KIF3AKIF3BKAP3

KIF3AKIF3BKAP3, commonly referred to as the KIF3 complex, is a heterotrimeric motor protein composed of two kinesin family member subunits (KIF3A and KIF3B) and a kinesin-associated protein subunit (KAP3). The complex is a member of the kinesin-2 family of microtubule-based motor proteins and functions primarily as an anterograde transporter, moving cargo toward the plus‑end of microtubules.

The KIF3A and KIF3B subunits each contain an N‑terminal motor domain responsible for ATP binding and hydrolysis, a central coiled‑coil stalk, and a C‑terminal tail domain that mediates cargo binding or regulation. KAP3 acts as a regulatory adaptor; it binds to the tail regions of both KIF3A and KIF3B, stabilizing the heterodimer and facilitating association with cargoes such as ciliary proteins, signaling molecules, and membrane vesicles. The combination of the two motor subunits and KAP3 allows the complex to generate processive movement along microtubules.

In mammalian cells, the KIF3 complex is essential for primary cilia biogenesis and maintenance. It transports intraflagellar transport (IFT) components from the basal body to the tip of the cilium, a process critical for ciliary signaling pathways and tissue development. In the nervous system, KIF3 mediates the delivery of synaptic vesicle precursors and neurotrophic receptors to axonal terminals, influencing axonal growth and guidance. Disruptions in KIF3 function have been linked to ciliopathies, developmental disorders, and certain cancers, underscoring its importance in cellular homeostasis.

The complex is regulated through phosphorylation of KIF3A and KAP3 by serine/threonine kinases, which modulates its motor activity and cargo specificity. Interactions with other proteins, such as Rab9 and the kinesin light chain, further fine‑tune its activity in distinct cellular contexts. Research continues to elucidate the precise mechanisms by which this motor complex coordinates transport with signaling and developmental processes.