Identification of AtPIS, a phosphatidylinositol synthase from Arabidopsis

Phosphatidylinositol synthase is the enzyme responsible for the synthesis of phosphatidylinositol, a key phospholipid component of all eukaryotic membranes and the precursor of messenger molecules involved in signal transduction pathways for calcium-dependent responses in the cell. Using the amino acid sequence of the yeast enzyme as a probe, we identified an Arabidopsis expressed sequence tag potentially encoding the plant enzyme. Sequencing the entire cDNA confirmed the homology between the two proteins. Functional assays, performed by overexpression of the plant cDNA in Escherichia coli, a bacteria which lacks phosphatidylinositol and phosphatidylinositol synthase activity, showed that the plant protein induced the accumulation of phosphatidylinositol in the bacterial cells. Analysis of the enzymatic activity in vitro showed that synthesis of phosphatidylinositol occurs when CDP-diacylglycerol and myo-inositol only are provided as substrates, that it requires manganese or magnesium ions for activity, and that it is at least in part located to the bacterial membrane fraction. These data allowed us to conclude that the Arabidopsis cDNA codes for a phosphatidylinositol synthase. A single AtPIS genetic locus was found, which we mapped to Arabidopsis chromosome 1.