Summary: Researchers have identified a previously unknown “hook-like” domain in the tail of the kinesin-2 motor protein that explains how these molecular machines select the right cargo inside cells. Using cryo-electron microscopy and simulations, the team mapped the HAC domain’s atomic structure and showed how it binds both adaptor proteins and cargo, forming a highly specific recognition interface.
The study also uncovered parallels between this cargo-binding architecture and those found in other motor families, suggesting a shared biological design. These findings illuminate the molecular logic of cellular transport and open new avenues for targeting motor–cargo interactions in disease.
Key Facts
- Cargo Recognition Solved: The newly identified HAC domain acts as a molecular hook that assembles adaptors and cargo with high specificity.
- Shared Architecture: The HAC/KAP3 structure resembles cargo-binding systems in dynein and kinesin-1, hinting at a universal transport framework.
- Medical Relevance: Errors in this transport system are tied to neurodegeneration, developmental disorders, and ciliopathies.
Source: Juntendo University
For decades, scientists have known that motor proteins like kinesin-2 ferry vital cargo along microtubule “highways” inside cells. But how these molecular vehicles identify and bind to the right cargo remained a mystery.
The new study provides a key piece of this puzzle by revealing the atomic-level structure of the kinesin-2 tail and its interaction with cargo and adaptor proteins.
This study, led by Professor Nobutaka Hirokawa from Juntendo University with Dr. Masahide Kikkawa from the University of Tokyo, Dr. Xuguang Jiang, a JSPS Postdoctoral Fellow, Dr. Radostin Danev from the University of Tokyo, and Mr. Sotaro Ichinose from Gunma University, was published in Science Advances on October 24, 2025.
Using cryo-electron microscopy and molecular dynamics simulations, the scientists reconstructed the structure of the heterotrimeric kinesin-2 complex (KIF3A/KIF3B/KAP3) bound to the cargo protein, adenomatous polyposis coli (APC).
They discovered a unique structural motif in the tail region of KIF3A and KIF3B (termed hook-like adaptor and cargo-binding (HAC) domain) that acts as a molecular “hook,” enabling the motor to assemble its adaptors and recognize cargo with high specificity.
“Our study has uncovered a previously unknown ‘hook-like’ structural element, the HAC domain, in the tail of the motor protein kinesin-2,” said Prof. Hirokawa.
“This domain acts as a molecular ‘connector’ that allows the motor to correctly recognize and transport its cargo inside cells.”
The HAC domain consists of a helix–β-hairpin–helix (H-βh-H) motif that forms a scaffold for the adaptor protein KAP3 and the cargo protein APC. The study revealed four distinct binding interfaces between KIF3 and KAP3, with KIF3A playing a dominant role in cargo recognition.
The researchers also found that the HAC/KAP3 structure resembles cargo-binding architectures of other motor proteins, such as dynein and kinesin-1, suggesting a shared recognition framework.
“This discovery builds on decades of research from our laboratory, which first identified and characterized the complete family of mammalian kinesin motor proteins in the 1980s and 1990s and later revealed how these molecular ‘vehicles’ move along the cytoskeletal ‘highways’ of the cell,” Prof. Hirokawa said.
“While we have long understood how these motors travel, the remaining mystery was how they know what to carry. Our new findings provide the first atomic-level insight into this ‘logistics code’ of cellular transport, the molecular rules that allow each motor to recognize and deliver its specific cargo with remarkable precision.”
The team validated their structural model using cross-linking mass spectrometry, biochemistry, and neuronal cell biology. They showed that the HAC domain binds specifically to the ARM repeat region of APC, a tumor suppressor protein involved in neuronal RNA transport. Notably, KIF3A contributed the majority of the binding energy, while KIF3B played a structural support role.
“Defects in intracellular transport are linked to a variety of human diseases, including neurodegenerative diseases, neurodevelopmental disorders, and ciliopathies,” Prof. Hirokawa said.
“Understanding how motor proteins accurately recognize and deliver their cargo provides a molecular basis for developing new diagnostic and therapeutic approaches.”
The study also highlights the potential for drug discovery targeting motor-cargo interactions and the design of artificial transport systems that mimic biological logistics.
However, the authors note that some regions of the protein complex remain unresolved due to structural flexibility, and further studies are needed to explore cargo diversity and regulatory mechanisms.
This research marks a major step toward decoding the cellular transport system and understanding motor-driven cargo delivery in neurons.
Key Questions Answered:
A: It reveals how kinesin-2 motors identify and bind their cargo using a newly discovered structural element in their tail called the HAC domain.
A: The HAC (hook-like adaptor and cargo-binding) domain is a helix–β-hairpin–helix motif that form
A: Understanding how motor proteins select cargo could lead to new treatments for diseases linked to faulty intracellular transport, including neurodegenerative and developmental disorders.
About this neuroscience research news
Author: Toshifumi Asano
Source: Juntendo University
Contact: Toshifumi Asano – Junetendo University
Image: The image is credited to Neuroscience News
Original Research: Open access.
“The hook-like adaptor and cargo-binding (HAC) domain in the kinesin-2 tail enables adaptor assembly and cargo recognition” by Nobutaka Hirokawa et al. Science Advances
Abstract
The hook-like adaptor and cargo-binding (HAC) domain in the kinesin-2 tail enables adaptor assembly and cargo recognition
Intracellular transport relies on motor proteins such as kinesins to deliver cargo along microtubules, yet how they recognize cargo remains unclear.
Here, we present high-resolution cryo–electron microscopy structures of the heterotrimeric kinesin-2 complex (KIF3A/KIF3B/KAP3) bound to the cargo protein APC.
Our findings reveal a previously uncharacterized KIF3 tail hook-like motif, termed the “HAC” domain, which mediates binding to both KAP3 adaptor and APC cargo.
Within this domain, the KIF3A helical regions ensure cargo specificity, while a β-hairpin and KIF3B provide structural support. Biochemical and neuronal experiments confirm its functional importance.
Notably, the HAC/KAP3 structure resembles hook-like architectures seen in kinesin-1 and dynein, suggesting a shared cargo recognition framework.
These findings also shed light on kinesin-2 cargo specificity and offer a structural framework for understanding related neuronal transport mechanisms.
