| Summary: | Biological membranes are predominantly composed of structural glycerophospholipids such as phosphatidylcholine, phosphatidylethanolamine, and phosphatidylinositol. Of the membrane glycerophospholipids, phosphatidylinositol (PtdIns) and its phosphorylated derivatives (phosphoinositides) constitute a minor fraction yet exert a wide variety of regulatory functions in eukaryotic cells. Phosphoinositides include PtdIns, three PtdIns monophosphates, three PtdIns bisphosphates, and one PtdIns triphosphate, in which the hydroxy groups of the inositol head group of PtdIns are phosphorylated by specific lipid kinases. Of all the phosphoinositides in eukaryotic cells, phosphatidylinositol 3,5-bisphosphate [PtdIns(3,5)P2] constitutes the smallest fraction, yet it is a crucial lipid in animal and yeast membrane trafficking systems. Here, we review the recent findings on the physiological functions of PtdIns(3,5)P2 and its enzyme—formation of aploid and binucleate cells (FAB1)—along with the regulatory proteins of FAB1 and the downstream effector proteins of PtdIns(3,5)P2 in Arabidopsis.
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