The Story of EXPO

Prof. Jiang Liwen (1st left) and his research team
EXPO outside the plasma membrane just before releasing its protein cargo; tiny black dots indicate presence of proteins in EXPO

“What you see today is built upon the work of several generations of students,” Prof. Jiang Liwen of the School of Life Sciences said humbly. He recounted how “lucky” he was in recruiting his first batch of graduate students when he joined CUHK in 2000. Together, he and his successive batches of students have opened new doors in plant cell biology including the discovery of EXPO (Exocyst-positive Organelle), a novel organelle that mediates an unconventional protein secretion pathway in plants.

Conceptually, one can think of the relationship between organelles and the cell as being similar to that between organs and the human body. Prevacuolar compartments (PVCs), Multivesicular Bodies (MVBs), Golgi apparatus, Trans-Golgi network (TGN) and EXPOs are all different organelles performing different “traffic control” functions in plant cells. By studying the transportation system in plants, one can gain insight into plant growth, development, survival, yield and value.

Starting with one small step

Little was known about the dynamics and molecular components of plant PVCs ten years ago. Together with a research group from the University of Heidelberg, Professor Jiang’s group first identified MVBs as being the PVCs and late endosomes in plant cells as well as the TGN being an early endosome merging the secretory and endocytic pathways to the lytic vacuole in plant cells. Endosomes are like a sorting house in the plant transport system. An analogy is that a freeway to transport cell materials was found and proved to be like a traffic depot that has some sorting function.

Using various tagging methods for new proteins, high pressure freezing technique and confocal laser scanning microscopy, the research group performed further research into other organelles and transport pathways in plants. They discovered EXPO which represents a form of unconventional secretion crucial in the transportation of Cytosolic proteins which could be antimicrobial agents crucial to the survival of plants in pathogen attacks or essential for cell wall biosynthesis.

Plant traffic control

It was not clear why some cell materials would go through the conventional transportation system while others would take unconventional secretion pathways like EXPO. In 2012, the group published a paper in The Plant Cell that partially answered that question. It was found that the Golgi-Localized Arabidopsis Endomembrane Protein 12 contains both endoplasmic reticulum export and Golgi Retention Signals at its C-Terminus. These sorting signals, like a GPS system for a vehicle, represent a general mechanism for EMP targeting to Golgi in plant cells.

It is all well if one assumes that protein secretion depends on these “GSP” signals but the discovery of leaderless secretory proteins found outside of the plasma membrane challenges this assumption. How did these proteins get there without a GPS guiding system? This unconventional protein secretion (“UPS”) was largely ignored by the plant community until a collaboration between Professor Jiang’s group and two German universities reviewed the evidence for UPS in plants, especially with regard to EXPO.

New generation of scientists

The puzzles regarding the function and mechanism of EXPO are being solved one by one. Graduate students like Tse Yu-chung, Lam Sheung-kwan, Miao Yansong, Wang Juan, Ding Yu and Gao Caiji who have participated in the above discoveries all have very promising prospects in international research/academic institutions now. Professor Jiang looks forward to the next generation of students to explore new frontiers in plant biology.