A research team at The Hong Kong University of Science and Technology (HKUST) has made a significant advance in understanding cellular transport mechanisms, which are crucial for maintaining life and are linked to various genetic disorders. Led by Prof. Guo Yusong, an Associate Professor in the Division of Life Science, the team has published their findings in the Proceedings of the National Academy of Sciences (PNAS).
Utilizing a novel vesicle proteomics platform, the researchers identified new cargo proteins and key accessory factors associated with two important cellular transport complexes, AP-1 and AP-4. This innovative approach combines vesicle reconstitution techniques with quantitative mass spectrometry-based proteomics, offering a detailed overview of previously unidentified cargo proteins and regulatory elements involved in protein transport.
The study highlights the essential role of the secretory pathway, often described as the cell’s postal service, in delivering proteins to their intended locations. Disruptions in this process can lead to severe physiological issues. According to Prof. Guo, “For years, the field has struggled to comprehensively map the cargo repertoire of adaptor complexes like AP-1 and AP-4, whose malfunctions are directly linked to serious human conditions such as MEDNIK syndrome, X-linked intellectual disability, and AP-4 deficiency syndrome.” Despite this, the complete list of proteins these complexes transport has remained largely unknown.
Through their research, the team developed a method to recreate the transport process in vitro. This allowed them to identify precisely which proteins rely on AP-1 or AP-4 for vesicle packaging. “Our study breaks new ground through its approach,” added Prof. Guo. “This enabled us to transition from a fragmented understanding to a more comprehensive view of their cargo landscape, revealing both new clients and the unexpected cellular machinery that AP-4 depends on.”
The integration of in vitro vesicle reconstitution with quantitative mass spectrometry is a powerful tool for researchers. It allows for the creation of transport vesicles in a controlled setting, followed by extensive analysis of their protein composition. Collaborating with Prof. Yao Zhong-Ping from The Hong Kong Polytechnic University (PolyU), the team identified specific cargo proteins that depend on AP-1 or AP-4 to transport materials from the trans-Golgi network, a key sorting hub in the cell.
Among their discoveries, the researchers confirmed that the protein CAB45 is an AP-1-dependent cargo, while ATRAP is a novel cargo for AP-4. A significant finding addressed the long-standing question of how AP-4 can form transport vesicles without the well-known protein clathrin. The research revealed that two cytosolic factors, WDR44 and PRRC1, are critical for AP-4-mediated trafficking. When these factors were removed, essential cargos like ATG9A and ATRAP could not exit their organelles, resulting in disruptions to cellular processes such as autophagy.
“Our findings not only reveal new cargo clients and essential co-factors for AP-1 and AP-4 but also provide a useful toolkit for the scientific community to dissect the mechanisms of vesicular trafficking,” said Prof. Guo. “This opens new avenues for researching the pathological mechanisms of related diseases and potentially identifying new therapeutic targets.”
The study underscores the importance of understanding cellular transport mechanisms and their implications for human health. With further exploration, this research could lead to advancements in treating genetic disorders linked to these transport complexes. The co-corresponding authors of the study are Prof. Guo Yusong of HKUST and Prof. Yao Zhong-Ping of PolyU, with Dr. Peng Ziqing, a postdoctoral researcher at HKUST, serving as the first author.
