For decades, the intricate mechanisms governing intracellular transport have captivated scientists, with particular focus on the motor proteins that haul vital molecular cargo along the complex microtubule networks within cells. Among these motor proteins, kinesin-2 has stood out due to its pivotal role in neuronal function and development. Yet, despite extensive study, the precise molecular code enabling kinesin-2 motors to accurately recognize and selectively bind an array of cargos remained elusive—until now.
In a pioneering study led by Professor Nobutaka Hirokawa of Juntendo University in Tokyo, alongside collaborators from the University of Tokyo and Gunma University, groundbreaking atomic-level insights into kinesin-2’s cargo recognition machinery have been unveiled. Utilizing a combination of cryo-electron microscopy and molecular dynamics simulations, the structural architecture of the kinesin-2 heterotrimeric complex—composed of KIF3A, KIF3B, and KAP3 subunits—was resolved in unprecedented detail, particularly focusing on its interaction with the adenomatous polyposis coli (APC) protein, a cargo essential for neuronal RNA transport and tumor suppression.
Central to this breakthrough is the identification of a previously unknown structural motif in the kinesin-2 tail domain, coined the hook-like adaptor and cargo-binding (HAC) domain. This domain comprises a distinctive helix–β-hairpin–helix (H-βh-H) configuration that forms a highly specialized scaffold, enabling the cooperative assembly of adaptor proteins, notably KAP3, alongside cargo recognition. The HAC domain acts analogously to a molecular hook, a precise connector that orchestrates the selective engagement and transport of cargo within the dense cellular environment.
Professor Hirokawa emphasizes that the discovery of the HAC domain marks a significant leap in decoding the molecular logistics system inside neurons. “Our findings reveal the crucial structural basis by which kinesin-2 motors meticulously recognize and transport specific cargos, a process that had defied molecular characterization until now,” he stated. This insight not only illuminates the sophisticated specificity of motor-cargo interactions but also elucidates how these transport processes are finely orchestrated to maintain neuronal function.
Elaborating on the mechanism, the study identified four distinct binding interfaces between the kinesin-2 complex and the KAP3 adaptor protein. Notably, the KIF3A subunit emerged as the primary driver for cargo binding, contributing the majority of the binding energy, whereas KIF3B provides essential structural support. This delineation of functional roles within the motor complex underscores a nuanced division of labor, ensuring stable yet flexible cargo attachment necessary for dynamic intracellular trafficking.
Furthermore, the HAC/KAP3 binding configuration shares structural resemblance with known cargo-binding regions from other motor proteins such as dynein and kinesin-1, suggesting the existence of a conserved cargo recognition framework across diverse motor systems. This revelation hints at an evolutionary convergence whereby molecular “hooks” have evolved as a universal solution for targeted cargo delivery, highlighting the fundamental nature of such adaptor-mediated specificity in cellular logistics.
Validating their structural model through complementary cross-linking mass spectrometry experiments and biochemical assays, the researchers confirmed that the HAC domain selectively engages with the ARM repeat region of APC. This interaction is critical for the transport of neuronal RNA cargo, demonstrating that disruptions in this process could have profound cellular consequences. Such specificity is vital for neuronal health, given that misregulation of cargo transport systems has been implicated in a multitude of neurodegenerative and neurodevelopmental disorders.
The authors also emphasize the broader biomedical implications of this discovery. Intracellular transport defects are increasingly recognized as a molecular underpinning for various ciliopathies, neurodegenerative diseases such as Alzheimer’s and Parkinson’s, and other neurological dysfunctions. Greater understanding of how kinesin-2 motors decipher their cargo “address” opens potential avenues for targeted drug design. By modulating motor-cargo interfaces or adaptor assembly processes, it could be possible to rectify transport defects or selectively interfere with pathogenic cargo trafficking pathways.
Beyond therapeutic prospects, this research heralds exciting possibilities in the field of synthetic biology. The detailed molecular blueprint of the HAC domain and its adaptor assembly offers a foundation for engineering artificial transport systems capable of mimicking the exquisite precision of natural intracellular logistics. Such biomimetic designs could revolutionize drug delivery, biosensing, and even nanoscale manufacturing platforms by leveraging engineered molecular motors with programmable cargo specificity.
Despite the monumental progress, the authors acknowledge remaining challenges. Certain regions within the kinesin-2 complex, particularly flexible segments, resisted structural resolution due to inherent conformational dynamics. Moreover, the diversity of cargos beyond APC and potential regulatory mechanisms modulating HAC domain interactions warrant further investigation to fully map the kinesin-2 transport repertoire within various cell types and physiological contexts.
The journey of kinesin research, which Professor Hirokawa’s lab pioneered since the 1980s by first identifying the kinesin superfamily and elucidating their motility along cytoskeletal highways, has now entered a new era. By decoding the atomic-scale “logistics code” that enables cargo recognition, this study transforms our comprehension of cellular transport from descriptive to mechanistic, promising to illuminate how molecular machines drive life’s essential logistics in health and disease.
As neurons rely on precise cargo delivery for normal function and survival, this study’s insights into the HAC domain unlock potential to understand—and eventually manipulate—the cellular highways that sustain brain health. The fusion of structural biology, biochemistry, and cell biology in this research exemplifies the integrative approach necessary to unravel the complexities of intracellular transport and pave the way for novel diagnostics and therapeutics in neurobiology.
The identification of the HAC domain thus represents a landmark finding that not only resolves a long-standing mystery in cell biology but also lays the conceptual and practical groundwork for future innovations spanning medicine, synthetic engineering, and fundamental neuroscience. This molecular “hook” imagery captures the elegant specificity by which kinesin-2 motors navigate cellular landscapes, inspiring new perspectives on the logistical precision inherent to life itself.
Subject of Research: Cells
Article Title: The hook-like adaptor and cargo-binding (HAC) domain in the kinesin-2 tail enables adaptor assembly and cargo recognition
News Publication Date: 24-Oct-2025
Web References:
https://doi.org/10.1126/sciadv.ady5861
References:
Xuguang Jiang, Radostin Danev, Sotaro Ichinose, Baichun Niu, Sumio Ohtsuki, Haruaki Yanagisawa, Satoru Nagatoishi, Kouhei Tsumoto, Nobutaka Hirokawa, and Masahide Kikkawa. “The hook-like adaptor and cargo-binding (HAC) domain in the kinesin-2 tail enables adaptor assembly and cargo recognition.” Science Advances, 24 October 2025. DOI: 10.1126/sciadv.ady5861
Image Credits: Professor Nobutaka Hirokawa from Juntendo University, Japan
Keywords: Intracellular transport, Cell biology, Molecular biology, Protein structure, Neurodegenerative diseases
