Analysis, regulation, and roles of endosomal phosphoinositides.

Phosphoinositides (PIs) are minor lipid components of cellular membranes that play critical roles in membrane dynamics, trafficking, and cellular signaling. Among the seven naturally occurring PIs, the monophosphate phosphatidylinositol 3-phosphate (PtdIns3P) and the bisphosphate phosphatidylinositol 3,5-bisphosphate [PtdIns(3,5)P2] have been mainly associated with endosomes and endosomal functions. Metabolic labeling and HPLC analysis revealed that a bulk of PtdIns3P is constitutively present in cells, making it the only detectable product of the enzymes phosphoinositide 3-kinases in unstimulated, normal cells. The use of specific tagged-PtdIns3P-binding domains later demonstrated that this constitutive PtdIns3P accumulates in endosomes where it critically regulates trafficking and membrane dynamics.