Inositol 5-phosphatase which may be involved in distinct membrane trafficking and signal transduction pathways. May mediate the inhibitory effect of Rac1 on endocytosis (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 Binds to GRB2 (By similarity). Isoform 2A binds to SYNJ2BP/OMP25. Localizes at presynapse terminals in brain and at bundles of microtubules surrounding the nucleus in the elongating spermatids corresponding to the manchette (PubMed:11498538). Translocates from the cytoplasm to membrane ruffles in a RAC1-dependent manner (PubMed:11498538). Interaction of isoform 2A with SYNJ2BP/OMP25 results in localization to the mitochondrion (PubMed:10357812). Additional isoforms seem to exist. Experimental confirmation may be lacking for some isoforms. Widely expressed. Isoforms 2B1 and 2B2 are concentrated at nerve terminals in brain and at spermatid manchette in testis. The PDZ domain of isoform 2A binds SYNJ2BP/OMP25. Mutagenesis of Ser-1246 or Val-1248 to Ala abolishes SYNJ2BP/OMP25 binding. Belongs to the synaptojanin family. In the central section; belongs to the inositol 1,4,5-trisphosphate 5-phosphatase family.