Dephosphorylates a number of phosphatidylinositols (PIs) like phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2), but also phosphatidylinositol 3-phosphate (PtdIns(3)P), phosphatidylinositol 4-phosphate (PtdIns(4)P), and phosphatidylinositol 3,5-bisphosphate (PtdIns(3,5)P2). Controls the cellular levels and subcellular distribution of phosphatidylinositol 3-phosphate and phosphatidylinositol 4,5-bisphosphate. Specifically functions within the early endocytic pathway and actin organization. a 1,2-diacyl-sn-glycero-3-phospho-(1D-myo-inositol-4,5-bisphosphate) + H2O = a 1,2-diacyl-sn-glycero-3-phospho-(1D-myo-inositol 4-phosphate) + phosphate Interacts with ABP1 and BSP1. The SAC1 domain is capable of hydrolyzing phosphatidylinositol 3-phosphate (PtdIns(3)P), phosphatidylinositol 4-phosphate (PtdIns(4)P), and phosphatidylinositol 3,5-bisphosphate (PtdIns(3,5)P2). Belongs to the synaptojanin family. In the central section; belongs to the inositol 1,4,5-trisphosphate 5-phosphatase family.