The Phosphoinositide Kinase PIP 5 K That Produces the Versatile Signaling Phospholipid PI 4 , 5 P 2

In mammalian cells, phosphatidylinositol 4-phosphate (PI4P) 5-kinase (PIP5K) generates phosphatidylinositol 4,5bisphosphate (PI4,5P2) by catalyzing the phosphorylation of PI4P at the D5 position of the inositol ring (Fig. 1) and plays important roles in a wide variety of signal transduction systems through its product PI4,5P2. To date, three mammalian PIP5K isozymes, a , b and g , and three splicing variants of the g isozyme, g635, g661, and g687, have been identified. The level of PI4,5P2 in the plasma membrane at the resting state of cells is only ca. 0.5% of total phospholipids. Upon cell stimulation by agonists, such as hormones, neurotransmitters, and growth factors, PIP5K is activated and thereby the PI4,5P2 level increases in the membrane. This increased PI4,5P2 pool appears crucial for the regulation of signal transduction and cell events. It is well known that PI4,5P2 functions as a precursor of two lipid second messengers, diacylglycerol (DG) and inositol 1,4,5-trisphosphate (IP3), which are produced by the action of phosphoinositide-specific phospholipase C: the former activates protein kinase C and the latter increases the intracellular Ca concentration by inducing the release of Ca from the endoplasmic reticulum. PI4,5P2 is further phosphorylated by phosphatidylinositol 3-kinase to generate another lipid second messenger, phosphatidylinositol 3,4,5trisphosphate, which is involved in signal transduction regulating cell growth, survival, and apoptosis. In addition, recent studies provide evidence that PI4,5P2 itself functions as a lipid signaling molecule by regulating activities of a wide variety of proteins and enzymes that are involved in signal transduction and cell events, or by recruiting them to the plasma membrane. First, PI4,5P2 directly binds to actinbinding proteins and regulates their activities, which in turn reorganizes the actin cytoskeleton, thereby resulting in the regulation of membrane shape and cell motility. Second, this lipid directly interacts with the pleckstrin homology (PH) domainand epsin N-terminus homology (ENTH) domain-containing proteins and enzymes to recruit them to the plasma membrane. Third, PI4,5P2 binds to several components of clathrin-dependent endocytic machinery to recruit them to the plasma membrane and thereby assemble the clathrin coat at the plasma membrane. Thus PI4,5P2 is a highly versatile phospholipid regulating various signal transduction systems and cell events. Why is PI4,5P2 so multifunctional? Although we do not have any evidence to answer this question clearly, the apparent multifunction of PI4,5P2 may be attributable to tempospatially different production of PI4,5P2 depending on types of cells and agonists. If each PIP5K isozyme differently localizes or translocates to distinct compartments in the cell, PI4,5P2 production by each isozyme is tempo-spatially different and PI4,5P2 produced at different times or in different compartments would interact with and regulate distinct downstream effectors, thereby exerting apparent multifunction. It is also conceivable that, if this is true, each PIP5K isozyme is activated by an activator specific to each isozyme. To address this issue, we have been hunting for a partner protein(s) specifically interacting with and activating each PIP5K isozyme. Quite recently, we identified the endocytic adaptor protein complex AP-2 as an activator specific to PIP5Kg661 and found that interaction of AP-2 and PIP5Kg661 is essential for synaptic vesicle (SV) endocytosis. In this article, we describe recent advances regarding the PIP5K isozyme-specific activation mechanisms and physiological functions. The Phosphoinositide Kinase PIP5K That Produces the Versatile Signaling Phospholipid PI4,5P2