소포체에서 지방 합성에 기여하는 지방산 수송체의 기능 연구

Abstract

Lipids are a major storage form of carbons and energy in oilseed plants such as canola, soybean, and palm. Seed oils are energy-dense and contain different forms of useful fatty acids. They serve as high calorie foods for people and animals, carbon-neutral fuels, and renewable raw materials for industry. Recently, the demand for seed oils has increased dramatically, as it has become a source for the production of clean, sustainable bio-diesel. It is estimated that the global consumption of vegetable oils will double by 2030. Accordingly, a critical need exists in increasing vegetable oil production by biotechnology. Previous genetic engineering biotechnology was to increase seed oil by changing the activity of enzymes and transcription factors that are involved in lipid metabolism. Fatty acids are synthesized in plastid and then transported into the endoplasmic reticulum (ER) for further lipid modification in plants. It has been postulated that there might be transporters that transport lipophilic fatty acids across the membranes. Although each step of fatty acid and lipid synthesis has been intensively studied for many decades, the process of fatty acid transport into the ER remains obscure. Such transporters responsible were predicted to be ATP-binding cassette (ABC) transporter proteins, since one ABC transporter, ABCD1/PXA1/PED3p/COMATOSE, has been reported to transport fatty acids into peroxisomes for β-oxidation.ABC transporter proteins are present in all living organisms and constitute a large gene family. Among eight subfamilies of ABC transporter proteins, ABCA subfamily is well known as lipid transporters in animals. Because ABC transporter proteins are highly conserved in their structures and functions among many different species ranging from prokaryotes to plants and humans, some members of ABCA family in plants might transport lipids and/or related compounds in lipid metabolism. However, the function of plant ABCA subfamily members has not been characterized yet.The aim of this study was to identify plant ABCA transporters which transport fatty acids from plastids into ER. To identify such ABCA transporters, I collected knockout mutants for 8 available ABCA subfamily members in Arabidopsis and tested their growth on half-strength Murashige and Skoog (1/2 MS) medium with or without sucrose. The logic of this screening is as follows: Arabidopsis seeds contain triacylglycerol (TAG) about 35 to 40% of their dry weight and uses this as an energy source for early seedling growth, thus decreased TAG in the mutant seeds by a defect in fatty acid transporter would not allow normal growth in the absence of additional carbon source such as sucrose.As a result of this screening, I found one ABC transporter, ABCA9, a member of the ABCA protein family in Arabidopsis thaliana, facilitates the transport of fatty acid required for oil production. ABCA9 is localized at the ER and loss-of-function plants contained approximately 35% less TAG, a major neutral lipid in seeds, than the wild type. Developing abca9 seeds incorporated 35% less 14C-oleoyl-CoA into TAG than did wild-type seeds. Furthermore, plants that overexpressed ABCA9 produced larger seeds that contained up to 40% more TAG. Production of siliques and seeds was not impaired in the ABCA9-overexpressing plants, and overexpression of ABCA9 thus resulted in a higher overall lipid production per plant. This study closes one of the long-standing mysteries of plant lipid biosynthesis, namely the supply of fatty acids to the ER. ABCA9 localizes at the ER and uptakes fatty acids for lipid synthesis during seed maturation. Overexpression of ABCA9 increased seed size and TAG deposition without any loss of protein and carbohydrates. The use of a lipid transporter is a novel approach for increasing the vegetable oil content of seeds, and can be combined with other methods enforcing seed metabolic functions.