Beyond Binding Affinity: How a Conformation-Specific Salt Bridge Tunes KIF1A Mechanochemistry.

Mutations at KIF1A residue R350 are linked to hereditary spastic paraplegia type 30 (SPG30), part of the spectrum of KIF1A-associated neurological disorders (KAND). Recent high-resolution cryo-EM structures of the KIF1A R350G and R350W mutants bound to microtubules in both nucleotide-free (apo) and AMP-PNP-bound states revealed a salt bridge between KIF1A R350 and α-tubulin E415 that forms specifically in the open motor domain conformation and is abolished by both substitutions. Single-molecule motility assays further showed that disruption of this conformation-dependent electrostatic interaction increases motor velocity while reducing processivity and microtubule affinity in the apo state. These findings identify a previously unrecognized mechanism regulating KIF1A motility and highlight how subtle changes in motor mechanochemistry can contribute to KAND pathology.