The KASH protein UNC-83 differentially regulates kinesin-1 activity to control developmental stage-specific nuclear migration.

Nuclear migration plays a fundamental role in development, requiring precise spatiotemporal control of bidirectional movement through dynein and kinesin motors. Here, we uncover a differential isoform-dependent mechanism for developmental regulation of nuclear migration directionality. The nuclear envelope Klarsicht/ANC-1/Syne homology (KASH) protein UNC-83 in Caenorhabditis elegans exists in multiple isoforms that differentially control motor activity to achieve tissue-specific nuclear positioning. The shorter UNC-83c isoform promotes kinesin-1-dependent nuclear movement in embryonic hyp7 precursors, while longer UNC-83a/b isoforms facilitate dynein-mediated nuclear migration in larval P cells. We demonstrate that the UNC-83a-specific N-terminal domain functions as a kinesin-1 inhibitory module by directly binding the kinesin heavy chain (UNC-116). This interaction prevents kinesin-1 activation and reduces the protein's affinity for kinesin light chain (KLC-2), allowing for dynein-mediated transport. By contrast, UNC-83c exhibits high-affinity binding to KLC-2, promoting kinesin-1 activation for plus-end-directed movement. AlphaFold structural predictions reveal that UNC-83 contains five spectrin-like repeats, with two located within the inhibitory N-terminal domain. Genetic analysis demonstrates that these spectrin-like repeats are essential for dynein-dependent P cell nuclear migration but dispensable for kinesin-1-dependent hyp7 migration. This isoform-specific inhibition, combined with differential affinity for KLC-2, establishes a mechanism for achieving directional control of nuclear positioning during development. Together, these interdisciplinary studies reveal how alternative isoforms of cargo adaptors can generate developmental stage-specific regulation of motor activity.