Catalytic inositol polyphosphate 5-phosphatase (INPP5c) domain of Type II inositol polyphosphate 5-phosphatase I, Oculocerebrorenal syndrome of Lowe 1, and related proteins
This subfamily contains the INPP5c domain of type II inositol polyphosphate 5-phosphatase I (INPP5B), Oculocerebrorenal syndrome of Lowe 1 (OCRL-1), 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), and to the large EEP (exonuclease/endonuclease/phosphatase) superfamily that contains functionally diverse enzymes that share a common catalytic mechanism of cleaving phosphodiester bonds. INPP5B and OCRL1 preferentially hydrolyze the 5-phosphate of phosphatidylinositol (4,5)- bisphosphate [PI(4,5)P2] and phosphatidylinositol (3,4,5)- trisphosphate [PI(3,4,5)P3]. INPP5B can also hydrolyze soluble inositol (1,4,5)-trisphosphate [I(1,4,5)P3] and inositol (1,3,4,5)-tetrakisphosphate [I(1,3,4,5)P4]. INPP5B participates in the endocytic pathway and in the early secretory pathway. In the latter, it may function in retrograde ERGIC (ER-to-Golgi intermediate compartment)-to-ER transport; it binds specific RAB proteins within the secretory pathway. In the endocytic pathway, it binds RAB5 and during endocytosis, may function in a RAB5-controlled cascade for converting PI(3,4,5)P3 to phosphatidylinositol 3-phosphate (PI3P). This cascade may link growth factor signaling and membrane dynamics. Mutation in OCRL1 is implicated in Lowe syndrome, an X-linked recessive multisystem disorder, which includes defects in eye, brain, and kidney function, and in Type 2 Dent's disease, a disorder with only the renal symptoms. OCRL-1 may have a role in membrane trafficking within the endocytic pathway and at the trans-Golgi network, and may participate in actin dynamics or signaling from endomembranes. OCRL1 and INPP5B have overlapping functions: deletion of both 5-phosphatases in mice is embryonic lethal, deletion of OCRL1 alone has no phenotype, and deletion of Inpp5b alone has only a mild phenotype (male sterility). Several of the proteins that interact with OCRL1 also bind INPP5B, for examples, inositol polyphosphate phosphatase interacting protein of 27kDa (IPIP27)A and B (also known as Ses1 and 2), and endocytic signaling adaptor APPL1. OCRL1, but not INPP5B, binds clathrin heavy chain, the plasma membrane AP2 adaptor subunit alpha-adaptin. In addition to this INPP5c domain, most proteins in this subfamily have a C-terminal RhoGAP (GTPase-activator protein [GAP] for Rho-like small GTPases) domain.
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