| Summary: | 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.
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