KIF5A binds RNA to orchestrate synaptic mRNA localization and stress granules in ALS.

Neuronal health depends on the precise transport and local translation of mRNAs to maintain synaptic function across highly polarized cellular architecture. While kinesin motor proteins are known to mediate mRNA transport, the specificity and direct involvement of individual kinesins as RNA-binding proteins (RBPs) remain unclear. Here, we demonstrate that KIF5A, a neuron-specific kinesin implicated in amyotrophic lateral sclerosis (ALS), functions as an RBP. We show that KIF5A directly binds mRNAs encoding synaptic ribosomal proteins and is required for their synaptic localization and for maintaining normal synaptic composition and function. Additionally, we show ALS-linked KIF5A mutations confer gain-of-function properties, enhancing mRNA binding, increasing synaptic ribosomal protein accumulation, inducing neuronal hyperexcitability, and impairing stress responses. These findings reveal a previously unrecognized mechanism by which mutant KIF5A disrupts synaptic homeostasis. Our work positions a kinesin motor protein as an RBP with critical roles in mRNA transport, local translation, and stress response. Highlights: KIF5A interacts with mRNA encoding synaptic ribosomal proteinsKIF5A is required for normal synaptic composition and functionKIF5A binds to G3BP1 and G3BP1 stress granule associated proteinsKIF5A mutant ALS patient-derived motor neurons have abnormal synaptic function and stress response.