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Catalytic inositol polyphosphate 5-phosphatase (INPP5c) domain of synaptojanins This subfamily contains the INPP5c domains of two human synaptojanins, synaptojanin 1 (Synj1) and synaptojanin 2 (Synj2), and related proteins. It belongs to a family of Mg2+-dependent inositol polyphosphate 5-phosphatases, which hydrolyze the 5-phosphate from the inositol ring of various 5-position phosphorylated phosphoinositides (PIs) and inositol phosphates (IPs). They belong to the large EEP (exonuclease/endonuclease/phosphatase) superfamily that contains functionally diverse enzymes that share a common catalytic mechanism of cleaving phosphodiester bonds. Synj1 occurs as two main isoforms: a brain enriched 145 KDa protein (Synj1-145) and a ubiquitously expressed 170KDa protein (Synj1-170). Synj1-145 participates in clathrin-mediated endocytosis. The primary substrate of the Synj1-145 INPP5c domain is PI(4,5)P2, which it converts to PI4P. Synj1-145 may work with membrane curvature sensors/generators (such as endophilin) to remove PI(4,5)P2 from curved membranes. The recruitment of the INPP5c domain of Synj1-145 to endophilin-induced membranes leads to a fragmentation and condensation of these structures. The PI(4,5)P2 to PI4P conversion may cooperate with dynamin to produce membrane fission. In addition to this INPP5c domain, Synjs contain an N-terminal Sac1-like domain; the Sac1 domain can dephosphorylate a variety of phosphoinositides in vitro. Synj2 can hydrolyze phosphatidylinositol diphosphate (PIP2) to phosphatidylinositol phosphate (PIP). Synj2 occurs as multiple alternative splice variants in various tissues. These variants share the INPP5c domain and the Sac1 domain. Synj2A is recruited to the mitochondria via its interaction with OMP25 (a mitochondrial outer membrane protein). Synj2B is found at nerve terminals in the brain and at the spermatid manchette in testis. Synj2B undergoes further alternative splicing to give 2B1 and 2B2. In clathrin-mediated endocytosis, Synj2 participates in the formation of clathrin-coated pits, and perhaps also in vesicle decoating. Rac1 GTPase regulates the intracellular localization of Synj2 forms, but not Synj1. Synj2 may contribute to the role of Rac1 in cell migration and invasion, and is a potential target for therapeutic intervention in malignant tumors. |
Jiyao W et al.(2023). "The conserved domain database in 2023.",