Dephosphorylates MAP kinase mpk-1. H2O + O-phospho-L-tyrosyl-[protein] = L-tyrosyl-[protein] + phosphate H2O + O-phospho-L-seryl-[protein] = L-seryl-[protein] + phosphate H2O + O-phospho-L-threonyl-[protein] = L-threonyl-[protein] + phosphate Localizes in rod-like structures at the plasma membrane of pachytene germline cells. Expressed in vulva cell precursors. Expression is increased in P5.p and P7.p and their descendants during vulva induction. Expressed in syncytium hyp7 (PubMed:11161219). Expressed in germline cells (PubMed:12169634, PubMed:16319922). Expressed at low level in the proximal mitotic region and transition zone of germline cells and at the pachytene stage of meiotic prophase I. The KIM (kinase interacting motif) region may be involved in the binding to MAP kinases. Pachytene exit and oocyte development are accelerated (PubMed:12169634, PubMed:21901106). These are associated with the accumulation of smaller oocytes in the proximal gonadal arm, the misalignment of oocytes in multiple rows, a premature nucleolus breakdown and an abnormal mpk-1 phosphorylation after pachytene exit and in diakinesis oocytes (PubMed:12169634, PubMed:21901106). 53 percent of mutants have endomitotic oocytes (PubMed:12169634). Embryo development is arrested before morphogenesis resulting in 50 percent embryonic lethality (PubMed:12169634). In adults, but not in larvae, reduced number of germ cells in the mitotic region of the gonad (PubMed:16319922). Moderate increase in the number of apoptotic corpses in the germline which is further enhanced following gamma irradiation (PubMed:21901106). Increased egl-1 mRNA expression in response to gamma irradiation (PubMed:21901106). Increased cep-1 expression in early pachytene germ cells (PubMed:21901106). Increased life span (PubMed:20624915). Increased MAP kinase activity (PubMed:11161219). Reduced levels of mpk-1 isoform b associated with an increase in mpk-1 isoform b phosphorylation (PubMed:21901106). In lip-1 and let-23, lip-1 and sem-5, and lip-1 and mpk-1 double mutants, normal vulva induction is restored (PubMed:11161219). In lip-1 and mpk-1 double mutants, normal oocyte development and fertility are restored (PubMed:12169634). lip-1 and puf-8 double mutant produces only sperm and has abnormal mpk-1 phosphorylation in the transition zone at the permissive temperature of 20 degrees Celsius. At the restrictive temperature of 25 degrees Celsius, has proximal germline tumors with high levels of mpk-1 phosphorylation (PubMed:22820175). Belongs to the protein-tyrosine phosphatase family. Non-receptor class dual specificity subfamily.