We have reported that two inositol 1,4,5-trisphosphate binding proteins, with molecular masses of 85 and 130 kDa, were purified from rat brain; the former protein was found to be the ∆1-isoenzyme of phospholipase C (PLC-∆1) and the latter was an unidentified novel protein [Kanematsu, Takeya, Watanabe, Ozaki, Yoshida, Koga, Iwanaga and Hirata (1992) J. Biol. Chem. 267, 6518-6525]. Here we describe the isolation of the full-length cDNA for the 130 kDa Ins(1,4,5)P3 binding protein, which encodes 1096 amino acids. The predicted sequence of the 130 kDa protein had 38.2% homology to that of PLC-∆1. Three known domains of PLC-∆1 (pleckstrin homology and putative catalytic X and Y domains) were located at residues 110-222, 377-544 and 585-804 with 35.2%, 48.2% and 45.8% homologies respectively. However, the protein showed no PLC activity to phosphatidylinositol 4,5-bisphosphate and phosphatidylinositol. The 130 kDa protein expressed by transfection in COS-1 cells bound Ins(1,4,5)P3 in the same way as the molecule purified from brain. Thus the 130 kDa protein is a novel Ins(1,4,5)P3 binding protein homologous to PLC-∆1, but with no catalytic activity. The functional significance of the 130 kDa protein is discussed.

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