Sulfolipid and Betaine Lipid Head Group Biosynthesis

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Non-phosphorous polar lipids are very abundant in membranes of photosynthetic organisms. In plants and bacteria, phosphate limitation increases the ratio of non-phosphorous lipids to phospholipids and a functional substitution of phospholipids by non-phosphorous lipids has been proposed. Examples are the sulfolipid sulfoquinovosyldiacylglycerol (SQDG) substituting for phosphatidylglycerol and the betaine lipid diacylglycerol-N,N,N-trimethylhomoserine (DGTS) replacing phosphatidylcholine. The sulfolipid is present in all plants and most photosynthetic bacteria while betaine lipids are abundant in non-seed plants, bacteria and fungi. The SQDG head group is a sulfonic acid derivative of glucose (sulfoquinovose). The SQD1 protein of Arabidopsis is the key enzyme involved in the formation of the sulfolipid head group from UDP-glucose and sulfite. The crystal structure for the protein has been solved and an in vitro assay is available. These are the prerequisites for a detailed analysis of the SQD1 reaction mechanism pursued in this project. Furthermore, the interaction of SQD1 with other proteins and its true activity in vivo are under investigation. Two genes essential for betaine lipid biosynthesis in the purple bacterium Rhodobacter sphaeroides have been isolated. Based on preliminary analysis, one gene, btaA, is proposed to encode an S-adenosylmethionine:diacylglycerol 3-amino-3-carboxypropyl transferase catalyzing the first reaction of betaine lipid biosynthesis. The second enzyme presumably encodes an S-adenosylmethionine:diacylgycerolhomoserine N-methyltransferase converting diacylglycerol-homoserine to DGTS. Experiments are underway to test this hypothesis and to understand the reaction mechanism of these two enzymes in greater detail. The chemical synthesis of sulfonic acids is well known, giving rise to numerous compounds encountered by humans during daily life. The sulfolipid of plants is a prominent example of a naturally occurring sulfonic acid. Carbohydrate and sulfur metabolism meet in the biosynthesis of sulfolipid. Understanding the biosynthesis of the sulfolipid head group will provide a model for the de novo formation of sulfonates in nature. This project focuses on the detailed analysis of the function of the Arabidopsis SQD1 protein in the test tube and inside the plant chloroplast. This protein is responsible for the introduction of the sulfonic acid into a precursor of sulfolipid biosynthesis. Betaine lipid biosynthesis is mediated by two interesting proteins. The first makes an unusual use of a common metabolite, S-adenosylmethionine. A similar reaction occurs in the biosynthesis of the antibiotic nocardicin. Elucidating the basic reaction mechanism of this enzyme is one of the goals and may provide clues for the synthesis of useful compounds. The second enzyme methylates the amino group of a lipid substrate. Experiments planned to understand the mechanism of this enzyme will provide fundamental insights into the general mechanism of lipid-linked N-methylation.