Phosphatidylcholine Biosynthesis in Soybeans: The Cloning and Characterization of Genes Encoding Enzymes of the Nucleotide Pathway

As the predominant phospholipid constituent of most eukaryotic cellular membranes, phosphatidylcholine (PC) plays a vital role in cellular structure and physiology. PC is also the substrate for C18 fatty acid polyunsaturation, and is therefore additionally involved in the processes that determine the polyunsaturation content of both membrane lipids and seed storage oils. Two distinct pathways for PC biosynthesis have been thoroughly characterized in both animals and yeast: (a) the methylation pathway, whereby PC is produced by the sequential methylation of phosphatidylethanolamine (PE); and (b) the nucleotide pathway, a three-step enzymatic process where free choline is incorporated into PC via the enzymes choline kinase (CKase), cholinephosphate cytidylyltransferase (CCTase) and cholinephosphotransferase (CPTase; Figure 1). In higher plants, however, recent studies of PC biosynthesis have provided compelling evidence suggesting that the majority of PC is produced through a single biosynthetic pathway possessing elements of both the methylation and nucleotide pathways (1). The isolation and characterization of the genes encoding enzymes of this consolidated pathway will accelerate the study of PC biosynthesis in higher plants and help elucidate the processes by which plants regulate the production of this important phospholipid at both the molecular and cellular levels.