A research team from The Hong Kong University of Science and Technology (HKUST) has achieved a significant breakthrough in understanding the complex internal transport mechanisms in cells. Led by Prof. GUO Yusong, an Associate Professor in the Division of Life Science, the team’s findings shed light on the transport of proteins, a process crucial for cellular function and linked to various genetic diseases. Their study was published in the Proceedings of the National Academy of Sciences (PNAS) on October 10, 2023.
Utilizing an innovative vesicle proteomics platform, the researchers systematically identified new cargo proteins and essential accessory factors for two key cellular transport complexes, AP-1 and AP-4. These complexes play a vital role in the secretory pathway, which functions as the cell’s postal service, ensuring proteins are delivered accurately to their designated locations. Missteps in this transport system can result in serious physiological defects.
“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,” explained Prof. Guo. He noted, “However, the full list of proteins they transport has remained elusive.”
The study advances the understanding of these complexes by employing a combination of vesicle reconstitution techniques and quantitative mass spectrometry-based proteomics. This innovative approach facilitated the creation of transport vesicles in a controlled environment, allowing the team to analyze their protein composition comprehensively.
In collaboration with Prof. YAO Zhong-Ping from The Hong Kong Polytechnic University (PolyU), the researchers identified specific cargo proteins that rely on AP-1 or AP-4 for transport from the trans-Golgi network, a central sorting hub within the cell. Notably, the protein CAB45 was confirmed as an AP-1-dependent cargo, while ATRAP emerged as a novel cargo for AP-4.
A key discovery addresses a longstanding question regarding AP-4’s ability to form transport vesicles without the well-known protein clathrin. The team found that two cytosolic factors, WDR44 and PRRC1, are critical accessories for AP-4-mediated trafficking. When these factors were depleted, vital AP-4 cargoes like ATG9A and ATRAP could not exit their organelles, resulting in defects in crucial 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 powerful toolkit for the scientific community to dissect the mechanisms of vesicular trafficking,” added Prof. Guo. “This opens new avenues for researching the pathological mechanisms of related diseases and potentially identifying new therapeutic targets.”
The study highlights the importance of understanding cellular transport mechanisms in the context of genetic diseases. By mapping the cargo landscape of AP-1 and AP-4, the researchers have laid the groundwork for future investigations into the underlying causes of various hereditary conditions.
The co-corresponding authors of the study are Prof. Guo Yusong of HKUST and Prof. Yao Zhong-Ping of PolyU. The first author, Dr. PENG Ziqing, is a postdoctoral researcher at HKUST. This collaborative effort underscores the significance of interdisciplinary research in advancing scientific knowledge and addressing complex biological questions.
