The type I phosphatidylinositol 4-phosphate 5-kinases (PI4P5K) phosphorylate phosphatidylinositol 4-phosphate [PI(4)P] to produce phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2]. PI(4,5)P2 has been implicated in signal transduction, receptor mediated endocytosis, vesicle trafficking, cytoskeletal structure, and membrane ruffling. However, the specific type I enzymes associated with the production of PI(4,5)P2 for the specific cellular processes have not been rigorously defined. Murine PI4P5K type Iβ (mPIP5K-Iβ) was implicated in receptor mediated endocytosis through the isolation of a truncated and inactive form of the enzyme that blocked the ligand-dependent downregulation of the colony-stimulating factor-1 receptor. The present study shows that enforced expression of mPIP5K-Iβ in 293T cells resulted in the accumulation of large vesicles that were linked to an endosomal pathway. Similar results were obtained after the expression of the PI(4,5)P2-binding pleckstrin homology (PH) domain of phospholipase-Cδ (PLC-δ). Analysis of the conserved domains of mPIP5K-Iβ led to the identification of dimerization domains in the N- and C-terminal regions. Enforced expression of the individual dimerization domains interfered with the proper subcellular localization of mPIP5K-Iβ and the PLC-δ-PH domain and blocked the accumulation of the endocytic vesicles induced by these proteins. In addition to regulating early steps in endocytosis, these results suggest that mPIP5K-Iβ acts through PI(4,5)P2 to regulate endosomal trafficking and/or fusion. J. Cell. Biochem. 85: 131–145, 2002. © 2002 Wiley-Liss, Inc.