Summary: Researchers have identified a previously unknown hook-like structural element in the tail of the kinesin-2 motor protein that explains how these molecular motors select and bind the correct cargo inside cells. Using high-resolution cryo-electron microscopy and molecular dynamics simulations, the team mapped the HAC domain at near-atomic detail and demonstrated how it organizes adaptor proteins and cargo into a highly specific recognition interface.
This work reveals parallels between the HAC-based cargo recognition architecture and cargo-binding systems in other motor families, suggesting a conserved biological design for intracellular transport. The findings clarify the molecular logic of cargo selection and point toward new strategies for targeting motor–cargo interactions in disease.
Key Facts
- Cargo recognition explained: The hook-like adaptor and cargo-binding (HAC) domain functions as a molecular hook that assembles adaptors and cargo with high specificity, resolving how kinesin-2 identifies its cargo.
- Shared architecture across motors: The HAC/KAP3 assembly shows structural similarity to cargo-binding architectures in dynein and kinesin-1, indicating a potentially universal transport framework.
- Medical relevance: Disruption of intracellular transport is implicated in neurodegeneration, neurodevelopmental disorders, and ciliopathies, so understanding HAC-mediated recognition may inform diagnostics and therapies.
This study, led by Professor Nobutaka Hirokawa of Juntendo University with collaborators Dr. Masahide Kikkawa and Dr. Radostin Danev (University of Tokyo), Dr. Xuguang Jiang (JSPS postdoctoral fellow), and Mr. Sotaro Ichinose (Gunma University), was published in Science Advances on October 24, 2025. The team reconstructed the heterotrimeric kinesin-2 complex (KIF3A/KIF3B/KAP3) bound to the cargo protein adenomatous polyposis coli (APC) using cryo-electron microscopy complemented by simulations and biochemical validation.
At the core of the discovery is a previously uncharacterized motif in the KIF3A/KIF3B tail that the authors name the hook-like adaptor and cargo-binding (HAC) domain. The HAC domain adopts a helix–β-hairpin–helix (H-βh-H) fold that serves as a scaffold to recruit the KAP3 adaptor and to engage APC cargo. Within the HAC, KIF3A contributes key helical regions that determine cargo specificity, while a β-hairpin and structural support from KIF3B stabilize the assembly.
Detailed structural analysis revealed four distinct interaction interfaces between KIF3 and KAP3, with KIF3A accounting for the majority of binding energy and KIF3B providing mechanical support. Cross-linking mass spectrometry, biochemical assays, and neuronal cell biology experiments corroborated the structural model and confirmed that the HAC domain specifically binds the ARM repeat region of APC, a protein previously implicated in RNA transport in neurons.
Professor Hirokawa commented that the discovery fills a longstanding gap in understanding how motor proteins recognize their cargo. “We have known for decades how molecular motors move along microtubules,” he said, “but the ‘logistics code’—the molecular rules that determine which motor carries which cargo—remained elusive. The HAC domain gives us the first atomic-level view of that code for kinesin-2.”
The resemblance of the HAC/KAP3 architecture to hook-like interfaces in kinesin-1 and dynein suggests that diverse motors may use similar structural principles to achieve selective cargo binding. This convergence implies a conserved strategy across motor families to combine adaptor proteins and motor-tail motifs into precise recognition modules.
Beyond fundamental insight into intracellular transport, the findings have practical implications. Because defects in motor-driven cargo delivery are linked to a range of diseases—particularly in neurons—defining the HAC-mediated recognition interface opens new avenues for drug discovery aimed at modulating motor–cargo interactions. The structural framework could also guide the design of synthetic transport systems that mimic biological logistics for therapeutic or biotechnological applications.
The authors note limitations: some regions of the complex remained unresolved due to intrinsic flexibility, and the full diversity of kinesin-2 cargoes and regulatory mechanisms will require further study. Nonetheless, this work provides a robust structural and biochemical foundation for future investigations into cargo specificity and motor regulation in neurons and other cell types.
Key Questions Answered
Q: What long-standing mystery does this study address?
A: It explains how kinesin-2 motors identify and bind their cargo by revealing the HAC domain in the motor tail that organizes adaptor and cargo interactions.
Q: What is the HAC domain and why is it important?
A: The HAC (hook-like adaptor and cargo-binding) domain is a helix–β-hairpin–helix motif in KIF3A/KIF3B tails that creates a specific docking platform for the KAP3 adaptor and APC cargo, enabling selective transport.
Q: How could this discovery affect medicine and biology?
A: Understanding HAC-mediated cargo recognition clarifies mechanisms underlying motor-based transport defects in neurodegenerative and developmental diseases and suggests new targets for therapeutic intervention and biomimetic transport design.
About this neuroscience research news
Author: Toshifumi Asano
Source: Juntendo University
Contact: Toshifumi Asano – Juntendo University
Image: Image credit: Neuroscience News
Original Research: Open access. Title: “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., published in Science Advances (DOI provided in the original publication).
Abstract
The hook-like adaptor and cargo-binding (HAC) domain in the kinesin-2 tail enables adaptor assembly and cargo recognition
Intracellular transport depends on motor proteins such as kinesins to deliver cargo along microtubules, yet the molecular basis for cargo recognition has been 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 structures reveal an uncharacterized KIF3 tail motif—the HAC domain—that mediates binding to both the KAP3 adaptor and APC cargo. Within this motif, helical elements from KIF3A confer cargo specificity, while a β-hairpin and contributions from KIF3B provide structural stability. Biochemical and neuronal experiments validate the functional importance of the HAC domain. Remarkably, the HAC/KAP3 assembly resembles hook-like cargo-recognition architectures in kinesin-1 and dynein, suggesting a shared framework for motor–cargo specificity. These results establish a structural basis for kinesin-2 cargo selection and provide a platform for future studies of neuronal transport mechanisms.