Nine currently identified phosphoinositide 3-kinases (PI 3-K) constitute a subfamily of lipid kinases that catalyze the addition of a phosphate molecule on the 3-position of the inositol ring of phosphoinositides. Phosphatidylinositol (PtdIns), the precursor of all phosphoinosi-tides (PI), constitutes less than 10% of the total lipid in eukaryotic cell membranes. Approximately 5% of cellular PI is phosphorylated at the 4-position (PtdIns-4-P), and another 5% is phosphorylated at both the 4- and 5-positions (PtdIns-4,5-P2 ). However, less than 0.25% of the total inositol-containing lipids are phosphorylated at the 3-position, consistent with the idea that these lipids exert specific regulatory functions inside the cell, as opposed to a structural function. Here we have chosen to highlight a group of the phosphoinositide targets of the PI3-Ks and their downstream targets. The downstream effects of these PI-3 targets are indicated in the lower band illustrating the important role the PI3Ks have in cell function and survival.
