Plants synthesize ethanolamine by direct decarboxylation of serine using apyridoxal phosphate enzyme

The established pathways from serine to ethanolamine are indirect and invol ve decarboxylation of phosphatidylserine. Here we show that plants can deca rboxylate serine directly. Using a radioassay based on ethanolamine (Etn) f ormation, pyridoxal 5'-phosphate-dependent L-serine decarboxylase (SDC) act ivity was readily detected in soluble extracts from leaves of diverse speci es, including spinach, Arabidopsis, and rapeseed. A putative Arabidopsis SD C cDNA was identified by searching GenBank (TM) for sequences homologous to other amino acid decarboxylases and shown by expression in Escherichia col i to encode a soluble protein with SDC activity. This cDNA was further auth enticated by complementing the Etn requirement of a yeast psd1 psd2 mutant. In a parallel approach, a cDNA was isolated from a rapeseed library by its ability to complement the Etn requirement of a yeast cho1 mutant and shown by expression in E. coli to specify SDC. The deduced Arabidopsis and rapes eed SDC polypeptides are 90% identical, lack obvious targeting signals, and belong to amino acid decarboxylase group II. Recombinant Arabidopsis SDC w as shown to exist as a tetramer and to contain pyridoxal 5'-phosphate. It d oes not attack D-serine, L-phosphoserine, other L-amino acids, or phosphati dylserine and is not inhibited by Etn, choline, or their phosphoesters. As a soluble, pyridoxal 5-phosphate enzyme, SDC contrasts sharply with phospha tidylserine decarboxylases, which are membrane proteins that have a pyruvoy l cofactor.