreased microtubule affinity in the open, apo conformation, as demonstrated by single-molecule assays. Our results reveal an electrostatic interaction at the motor-microtubule interface that regulates KIF1A's motility behavior.