A team of researchers led by Professor Nobutaka Hirokawa from Juntendo University announced on Apr. 10 the discovery of a new mechanism that regulates how motor proteins selectively transport specific cargo within neurons. The findings, published online on March 30 and set to appear in the Journal of Cell Biology in May, shed light on a long-standing question in cell biology regarding intracellular transport.
The study is significant because precise protein delivery is essential for neuron function and development. Neurons have complex structures that require accurate placement of proteins such as TRIM46 at specialized sites like the axon initial segment (AIS), which initiates electrical signals.
The research focused on kinesin superfamily proteins, particularly the kinesin-2 family, which includes KIF3A, KIF3B, and kinesin-associated protein 3 (KAP3). Using cultured neurons and mouse brain samples, the team examined different assemblies of these motor complexes. They found that kinesin-2 forms multiple subtypes with distinct roles. In addition to the standard KIF3A/B/KAP3 complex, they identified a KIF3B/B/KAP3 variant that specifically associates with TRIM46 to facilitate its delivery to the AIS. When KIF3B was depleted in experiments, TRIM46 failed to accumulate at its intended site despite normal production levels elsewhere in the cell.
Professor Hirokawa said, "While many studies have revealed how kinesin motor proteins move along microtubules, a key unanswered question has been how they recognize and selectively transport specific cargo molecules." He added that "neurons provide a particularly compelling system to study this because they require extremely precise intracellular transport to maintain their highly polarized structure."
Structural analyses suggested differences in tail domains among these complexes may determine their ability to bind specific cargoes. This discovery provides new insight into how cells organize internal logistics and could inform future therapeutic strategies for neurological disorders linked to defective intracellular transport.
"By identifying how kinesin-2 motors selectively transport proteins to specific neuronal regions, our study provides important insights into the molecular mechanisms that organize neuronal architecture," Hirokawa said. "In the long term, understanding how motor proteins recognize and deliver specific cargo could help guide the development of therapeutic strategies targeting transport defects."
