We have previously identified a novel 130 kDa protein (p130) which binds Ins(1,4,5)P3 and shares 38% sequence identity with phospholipase C-Δ1 [Kanematsu, Misumi, Watanabe, Ozaki, Koga, Iwanaga, Ikehara and Hirata (1996) Biochem. J. 313, 319–325]. We have now transfected COS-1 cells with genes encoding the entire length of the molecule or one of several truncated mutants, in order to locate the region for binding of Ins(1,4,5)P3. Deletion of N-terminal residues 116–232, the region which corresponds to the pleckstrin homology (PH) domain of the molecule, completely abolished binding activity. This result was confirmed when the PH domain itself (residues 95–232), isolated from a bacterial expression system, was found to bind [3H]Ins(1,4,5)P3. We also found that Ins(1,4,5,6)P4 was as efficacious as Ins(1,4,5)P3 in displacing [3H]Ins(1,4,5)P3, suggesting that these two polyphosphates bind to p130 with similar affinity. This conclusion was confirmed by direct binding studies using [3H]Ins(1,4,5,6)P4 with high specific radioactivity which we prepared ourselves. Binding specificity was also examined with a variety of inositol phosphate derivatives. As is the case with other PH domains characterized to date, we found that the 4,5-vicinal phosphate pair was an essential determinant of ligand specificity. However, the PH domain of p130 exhibited some novel features. For example, the 3- and/or 6-phosphates could also contribute to overall binding; this contrasts with some other PH domains where these phosphate groups decrease ligand affinity by imposing a steric constraint. Secondly, a free monoester 1-phosphate substantially increased binding affinity, which is a situation so far unique to the PH domain of p130.
These authors contributed equally to this work.
Present address: Department of Pharmacology, Faculty of Medicine, University of Malaya, Kuala Lumpur, Malaysia.