Phosphatase that acts on various phosphoinositides, including phosphatidylinositol 4-phosphate, phosphatidylinositol (4,5)-bisphosphate and phosphatidylinositol (3,4,5)-trisphosphate (By similarity). Has a role in clathrin-mediated endocytosis (PubMed:9428629). Hydrolyzes PIP2 bound to actin regulatory proteins resulting in the rearrangement of actin filaments downstream of tyrosine kinase and ASH/GRB2 (By similarity). 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 ASH/GRB2. Interacts with PACSIN1, PACSIN2 and PACSIN3 (By similarity). Binds AMPH, SH3GL1, SH3GL2 and SH3GL3 (PubMed:9238017, PubMed:9341169). Interacts with MYO1E (via SH3 domain) (PubMed:17257598). Interacts with BIN1 and DNM1 (PubMed:9341169). A stop codon is created in position 1309 of isoform 1 due to alternative splicing. Isoform 1 is found in neonatal brain, and in a wide variety of adult non-neuronal tissues. Isoform 2 is expressed predominantly in the neurons, but is also found in all other tissues at much lower levels. Isoform 1 and isoform 2 are detected in the lung and heart. Isoform 1 is expressed at higher levels than isoform 2 in the testis and liver and both isoforms are not detected in the skeletal muscle. Isoform 3 with the 16-amino-acid insert is only found in the brain while isoform 3 without the 16-amino-acid insert is found in the lung. At embryonic day 12 (E12) only isoform 1 is seen while at E16 and E18 isoform 1 and isoform 2 are seen. In the adult brain expression of isoform 2 increases dramatically as compared with its expression in embryonic brain where as isoform 1 decreases to undetectable levels. Binds to EPS15 (a clathrin coat-associated protein) via a C-terminal domain containing three Asn-Pro-Phe (NPF) repeats. The C-terminal proline-rich region mediates binding to a variety of SH3 domain-containing proteins including AMPH, SH3GL1, SH3GL2, SH3GL3 and GRB2. Splicing of the SAC1 domain does not alter the catalytic activity of synaptojanin 1. Belongs to the synaptojanin family. In the central section; belongs to the inositol 1,4,5-trisphosphate 5-phosphatase family.