Protein tyrosine phosphatase that negatively regulates oligodendrocyte precursor proliferation in the embryonic spinal cord. Required for normal differentiation of the precursor cells into mature, fully myelinating oligodendrocytes. May play a role in protecting oligondendrocytes against apoptosis. May play a role in the establishment of contextual memory, probably via the dephosphorylation of proteins that are part of important signaling cascades (By similarity). Isoform 3 (phosphacan), previously designated 3F8 chondroitin sulfate proteoglycan or 3H1 keratan sulfate proteoglycan depending on the glycosylation status, is a soluble nervous tissue-specific proteoglycan. It is synthesized by glia and binds to neurons and to the neural cell adhesion molecules tenascin, N-CAM or NG-CAM but not to laminin and fibronectin. Phosphacan acts as a potent inhibitor of cell adhesion and neurite outgrowth. H2O + O-phospho-L-tyrosyl-[protein] = L-tyrosyl-[protein] + phosphate Interacts with tenascin (PubMed:7512960). Interacts with N-CAM and NG-CAM (PubMed:7528221). The carbonic-anhydrase like domain interacts with CNTN1 (contactin) (PubMed:7628014). Interacts with PTN (PubMed:16814777). Interaction with PTN promotes formation of homooligomers; oligomerization impairs phosphatase activity (PubMed:16814777). Interacts (via chondroitin sulfate chains) with MDK (via C-terminal); this interaction is inhibited by PTN; this interaction promotes neuronal migration (PubMed:10212223). A secreted form is apparently generated by shedding of the extracellular domain. Nervous tissue specific. Belongs to the protein-tyrosine phosphatase family. Receptor class 5 subfamily.