Phosphatase that hydrolyzes phosphate groups from the inositol ring of phosphoinositides and inositol phosphates, in a domain-specific manner (PubMed:10224048, PubMed:18093523, PubMed:23804563, PubMed:27435091, PubMed:33349335, PubMed:40969890). The 5-PPase domain catalyzes removal of the 5-phosphate from substrates such as phosphatidylinositol-4,5-bisphosphate (PtdIns(4,5)P2), phosphatidylinositol-3,4,5-trisphosphate (PtdIns(3,4,5)P3), inositol-1,4,5-trisphosphate (Ins(1,4,5)P3) and inositol-1,3,4,5- tetrakisphosphate (Ins(1,3,4,5)P4) (PubMed:18093523, PubMed:23804563, PubMed:27435091, PubMed:33349335, PubMed:40969890). The SAC domain hydrolyzes phosphates at the 3- and 4-positions of the inositol ring, targeting phosphatidylinositol-3-phosphate (PI(3)P), phosphatidylinositol-4-phosphate (PI(4)P), and phosphatidylinositol-3,5-bisphosphate (PI(3,5)P2) (PubMed:10224048, PubMed:18093523, PubMed:23804563, PubMed:27435091). Plays a role in the phosphatidylinositol metabolic process during an unconventional endocytic mechanism, the ultrafast endocytosis where it participates in the rapide neck formation of the newly generated synaptic vesicle through its phosphoinositide 5-phosphatase activity (By similarity). Also, participates in the clathrin uncoating of synaptc vesicle formed via budding from synaptic endosomes after ultrafast endocytosis through combined activity of its phosphatase domains (By similarity). Additionally, regulates actin filament rearrangement by hydrolyzing PIP2 bound to actin regulatory proteins (By similarity). {ECO:0000250|UniProtKB:O18964, ECO:0000250|UniProtKB:Q8CHC4, ECO:0000269|PubMed:10224048, ECO:0000269|PubMed:18093523, ECO:0000269|PubMed:23804563, ECO:0000269|PubMed:27435091, ECO:0000269|PubMed:33349335, ECO:0000269|PubMed:40969890}. This is the function of Ensembl gene identifier ENSG00000159082 (SYNJ1, synaptojanin 1).